Journal of Conference Abstracts

Session O08:4A

O08 : 4A/01 : H2

Reconstructing the Liquid Line of Descent of the Skaergaard Intrusion

Michael Toplis (mtoplis@crpg.cnrs-nancy.fr)

CRPG-CNRS, Vandoeuvre-lès-Nancy, F-54501, France

The Skaergaard intrusion, East Greenland, has held an important place in the petrologic literature ever since its first description in 1939, often being cited as key field evidence for Fenner's proposal that basaltic differentiation may lead to liquids strongly enriched in iron with little or no enrichment in silica. However, for the last ten years controversy has surrounded the compositional evolution of the Skaergaard liquid since Hunter and Sparks (1987) proposed that the appearance of cumulus magnetite resulted in strong silica enrichment and iron depletion of the liquid, as observed in volcanic systems and experimental studies. On the other hand, a variety of evidence from the Skaergaard rocks including contemporaneous dyke compositions, iron content of plagioclase, and melting experiments (e.g. McBirney and Naslund, 1990; Brooks and Nielsen, 1990; Tegner, 1997) has been used to support the contention that the liquid from which the middle zone of the intrusion crystallized (after the appearance of cumulus magnetite), was rich in iron and low in silica, as originally proposed by Wager and coworkers (e.g. Wager, 1960). This lack of consensus is disturbing given that the compositional evolution of the liquid is clearly of fundamental importance for understanding the physical and chemical interactions between melts and crystals during fractionation. A novel method for estimating the compositional evolution of the Skaergaard liquid will be presented, which combines mass-balance constraints (average rock compositions) and major-element mineral melt equilibria, without making any assumptions concerning relative sub-zone masses, or the initial or final liquid composition. The results suggest that after magnetite saturation iron depletion of the liquid was minimal, but strong silica enrichment occurred. The calculated liquid line of descent is consistent with compositions of chilled margin rocks, contemporaneous dykes, as well as granophyric pods preserved in the upper zone of the intrusion. Furthermore, the liquid evolution is similar to independently modelled liquid lines of descent of ferrobasaltic systems under conditions closed to oxygen (Toplis and Carroll, 1996). This method may be applied to a range of other basaltic layered intrusions in order to estimate liquid lines of descent.

Brooks CK & Nielsen TFD, Contrib Mineral Petrol, 104, 244-247, (1990).

Hunter RH & Sparks RSJ, Contrib Mineral Petrol, 95, 451-461, (1987).

McBirney AR & Naslund HR, Contrib Mineral Petrol, 104, 235-240, (1990).

Tegner C, Contrib Mineral Petrol, 128, 45-51, (1997).

Toplis MJ & Carroll MR, J Petrol, 37, 837-858, (1996).

Wager LR, J Petrol, 1, 364-398, (1960).

O08 : 4A/02 : H2

Late Cretaceous Plutonic Rocks within the Ophiolitic Melange of Crete: Constraints on Their Origin and Evolution

Alexander Langosch

(langosha@gwp-min.min.uni-koeln.de)¹,

Heinz-Günter Stosch

(stosch@ipg003.bio-geo.uni-karlsruhe.de)²,

Eberhard Seidel

(E.Seidel@granat.min.uni-koeln.de)¹ &

Martin Okrusch

(okrusch@mail.uni-wuerzburg.de)³

¹ Institut für Mineralogie und Geochemie, Zülpicher Str. 49 b, 50674 Köln, Germany

² Institut für Petrographie und Geochemie, Kaiserstr. 12, 76131 Karlsruhe, Germany

³ Mineralogisches Institut, Am Hubland, 97074 Würzburg, Germany

The uppermost tectonic unit of the Cretan nappe pile consists of Jurassic ophiolites and an underlying ophiolitic melange. High-temperature crystalline slices form a prominent constituent of the melange. They are composed of upper amphibolite facies metamorphic rocks and metaluminous to slightly peraluminous plutonic stocks. All rock types yield Late Cretaceous K-Ar ages of approximately 70 Ma. Remnants of this formerly widespread HT belt crop out at several localities in Eastern and Central Crete and on some Aegean islands. Petrographic and geochemical features separate the plutonic rocks of Crete into two suites.

The magmatic rocks from Eastern Crete exposed at a few localities south of Agios Nikolaos range from gabbrodiorite to granite with a clear predominance of diorites and quartz diorites. The country rocks are represented by pure marbles and siliceous marbles. Sharp intrusive contacts to the country rocks and an excellent preservation of igneous textures are typical characteristics of this suite which defines a calc-alkaline trend on an AFM diagram. Element variations indicate that fractionation (of hornblende, plagioclase, ilmenite, apatite, biotite and finally K-feldspar) was the major process during magma evolution.

The main outcrop area of plutonites in Central Crete is located in the western part of Asterousia Mountains near the south coast. The intrusive rocks are mostly granites and granodiorites while mafic samples are subordinate. They are all more or less deformed and some appear even as augen gneisses. The contacts to country rocks (including metapelitic gneisses, amphibolites, marbles) are diffuse in some cases, especially in the vicinity to migmatic metapelites. Evidence of high to moderate temperature deformation suggests a syntectonic intrusion. Element variations display some differences to the rocks from Eastern Crete. The calc-alkaline trend shown on an AFM diagram is shifted to a higher Fe/Mg ratio for the whole suite. Intrusive rocks from Central Crete are richer in Ti, P, Zr, Nb, Y, Zn and REE for a given SiO₂ content. Fractionation of pyroxene, plagioclase and further phases played an important role in the early magma evolution. Additionally, geochemical characteristics require a crustal contribution, probably by assimilation of metasedimentary rocks with abundant zircon.

Recent Rb-Sr data of both, igneous and metamorphic rocks confirm the Late Cretaceous age and support contemporaneous intrusions and metamorphism. Nd and Sr isotopic compositions reveal similarities between the two Cretan plutonic suites. Mafic samples have compositions in the upper range of arc basalts or the lower range of MORB while intermediate to felsic rocks occupy an array between MORB and sediments, consistent with their formation by an AFC process. The combined geochemical evidence suggests an active plate margin as the most likely tectonic setting for generating melts of the Cretan granitoids.

O08 : 4A/03 : H2

Geochemical Variations in the Clasts and Matrix of Polymict Peridotites

Lisa Morfi (lmorfi@glg.ed.ac.uk)¹,

Ben Harte (Ben.Harte@glg.ed.ac.uk)¹,

Peter Hill (Peter.Hill@glg.ed.ac.uk)¹ &

John Gurney (john.gurney@minserv.co.za)²

¹ Grant Institute of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh, Scotland

² Department of Geological Sciences, University of Cape Town, Cape Town, South Africa

Polymict peridotites are an exceptional suite of mantle xenoliths recovered from kimberlites in Kimberley, South Africa. They have breccia-like characteristics and contain rock and mineral fragments derived from the mantle. These fragments are embedded in a very variable matrix often rich in phlogopite together with other silicates, ilmenite, rutile and sulphides. A two stage petrogenesis has been suggested for these xenoliths. This involves creation of the polymict rocks through sampling of the upper mantle by a fluid which migrated from depth and eventually crystallises the matrix assemblage. The consolidated polymicts rocks were later sampled and transported to a near-surface environment by a younger kimberlite intrusion. The polymict rocks provide an excellent opportunity to study aspects of melt infiltration and mantle metasomatism.

Major element analyses of orthopyroxene and phlogopite clasts in three of the xenoliths indicate core-rim variations. The orthopyroxene clasts have variable core compositions whereas the rims show a common Fe/Fe+Mg value of approximately 0.11. Often, individual orthopyroxenes show two rim compositions - an inner rim with higher Fe/Fe+Mg values and an outermost rim of slightly lower Fe/Fe+Mg values. This feature is not necessarily consistent within one nodule. Analyses of matrix orthopyroxenes in one of the nodules reveal that these crystals have a similar Fe/Fe+Mg value to the rims of the larger orthopyroxene clasts. The phlogopite clasts also show variable core compositions and two distinct rim compositions - both of higher Fe/Fe+Mg values than the cores. With respect to other elements, the phlogopite rims are consistently higher in Na, Cr, Al and Ti than the cores. This is not true of the orthopyroxene clasts. These features are suggestive of metasomatic processes operating on clasts derived from a variety of sources and preliminary interpretations involve the possibility of more than one pulse of a metasomatising fluid which has resulted in the heterogeneous rim compositions.

Trace element analyses of garnet clasts also show a variety of sources are present in the xenoliths. One polymict peridotite nodule contains small yellow-orange garnets intimately intergrown within a fine-grained matrix of orthopyroxene, phlogopite and ilmenite. The REE profile of these garnets is typical of megacrystic garnets from kimberlite sources. High TiO₂, low Cr₂O₃ and moderate CaO support this. A late-stage garnet overgrowth surrounding a large garnet fragment shows similar compositions to these matrix garnets. These features suggest that the melt which has transported the clasts and crystallised to form the matrix of the polymict peridotites is a relatively primitive ultrabasic melt of probably asthenospheric origin (Harte et al., 1993).

Harte B, Hunter RH, Kinny PD, Philosophical Transactions of the Royal Society of London A, 342, 1-21, (1993)

O08 : 4A/04 : H2

Subjotnian (1.53-1.50 Ga) Alkaline Magmatism in Central Sweden: Origin and Evolution of the Magmas

Ulf B. Andersson (ulf.andersson@geo.uu.se)

Institute of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Suède

A number of minor Subjotnian intrusive complexes in central Sweden, belonging to the westernmost and youngest (1.53-1.50 Ga) part Fennoscandian rapakivi magmatism, contain alkaline rocks as a major constituent. The alkaline rocks comprise saturated and over-saturated syenites, some trachytic dykes, and silicic peralkaline granites and dykes. Gabbros and met- to peraluminous granites are also an essential part of these complexes. Trachyte dykes have been found, particularly associated with the small Nordsjö intrusion. They display trachytic flow structures and contain phenocrysts of anorthoclase, Fe-Ti oxides, augite, and apatite, providing evidence that syenitic magmas were originally present, precipitating these phases. The syenites are particularly high in K, Na, Ba, and Eu, compared with the associated rocks, suggesting feldspar accumulation. This is also evident from the REE patterns, which show positive Eu anomalies, in contrast to those from the trachytes. The relative Fe enrichment is high in the syenites (Fe/(Fe+Mg)=0.85-0.96, av. c. 0.92), compared with the met-peraluminous granites (<0.90), while the peralkaline silicic rocks are all higher than 0.95. The latter contain typically one feldspar, a hypersolvus mesoperthite with no Ca, and alkali-rich amphiboles (riebeckite and ferrorichterite). Field relations promted geochemical testing of the cogenetic relation between the syenites and peralkaline silicic rocks. Least-square major element modelling was perfomed, using different syenites or trachytes as parental magmas arriving at peralkaline residual compositions. The calculations suggest that this evolution can be achieved by 50-70% crystallization of 75-85% anorthoclase, 7-15% Fe-Ti oxides, 3-10% augite, 2-7% fayalite, and 1-2% apatite, depending on start and end compositions, consistent with the modal phenocryst mineralogy observed in the trachytes (except for fayalite which, on the other hand, is abundant in the syenites). Independent tests of the major element models can be obtained by trace element modelling using crystal/liquid partion coefficients. Such calculations yield overall very good agreement, particularly for the REE but also for the HFS elements. Poorer fits are noted for the feldspar-related elements Eu, Ba, and Sr, which is understandable considering the uncertainty in the degree of feldspar accumulation in the syenites.In conclusion: the phenocryst mineralogy observed in the trachyte dykes probably represents the crystallizing assemblage that drives magma evolution from saturated syenitic to peralkaline silicic composition. In contrast, no particular evidence suggests a cogenetic connection between the alkaline rocks and the associated basic or met-peraluminous granites. A probable scenario: three different protoliths (mantle, mafic lower crust, and felsic calcalkaline crustal) gave rise to three different primary magmas (gabbroic, syenitic, and metaluminous granitic) simultaneously in time, which cointruded along the same crustal channelways to their present emplacement levels.

O08 : 4A/05 : H2

F-Li-B-Rich Two Mica S-Monzogranite Cored at 3.5 km Depth in the Larderello Geothermal Field (Italy)

Giuseppe Cavarretta (gc@gea.geo.uniroma1.it)¹ &

Mariano Puxeddu²

¹ CNR - C.S. Quaternario E. A., Dip. Scienze della Terra, P.le A. Moro, 5, 00185 Roma, Italy

² CNR - Istituto Internazionale Ricerche Geotermiche, Piazza Solferino, 2, 56126 Pisa, Italy

The well Monteverdi 7 (MV7) crossed at 3.5 km depth a finegrained, heterogranular, hypidiomorphic, 3.8 Ma old, two mica S-monzogranite. Quartz, K-feldspar, plagioclase, biotite, muscovite, cordierite and and alusite are the main phases. Tourmaline, titanite, apatite, zircon and fluorite are the accessory minerals. A metapelitic xenolith is rich in sillimanite and green spinel. Two generations of plagioclase were recognized: early limpid intratelluric crystals (An₄₀-An₆₀) and late fine grained turbidones (An₄-An₂₃). K-feldspar occurs with perthitic hypersolvus albite-rich microphenocrysts and fine grained albite-poor crystals. Biotite is a siderophillite-rich variety plotting within the "clusters" field in the Mg vs. F diagram (Mason, 1992). Muscovite includes an early primary paragonite-rich magmatic type with FeO_tot=1-1.5%, MgO=0.8-1.1% and a paragonite-poor late magmatic one with FeO_tot=3-7%, MgO=1.5-4%; both types are F-rich (up to 0.95%). A gradual increase of fO₂ is observed from early primarymuscovites with Fe³⁺/(Fe²⁺ + Fe³⁺)=0.05-0.10 to late primary ones with Fe³⁺/(Fe²⁺ + Fe³⁺)=0.10-0.50. Fluorine intercept IV(F) for adiacent crystals of biotite and muscovite are 1.56 and 1.62 respectively, an evidence of equilibrium growth. Very high F contents characterize titanite (up to 4.66%) and apatite (up to 4.34%). Tourmalines plot in the field of Li-poor granitoid pegmatites and aplites (Cathelineau et al., 1992). Fluorite accur as anhedral blades (5-20 µm) associated with very late muscovite crystals (5-20 µm) within cavities, and along fractures and cleavages of calcicplagioclase.

The bulk composition of the MV7 rock is comparable to that of F-Li-B-rich peraluminous two-mica rhyolites and granites of Utah, Alaska, New Mexico, Peru, Australia, SW England, Portugal, Czech Republic, Mongolia and Himalaya. Liquidus temperatures of 850^o-825^oC and initial 4-5 wt% H₂O are hypothesized. The abundance of LiCl up to 30 wt% eq. in fluid inclusions (Cathelineau et al, 1994; Valori et al., 1992), the strong final enrichment in F and B testified by the occurrence of late tourmaline and fluorite, and the geothermometric indications supplied by the biotite-apatite pair and by the muscovite compositions suggest solidus temperatures as low as 540^o-600^oC and P_H2O=P_Load=115 MPa (calculated from rock densities in the MV7 well). The last residual melts were strongly enriched in Na as suggested by a local pervasive albitisation of the whole rock without any evidence of a hydrothermal post-magmatic nature of the albite crystals. Finally the downward percolation of fluorine-free meteoric waters led to chloritisation of biotite and sericitisation of feldspars.

Mason RA, Can Mineral, 30, 343-354, (1992).

Cathelineau M, Marignac C, Boiron MC, Gianelli G, Puxeddu M, Geochim Cosmochim Acta, 58, 1083-1099, (1994).

Valori A, Cathelineau M, Marignac C, J Volcanol Geotherm Res, 51, 115-131, (1992).

O08 : 4A/06 : H2

Multi-Generation, Multi-Source Dyke Swarms in the Area of the Central Bohemian Plutonic Complex

Franticek V. Holub (frholub@prfdec.natur.cuni.cz)

Inst. of Petrology and Struct. Geology, Fac. Science, Charles University, Praha 2, Czech Republic

The Central Bohemian Plutonic Complex (CBPC) of Lower Carboniferous age intruded into the Central Bohemian Suture Zone (CBSZ) separating the relatively cold Barrandian block at NW from the hot, rapidly exhumed metamorphic rocks of the Moldanubian block at SE. CBPC comprises various types of granitoid and less voluminous mafic rocks ranging in composition from calc-alkaline (CA) and high-K calc-alkaline to shoshonitic (HK) to ultrapotassic (UK).

Dyke swarms that are highly variable in composition, orientation and temporal relations to plutonic rocks intrude the area. Pre-CBPC dykes (N-S trending diabases, variously oriented spessartites, and felsic granitoid porphyries) are known from the adjacent parts of the Barrandian block only. Within major parts of CBPC itself, there occurs a conspicuous regional swarm of innumerable E-W to SE-NW trending dykes formed by multiple injections of highly diverse magmas. They correspond to dolerite, microdiorite and spessartite (CA), kersantite, granodiorite to granite porphyries (HK), minette (UK), melasyenite to melagranite porphyries (UK, probably of hybrid nature), etc. Many minettes and UK porphyries are well comparable in composition to and perhaps broadly coeval with the ultrapotassic plutonic rocks of the durbachite group (zircon ages about 343-346 Ma, Holub et al. 1997).

Among mafic dykes the minettes are the youngest and most widespread. Accompanied by UK porphyries, they occur even in a part of the Moldanubian adjacent to S margin of CBPC. Older small magma batches were probably unable to penetrate the hot, low-viscosity, anatectic Moldanubian crust. The "peri-Moldanubian" zone of CBPC inclusive of the durbachitic rocks and also the Moldanubian complex itself are intruded by numerous dykes of tourmaline leucogranites originated in a crustal source. They should be younger than almost all the "typical" dyke rocks, perhaps excepting some minettes and the youngest, N-S striking dyke swarm of highly evolved sub-volcanic granite porphyry (rhyolite).

Multiple intrusive pulses forming the dyke swarms should be derived from distinct mantle and crustal sources. The highly variable geochemistry of mafic dykes reflects a strongly heterogeneous nature of the lithospheric mantle, which was depleted to various degrees by previous extraction of basaltic melts and subsequently enriched in hygromagmatophile elements. High LILE/HFSE elemental ratios suggest enrichment by hydrous fluids released from a subducted slab due to its metamorphic dehydration. The highly refractive nature combined with the most pronounced enrichment are characteristic for the mantle sources of ultrapotassic magmas. Some systematic variations in geochemistry of comparable mafic dykes from both sides of CBSZ indicate its first-rate significance and a different compositional development of the two lithospheric mantle domains before the final juxtaposition of the Barrandian and Moldanubian blocks.

Holub FV, Rossi Ph, Cocherie A, CR Acad. Sci. Paris Sci. Terre. Planet, 325, 19-26, (1997).

O08 : 4A/07 : H2

Partially Molten Lower Crust beneath Northern Tibet Triggered by Underplating of Melts Derived from Lithospheric Mantle?

Sun-Lin Chung (sunlin@ccms.ntu.edu.tw)¹,

Ching-Hua Lo¹ &

Tung-Yi Lee (t44001@cc.ntnu.edu.tw)²

¹ Department of Geology, National Taiwan University, 245 Choushan Road, Taipei 106, Taiwan

² Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan

The INDEPTH team has provided evidence for a partially molten layer (10-15 km) at the mid-crustal level in southern Tibet. Owens and Zandt (1997) reported seismic data suggesting that from south to north in the Tibetan plateau the crust thins by up to 20 km and pervasive partial melting of the lower half of the crust (>30 km) occurs in the north as a result of the India-Asia collision. An apparent drawback, however, is that in northern Tibet post-collisional, crustally-derived magmas, such as the leucogranites widespread in the Himalayas and southern Tibet, have been rarely identified. On the other hand, geochemical investigations of post-collisional potassic igneous rocks over the entire plateau have repeatedly suggested that partial melting has taken place in the underlying lithospheric mantle. The potassic magmatism may have resulted from convective removal (or "delamination" as often loosely referred) of the thickened Asian lithosphere, a process that could have also accounted for rapid uplift and development of north-striking normal faults in the Tibetan plateau.

We therefore envision that the proposed partially molten layer in the lower crust of northern Tibet does not necessarily only indicate melting of the lower crust. It may instead be interpreted in terms of extensive underplating of magmas derived from the lithospheric mantle, a process capable of upward influxing heat for the unusually high Poisson's ratios reported in the region. Whereas in the Himalayas and southern Tibet a thicker crust allows large scale crustal anatexis to form the leucogranites, delamination-induced partial melting in the north could have occurred preferentially in the lithospheric mantle because of a thinner crust (and lithosphere) and hotter geothermal structure. If substantial amounts of mantle-derived melts intrude into the lowermost part of crust, i.e., close to and above the Moho, they would be able to induce significant crustal melting. Then the abnormal lower crustal features observed in northern Tibet may be caused by combined effect of mantle and crustal melts. Considering the fact that so far crustally-derived igneous rocks have been identified only in a restricted area in central northern Tibet, a question remaining is why crustal melts do not erupt more widely, as did mantle-derived potassic rocks, or intrude into shallow depths and thus can be exposed by now. Perhaps crustal melts do exist in a thick range in the lower crust of northern Tibet, but they need more time to move upward and/or are awaiting a more effective mechanism to enhance melt migration and eruption.

O08 : 4A/09 : H2

The Kraemer Island Macrodyke, Skaergaard Region, East Greenland ­ Trapped Liquid Shift During Crystallization of a Tholeiitic Intrusion

Peter Momme (MOMME@geo.aau.dk) &

Richard Wilson (JRW@geo.aau.dk)

Geologisk Institut, Aarhus Universitet, DK-8000 Århus C, Denmark

The Kraemer Island Macrodyke (KIMD) is situated in central east Greenland, about 7 km west of the Skaergaard Intrusion. KIMD is exclusively exposed against Precambrian gneiss on Kraemer Island where it has a length of ~5 km and a width varying from 600 to 1000 m. The tholeiitic KIMD consists mainly of olivine gabbros, some with well developed clinopyroxene oikocrysts. KIMD is part of a dyke swarm that shows chilled margin whole-rock compositions similar to calculated Skaergaard liquids ('Skaergaard-like' dykes in Brooks & Nielsen, 1978). Weak modal layering (N-S/~30^o W) is developed in the northern outcrops on Kraemer Island. The most primitive clinopyroxene and plagioclase compositions in the gabbros (An₆₉ : Mg#cpx80) are slightly more primitive than in Skaergaard LZa. In contrast to clinopyroxene and plagioclase, olivine grains are unzoned and the most primitive composition is Fo₆₅. Clinopyroxene oikocryst cores are chadacryst-free and are believed to have grown from a large magma reservoir during the cumulus stage. Some KIMD olivine gabbros have abnormally high modal contents of olivine (20-25%) compared to the majority of the KIMD cumulates (5-10%). These olivine-rich gabbros occur near the eastern margin of the intrusion, suggesting that this could be a feature related to a relatively high degree of undercooling at the intrusion margin. In bivariate plots of major- and trace elements vs. wt% MgO, the compositional variance in the olivine-rich samples is covered by simple weighted average addition of olivine (Fo₇₈) alone to an appropriate magma composition (from the cogenetic 'Skaergaard-like' dyke swarm) inferring that accumulation of clinopyroxene and plagioclase did not play a role in formation of these rocks. This excludes the possibility that the olivine-rich gabbros simply are the result of crystallization of cotectic proportions of the mineral assemblage. Furthermore, Fo₇₈ coexists with the observed An₆₉ and Mg#₈₀ clinopyroxene in calculated "COMAGMAT" (Ariskin et al., 1993.) models of fractional crystallisation. In 'intra-plutonic chill zones' (ICZ) which occur near the western margin of KIMD new influxes of hot magma crystallized against pre-existing macrodyke walls to form olivine-rich gabbros. The olivine content here gradually decreases towards the intrusion centre to form 'normal' olivine-gabbros. Redistribution of olivines (initially produced at an ICZ) during a prolonged replenishment episode, could result in addition of olivine to a KIMD parental magma, making the observed compositional variations of the olivine-rich KIMD gabbros possible. All in all, combination of thin section observations, mineral- and whole-rock compositions as well as computer models suggest that olivine in KIMD has re-equilibrated to more Fe-rich compositions as a result of "trapped liquid shift" with Fo ~ 13 - 20 mol%.

Brooks CK & Nielsen TFD Lithos, 11, 1-14, (1978).

Ariskin AA, Frenkel MY, Barmina GS & Nielsen RL, Computers and Geosciences, 19, 1155-1170, (1993).

O08 : 4A/10 : H2

Separation of Bulk and Interstitial Liquid Fractionation Recorded by Pigeonite-Augite Intergrowths in the Graveyard Point Sill, Oregon

Gregor Markl (markl@ruf.uni-freiburg.de)¹ &

Craig White²

¹ Institut für Mineralogie, Albertstrasse 23 B, 79104-Freiburg, Germany

² Dept. of Geosciences, Boise State University, Boise, ID 83725, USA

The 150 m thick late Miocene Graveyard Point sill (GPS) is situated at the Idaho-Oregon border near the southwestern edge of the western Snake River Plain. It records from bottom to top continuous fractional crystallization of a tholeiitic parent magma (lower chilled border, FeO/(FeO+MgO)=0.59, Ni=90 ppm) towards granophyrs (late pods and dikes, FeO/(FeO+MgO)=0.98, 78 wt.% SiO₂ 3.5 wt.% K₂O, <4 ppm Ni) showing a typical trend of Fe and P enrichment. Fractionating minerals are olivine (Fo₇₉-Fo₂), augite (X_Fe=0.18-0.95), feldspars (An₈₀Or₁-An₁Or₆₂), Fe-Ti oxides (Ti-rich magnetite and ilmenite), apatite and in two samples pigeonite (Wo_18-28Fs_41-54). The granophyrs may bear some quartz. Compositionally zoned minerals record a large interval of the fractionation process in every single sample, but this interval changes with stratigraphic height. In pigeonite-bearing samples, olivine is scarce or lacking and because pigeonite occurs as characteristic overgrowths on augite, its formation is interpreted to be related to the schematic reactionaugite + olivine = pigeonite that defines the cotectic between augite and pigeonite in olivine-saturated basaltic systems. Line measurements with the electron microprobe reveal that the transition from augite to pigeonite is continuous. However, some crystals show an abrupt "reversal" towards augite after pigeonite growth. These observations appear to be similar to some textures described from lunar basalts, but they have not yet been described from terrestrial rocks. We interpret the textures to record the competition of two processes during fractionation in the GPS: fractional crystallization of the bulk liquid (the bulk melt separates from solids and interstitial liquids in the solidification front) and fractional crystallization of interstitial melt in the solidification front itself. Interplay between those two processes is proposed to account for the observed variations in mineral chemistry and mineral textures and is consistent with pigeonite stability field variations in published phase diagrams of basaltic systems during fractionation.

O08 : 4A/11 : H2

Petrography and Petrochemistry of the Upper Cretaceous Volcanic and Subvolcanic Rocks in Olucak Region (Gumushane-Torul/NE Turkey) and Their Genetic Relation to Torul Granitoide

Hakan Coban (coban@sdu.edu.tr)

Suleyman Demirel University Geology Dept. Isparta, Turkey

The study area includes the Olucak (Gumushane-Torul/NE Turkey) and its surrounding areas located at between the northern and southern zones of the East Pontide. Upper Cretaceous outcrops wide spreadly in the region and consist of magmatic rocks (Karaburun dacite-Torul granitoide) and volcano-sedimentary rocks (Kermutdere-Tepekoy formations). Two phases were determined in the Upper Cretaceous volcanism. The first phase (Early phase) includes the volcanics Kermutdere Formation. In this phase, rocks have basaltic-trachyandesite/tranhyandesite and phonolitic composition and indicate silica-saturated and silica-undersaturated magmas. The bottom level of the phase consist of the rocks having plagioclase (neutral) + sanidine + hornblende + clinopyroxene (augite). The upper part of the phase is composed of the rocks having plagioclase (acid-neutral) + sanidine + nepheline + sodalite (nosean) + clinoamphibole (arfvedsonite) and olivine. The second phase (late stage) is chracterized by a silica over-saturated magma and indicates rhyolitic to dacitic rocks in composition (Tepekoy formation). The rhyolites in the second phase have a perlitic texture and are a rapidly cooled magma origin. Some textural features, observing in Upper Cretaceous volcanites, such as melting inclusions in plagioclases, occurrence of needle-shaped apatites, ocellar texture (maphic mineral inclusion bearing quartz) are the evidences of the same aged homogene magma-mixing between both maphic and felsic magmas. The petrochemical analysis of volcanics showed that the volcanics of Kermutdere have alkaline (shoshonitic)-calc-alkaline magma character and the Karaburun dacite-Tepekoy formation volcanics have calc-alkaline magma character. These characters are associated with their mineralogic composition. These petrographic-petrochemical aspects and especially petrogenetic trace elements parameters of Upper Cretaceous volcanics indicate that volcanics are derived from different magma origines and Kermutdere formation volcanics have hybridic magma genesis related to partial melting of upper mantle and partially crustal material. Tepekoy formation and Karaburun dacite have dominantly crustal genetic magma and are interpreted that as Karaburun dacite and Tepekoy formation volcanics are the marginal sub-volcanic and volcanic extrusion facies of the Torul granitoide, which intruded in the Upper Cretaceous age.

O08 : 4A/12 : H2

Mantle Fluids and Genesis of Explosion and Impact Circulars Structures. Problems of Impact Volcanism

Vladimir Sakhno (olegavchenko@geocities.com)

Kirova 70-126, Vladivostok, Russia

Analyses of geological, petrologic and geochemical data on the El'gygytgyn caldera in the Eastern Chukotka and the Popigai and other circular structures in the Siberian North of Russia, which are assumed as astroblemes, revealed that they all were of endogenic origin. There is regularity in their development - all are located in the center of large arched uplifts composed of volcanic rocks. Processes of the structure formation was associated with volcanism of acid-to-basic character, took long interval from Early Cretaceous to Cenozoic and culminated in a powerful explosion in the center of uplift and caldera formation. Endogenic impactite and impact breccia, slag and glass characterize this sage. Rocks resulted during the explosion usually have mineral associations with planar features completely or partly replaced by diaplectic glass. The bulk of the impact melt is represented by fresh glass with fluidal texture separate sprites of which are differ from each other by color and composition. Planar features and diaplectic glasses in minerals as well as finding of coesite permitted defenders of the astrobleme theory to consider these structures to be impact, produced by impact of cosmic bodies. Thorough investigation of the rocks has not revealed any geochemical anomalies, whilst study of liquid phase of minerals and rocks revealed a high content of H and a high hydrogen-carbon ratio. The relation was close to 2. Rocks before the caldera stage (pre-impact explosion) have the marked predominance of carbonic oxides in fluid. The simulation of chemical reactions of gases with the "Selector" program (Karpov et. al., 1997, 1998) has demonstrated that the temperature and pressure these rocks having been formed under were more than 2000°C and 200 kb respectively. That did not correspond to the real adiabatic conditions of acid melts generating in the focuses of volcanism, but conform to conditions of gaseous chemical reactions developing through the mechanism of chain reaction of explosive mixture (hydrogen with oxygen) with the spasmodic velocity increase towards the higher values. That brings to a powerful explosive release of energy, high temperature and pressure, to an occurrence of minerals high in density (diamond, coesite) and minerals with planar features. Isotopic geochemistry of fluid as well as petrologic characteristics of the rocks supports the possibility of impact origin of the circular structures. It is also probable that impact volcanism was of widespread occurrence throughout the Earth's history (Marakushev et. al., 1993, 1997, and others).

O08 : 4A/13 : H2

Oxygen Isotope Composition of Plume-Related Magmatism in the Manus Basin, S.W. Pacific

Colin G. Macpherson (c.macpherson@gl.rhbnc.ac.uk)¹,

David R. Hilton (drhilton@ucsd.edu)²,

David P. Mattey (d.mattey@gl.rhbnc.ac.uk)¹ &

John M. Sinton (sinton@soest.hawaii.edu)³

¹ Department of Geology, Royal Holloway University of London, United Kingdom

² Geosciences Research Division, Scripps Inst. Oceanography, University of California San Diego, USA

³ Department of Geology and Geophysics, University of Hawaii, USA

The Manus Basin is a site of extension in the eastern Bismarck Sea, north of the active New Britain Arc. Magmatism occurs along the north-east striking Manus Spreading Centre and east striking Extensional Transform Zone. Smaller rift segments to the south and east are also volcanic. Like many other back-arc basins recent magmatism in the Manus Basin displays a range of petrogenetic affinities. Basaltic glasses form a spectrum between compositions typical of mid-ocean ridges and rocks that are arc-like. However, helium isotope analyses have revealed the presence of an additional component derived from a deep mantle plume.In this study we present a comprehensive oxygen isotope study of back-arc magmatism in the Manus Basin. ¹⁸O/¹⁶O ratios of basic glasses display a negative correlation with helium isotopes ratios. Higher ¹⁸O values are associated with ³He/⁴He ratios typical of the upper mantle. These high ¹⁸O values result from interaction between residual upper mantle and fluids recycled through the subduction zone. Glasses with high ³He/⁴He ratios, characteristic of a mantle plume, have ¹⁸O values that are lower than average mid-ocean ridge basalt ¹⁸O/¹⁶O ratios. Crustal contamination can not produce the observed correlation suggesting an ¹⁸O-depleted reservoir is associated with the mantle plume. This reservoir may represent parts of recycled oceanic crust that were hydrothermally altered at high temperatures. Alternatively, interactions between silicate and iron at the core mantle boundary may generate an ¹⁸O-depleted reservoir. Entrainment of such material in a plume, upwelling from the base of the lower mantle, could explain the relationship between the He and O isotope systems in the Manus Basin.

O08 : 4A/14 : H2

Zoning Patterns in Volcanic Feldspars as a Clue to Magmatic Processes: A New Approach Using Electron Microprobe

Catherine Ginibre (cginibr@ugcvax.dnet.gwdg.de) &

Gerhard Wörner (gwoerne@gwdg.de)

Geochemisches Institut, Goldschmidtstr. 1, D-37077 Göttingen, Germany

Zoning in phenocrysts, especially plagioclase, has been used as a tool to understand the history of the magma chamber in which they grew. However, the relationships between observed zoning patterns and processes and parameters which control them (growth, resorption, change in temperature, pressure or composition in the magma) are not well established. Here we investigate feldspars phenocrysts from Parinacota (Chile) and Laacher See (Eifel, Germany) volcanoes in order to understand these relationships. We use a combination of textural observation (optical microscopy and BSE images) and electron microprobe analysis of major, minor and trace elements and element mapping (Ca, Na, K, Al, Si, Ba, Sr, Fe, Mg, Ti).

A large number of feldspar crystals were investigated and show great diversity in textures (oscillatory zoning, resorption surfaces, and patchy zoning) but also some common features. Oscillatory zoning is present in most crystals and were probably destroyed by later dissolution and diffusion in others. Grey-value profiles across BSE images of oscillatory zoned crystals are representative of An content and show high spatial resolution (sub-µm). Zoning parameters which vary along a profile (or between crystals) are wave length, amplitude and shape of the oscillations as well as shape of growth zones boundaries (former crystal surfaces?). An aim of the study is to relate these oscillation parameters to growth conditions or igneous events in the magma chamber. Different styles of resorption surfaces correlate with different styles of zoning: low amplitude high frequency resorption surfaces are typical for oscillatory zoned plagioclase while high amplitude low frequency surfaces correlate with patchy zoning. Zoning and resorption patterns in sanidine are similar to those in plagioclase but less pronounced and larger scaled.

Minor and trace elements do not neccesarily correlate with each other and with major elements. For example, Ba and An-content correlate while Sr and An do not, suggesting fast diffusion for Sr in the crystal. Mg and Fe increase strongly but An decreases at the outermost rims which probably grew fast after eruption. This may indicate kinetic effects. Diffusion and growth kinetics are time dependent and thus may provide information for rates of processes and histories of the crystals in the magma chambers.

Session O08:4B

O08 : 4B/25 : H2

Eclogite and Granulite Nodules from the Mbuji Mayi Kimberlites (Kasai, R.D. Congo): Products of the Isobaric Cooling of an Underplated Mafic Gabbronoritic Complex

Daniel Demaiffe (ddemaif@ulb.ac.be)¹,

Saïd El Fadili (selfadil@ulb.ac.be)¹,

Luc Andre (lucandre@africamuseum.be)² &

Nicolas Coussaert

¹ Universitè Libre de Bruxelles, Géologie CP160/02, 50, Avenue Roosevelt B 1050 Bruxelles, Belgium

² Musée Royal de l'Afrique Centrale, Département de Géologie, 3080 Tervuren, Belgium

The Mbuji Mayi diamondiferous kimberlites have intruded (71 Ma ago) the ArcheanKasai craton that consists of high-grade migmatitic gneisses and a huge metamorphosed gabbronoritic complex.

Four main groups of rocks have been distinguished petrographically among the 160 deep-seated nodules : 1) bimineralic eclogites: omphacite + garnet ± accessory minerals; 2) kyanite-bearing eclogites: kyanite occurs either as symplectitic intergrowths with omphacite or as subhedral laths included in garnet; 3) diamond-bearing eclogite (one sample); 4) mafic granulites characterized by primary granulite assemblage (plagioclase +diopside +garnet) and secondary eclogite assemblage which consists of garnet and kyanite-omphacite intergrowths similar to those observed in kyanite eclogites. The nodules were variously affected by metasomatic process(es).

The bimineralic and kyanite-bearing eclogites were equilibrated under similar P-T conditions: 861-1091°C; 17-30 Kb and 815-946°C; 20-25 Kb respectively. The kyanite-bearing granulites were equilibrated under the same conditions (920°C and 20-21 Kb).

Detailed petrographic observations and mineral chemistry of the granulites show that the kyanite-omphacite intergrowths result from the breakdown of plagioclase (An₄₈) reacting with Al-rich diopside. This reaction possibly proceeded by isobaric cooling at relatively high P. The coexistence of relict diopside and newly formed omphacite and of two chemically distinct garnets in the granulites together with partial breakdown ofplagioclase suggest that these rocks have recorded the eclogite-granulite facies transition under the Kasai craton.

Major element compositions of bimineralic eclogites are broadly basaltic (Mg#= 42-72). Kyanite-bearing eclogites and kyanite-bearing granulites are peraluminous (17-28 and 21-24.4 wt% Al₂O₃) which suggests that their protoliths were plagioclase-rich (cumulate gabbros and/or anorthosites) This is confirmed by positive Ba, Sr, Pb and Eu anomalies in the trace element diagrams; these anomalies are higher for kyanite-bearing eclogites and kyanite-bearing granulites than for bimineralic eclogites.

The initial (at 2.5 Ga) Sr and Nd isotopic composition of the least metasomatized eclogites and of the kyanite-bearing granulites are similar (0.70102-0.70175; <epsilon>Nd= +2.80-+5.97) and comparable to those of the gabbronoritic complex (0.70156-0.70186; <epsilon>Nd= +1.3-+5.40) that outcrops 100 km to the SW of the Mbuji Mayi area. The isotopic data indicate a slightly depleted mantle source-region. The estimated age of the eclogite metamorphism (2.65±0.26 Ga; Sm-Nd on 5 ga-cpx pairs) is, within error limits, similar to the age of granulite metamorphism (2.4±0.1 Ga) that affected the gabbronoritic complex of Kasaï.

The Mbuji Mayi granulite and eclogite nodules could be the deep lower crust and the uppermost mantle equivalents of the metamorphosed gabbronoritic and anorthositic complex. Prolonged cooling, at high P, of this complex could generate the granulites and the two main groups of eclogites : the bimineralic eclogites being derived from protoliths of gabbronoritic compositions and the kyanite-bearing eclogites from gabbroic anorthosites and/or anorthosites.

O08 : 4B/26 : H2

Composition and Processes of the Mantle Lithosphere in Northeastern Brazil and Fernando de Noronha: Evidences from Mantle Xenoliths

Giorgio Rivalent (riva@unimo.it)¹,

Maurizio Mazzucchelli (mazzuc@unimo.it)¹,

Vicente A.V. Girardi (girardi@usp.br)²,

Maria A. Barbieri¹,

Alberto Zanetti (zanetti@crystal.unipv.it)³,

Steve L. Goldstein (steveg@ldeo.columbia.edu)⁴,

¹ Dipartimento di Scienze della Terra, Università di Modena, Italy

² Instituto de Geociencias, Universidade de Sao Paulo, Brazil

³ CNR, Centro di Studio per la Cristallochimica e la Cristallografia, Pavia, Italy

⁴ Lamont-Doherty Earth Observatory, Columbia University, Palisades, USA

Cenozoic alkali basalts in the Rio Grande do Norte State and the Fernando de Noronha Island contain abundant spinel facies mantle xenoliths. At Pico Cabuji (NE Brazil) both protogranular and porphyroclastic xenoliths occurr. The former mainly consists of lherzolites and the latter of harzburgites, with equilibrium temperatures of 1064±6°C and 1238±7°C, respectively. Pressure is assumed essentially equal in the two groups (~1.8 GPa). Porphyroclastic xenoliths have more refractory bulk rock and mineral phase compositions than the progranular ones, in keeping with their mode. Both groups show chemical variation trends consistent with fractional melting and basalt extraction. REE profiles of clinopyroxenes vary from LREE enriched (spoon shaped) to LREE depleted in the progranular group, whereas they are dominantly slightly convex upwards in the porphyroclastic clinopyroxenes. HFSE (Ti and Zr) anomalies are in general modest in the clinopyroxenes of both groups. At Fernando de Noronha, xenoliths are variably-textured lherzolites and harburgites. Their chemical variation trends overlap and extend those of the Pico Cabuji samples. Average temperature is 1056±87 and pressure 1.6±0.2 GPa. Clinopyroxenes have spoon-shaped to LREE depleted profiles similar to those of the Pico Cabuji protogranular type, but have higher REE concentrations and more marked negative HFSE spikes. Nd and Sr isotopes of the Pico Cabuji porphyroclastic clinopyroxenes (¹⁴³Nd/¹⁴⁴Nd = 0.51339 - 0.51255, ⁸⁷Sr/⁸⁶Sr = 0.70275 - 0.70319) and of Fernando de Noronha (¹⁴³Nd/¹⁴⁴Nd = 0.51323 - 0.51285, ⁸⁷Sr/⁸⁶Sr = 0.70323 - 0.70465) plot on distinct arrays originating from a similar, isotopically depleted composition and trending to lowNd-lowSr and lowNd-highSr, respectively. ¹⁴³Nd/¹⁴⁴Nd decreases, and ⁸⁷Sr/⁸⁶Sr increases, with increasing mg# of clinopyroxene and with all the parameters indicative of previous melting episodes (modal clinopyroxene, bulk rock MgO) The opposite correlations are observed with LREE concentration, La_n/Sm_n, and with all the parameters reflecting metasomatic enrichment. As a whole, chemical and isotopic data indicate that two isotopically and chemically distinct agents acted at Pico Cabuji and Fernando de Noronha and that the second is consistent with the presence of recycled crust in its source. Protogranular xenoliths from these two occurrences underwent chromatographic enrichment by porous flow percolation at low melt/rock ratio. At Pico Cabuji, porphyroclastic xenoliths are likely to be produced by fluid-assisted melting in shear zones of a former protogranular mantle. It is concluded that metasomatic processes have affected a heterogeneous lithospheric mantle with a depleted MORB-like composition. The metasomatic end-members identified in the xenoliths are recognized also in the host basalts.

O08 : 4B/27 : H2

CO₂ Rich High Pressure Melts Included in Minerals from Olivine Pyroxenite Xenoliths (West Eifel, Germany): Experimental and Ion Probe Study

Valentina Batanova (batanova@gs.ucsd.edu)¹,

Hans Seck (H.Seck@min.uni-koeln.de)²,

Alexander Sobolev (asobolev@glas.apc.org)¹,

Markus Klein (kleinm@geocip.geo.uni-koeln.de)² &

Marc Chaussidon (chocho@crpg.cnrs-nancy.fr)³

¹ Vernadsky Institute of Geochemistry, Russian Academy of Science, Kosigin 19, Moscow, 117975, Russia

² Universitaet zu Koeln, Institut fuer Mineralogie und Geochemie, Zuelpicher Str. 49b, Koeln, D-50674, Germany

³ CRPG-CNRS, BP 20, Vandoeuvre-les-Nancy Cedex, 54501, France

Abundant magmatic inclusions have been found in mineral phases of cumulative olivine pyroxenite xenolith (DW-905) occurring in the ash-tuffs of the Dreiser Weiher maar-type volcano, West Eifel, Germany (Witt-Eickschen & Kramm, 1998). Here we report the results of complex study of these inclusions by high temperature experiments both at low (1 atm, optical heating stage) and high pressures (10-12 kb piston cylinder apparatus). Quenched melt inclusions were analyzed by electron probe (major elements) and by ion probe (trace elements, H₂O and C) techniques.

Silicate melts, high density CO₂, sulfide melt and their combinations occur as inclusions in well-defined planes along healed intragranular fractures in olivine and clinopyroxene. These inclusions differ significantly in composition from secondary inclusions, which correspond to transporting melanephelinite. Thus they are interpreted as pseudosecondary inclusions trapped in xenolith minerals before its transportation. Abundant Cpx inclusions in olivine and olivine inclusions in Cpx are also present.

T-P conditions of formation of xenolith minerals estimated from the compositions of Cpx inclusions in olivine are as following: T=1200-1250°C; P more than 10-15 kb. These correspond to the temperature (T=1190-1250°C) estimations from the partial and full homogenization of melt inclusions and minimum pressure obtained from the density of CO₂ fluid inclusions (more than 9 kbar).

Compositions of melt inclusions corrected for quench olivine crystallization on the walls of cavity correspond to alkali rich highly silica undersaturated melts. They show significant range in respect to silica contents (SiO₂=33-41 wt.%), concentration of alkalis, Ca and P covering and exceed (toward lower Si and higher Ca, P and alkalis) all entire compositional range of reported Eifel magmas (Mertes & Schmincke, 1985). High sodium (up to 11 wt.% Na₂O) compositions not reported yet are also present. All inclusions show extreme enrichment (up to 1500X chondrite for Ba) by highly incompatible elements (light REE, Ba, Nb and Sr) and are relatively depleted in HREE (10-25 X chondrite for Yb). Again, as for major elements, trace elements concentrations cover all entire range reported for Eifel magmas (Mertes & Schmincke, 1985) and exceed it toward higher concentrations of all highly incompatible elements. Preliminary ion probe study of quench melts shows that major volatile element in the inclusions is C with concentration level exceeds 10000 ppm. H₂O concentrations in melts are lower than 0.50 wt%. Chlorine varied in the range of 0.2-0.4 wt%, sulfur in the range of 0.10 to 0.30 wt%.

Obtained data suggests that studied inclusions correspond to the melts of variable composition coexisting with olivine and clinopyroxene, CO₂-rich fluid and sulfide melt at temperature 1200-1250°C and pressure more than 10-15 kbar. Low viscosity of these melts and their extreme enrichment in highly incompatible elements make them effective metasomatic agents in lithospheric mantle.

Witteickschen G & Kramm U, Contributions to Mineralogy & Petrology, 131, 258-272, (1998).

Mertes H & Schmincke HU, Contributions to Mineralogy & Petrology, 89, 330-345, (1985).

O08 : 4B/28 : H2

Experimental Petrology of the Kiglapait Intrusion, Labrador, at 5 Kilobars

S. A. Morse (tm@geo.umass.edu)¹,

B. A. Sporleder (bsporled@u.washington.edu)²,

J. B. Brady (jbrady@science.smith.edu)³ &

A. L. Peterson (peterson@geo.umass.edu)¹

¹ Geosciences, UMass, Amherst, MA, USA

² Geological Sciences, Univ. Washington, Seattle, WA, USA

³ Geology, Smith College, Northampton, MA, USA

The liquid line of descent (LLD) has been determined for most of the Kiglapait intrusion. Samples were made up from analyzed whole rocks and separated minerals, adjusted to yield the observed crystal compositions at a given stratigraphic level, e.g. An₆₇, Fo₇₄ at the start of crystallization. Samples were melted and crystallized in graphite in piston-cylinder apparatus with pressure monitored by the solidus of sanidine. Experimental liquidus temperatures range from 1245°C to 1180°C in the Lower Zone (LZ), and from there down to a final solidus at 1065°C at the top of the Upper Zone (UZ). The LLD was calculated from cotectic glasses as oxygen norms. For the LZ it runs parallel to the modal track (Morse, 1979) from OL:AUG 80:20 at the start to 38:62 at the end, where saturation with AUG occurs. The path is slightly concave toward PL, as in the modal case. The provisional UZ path shows a steady increase in FSP content. The long LZ path over a small T range shows that large volumes of troctolite are produced by such a magma; both the lever rule and the volume estimates agree that augite saturation occurs at about 80% crystallized. Calculation with the MELTS subroutine gives only pyroxenes + PL, without OL, unless run at low pressure. The contrast with the experimental results indicates deficiencies in the model. Partition coefficients for Sr are 1.63 to 1.31 (An₆₈ to An₅₆), in strong contrast to a 1-atm study (Blundy, 1997) in which the SIMS analyses fail of mass balance in melts of known composition. For K, the D's are 0.39, 0.85 over the same PL composition range, much higher than any known 1-atm determinations. D for FeO in PL is 0.04, as commonly found. Ca was transferred rapidly into OL from PL in sintered OL+PL runs, showing the facility with which OL can gain or lose Ca against PL alone. The new data form the foundations for forward modelling of the intrusion history to compare with observed rocks. So far, most older models look quite good.

Morse SA, J. Petrol, 20, 591-624, (1979).

Blundy JD, Ch. Geol, 141, 73-92, (1997).

O08 : 4B/29 : H2

Experimental Investigation of Titanite Solid Solution Ca(Ti,Al)(O,F)SiO₄ in the Assemblage Titanite-Anorthite-Fluorite

Ulrike Troitzsch (ulrike@geology.anu.edu.au) &

David Ellis

Australian National University, Geology Department, Canberra A.C.T. 0200, Australia

The exchange reaction Ti⁴⁺ + O^2- = Al³⁺ + F^- in titanite has long been recognised to be strongly dependent on pressure, temperature as well as fluorine fugacity. Even though the presence of F has the potential to significantly increase the stability of titanite, especially towards higher pressure and lower temperature, most theoretical and experimental studies of phase equilibria have focused on pure Al- and F-free titanite, thus limiting their application to F-free environments. The only previous experimental study of AlF-rich titanite we are aware of (Smith, 1981) was carried out in an unbuffered, supersolidus assemblage, thus not allowing for the separation of the influence of fluorine fugacity on the titanite composition from that of pressure and temperature. We are investigating the pressure and temperature dependence of the Al-content of titanite in the subsolidus assemblages titanite-fluorite-anorthite (between 5 and 20 kbar) and titanite-fluorite-zoisite (between 20 and 35 kbar). The entire range of binary titanite solid solution [Ca(Ti,Al)(O,F)SiO₄] has been synthesised. Titanite coexisting with anorthite and fluorite reaches Al-contents up to X_Al=0.6 [X_Al=Al/(Al+Ti)], that coexisting with zoisite up to X_Al=1, thus clearly exceeding the maximum Al-content X_Al=0.53 of the previous experimental study (Smith, 1981). Since we were able to determine the change in unit cell volume of titanite with X_Al using X-ray diffraction data, the reaction anorthite + fluorite = CaAlFSiO₄ has the potential to yield thermodynamic data for CaAlFSiO₄, the Al-F end-member of titanite solid solution, as well as activity-composition relationships for intermediate compositions. This can then be applied to other equilibria involving titanite, and represents a first step towards the application of phase relations of titanite determined in experimental, F-free environments to F-bearing rocks. Phase equilibria including zoisite at higher pressure are not suited for the extraction of thermodynamic data for CaAlFSiO₄, because the zoisite in these experiments contains considerable amounts of F (about X_F=0.5 [X_F=F/(F+OH)); however, thermodynamic data for fluoro-zoisite are not available.

Smith DC, Progress in Experimental Petrology N.E.R.C. Publication Series, D-18, 193-197, (1981).

O08 : 4B/30 : H2

Low-Pressure Larnite-Normative Trend of Melt Evolution: Evidence from Synthetic Systems and Melt Inclusions

Leonid D. Krigman (elkor@geokhi.msk.su)¹,

Robert A. Ishbulatov²,

Troels F. D. Nielsen (nielsent@dlc.ku.dk)³ &

Ilya V. Veksler (ivv@dlc.ku.dk)³

¹ Vernadsky Institute of Geochemistry, Kosygin str. 19, 117975, Moscow, Russia

² Institute of Experimental Mineralogy, Chernogolovka, Noginsk region, Russia

³ Danish Lithosphere Centre, Oester Voldgade, 10L, Copenhagen, 1350, Denmark

Normative larnite in an igneous rock requires a silica-undersaturated bulk composition with high CaO/Al₂O₃ and CaO/MgO. In terms of mineral composition the rocks are usually melilite-bearing and, in some cases, approach monomineralic melilitolites. Larnite-normative liquids are documented in chilled dykes and lavas and by data on melt inclusions.

The vast majority of natural basaltic liquids do not evolve to larnite-normative compositions, because low-pressure crystallization of common SiO₂ - saturated Ca-silicates (e.g., plagioclase and diopsidic clinopyroxene) decreases the CaO content of the liquid and drives it to SiO₂-rich, granitic residuals. Thus, a SiO₂-undersaturated (e.g., melanephelinitic) parental liquid in which plagioclase is unstable is the first requirement for the larnite-normative trend.

The simple and basic model which demonstrates how low SiO₂ activity in a dry system results in the larnite-normative trend is the well-studied Nepheline (NaAlSiO₄) - Diopside (CaMgSi₂O₆) join. Reaction between the components drives the liquid to larnite-normative compositions, but a further increase in CaO in the liquid is soon halted by the onset of melilite crystallization.

The larnite-normative trend is greatly enhanced in the presence of water and fluorine. Stabilization of phlogopite and/or amphibole in equilibrium with SiO₂-undersaturated liquids provides an effective mechanism to produce highly larnite-normative, melilitic residual liquids. We have demonstrated this in experimental studies of several joins in the SiO₂-undersaturated part of the F-doped kalsilite-based normative tetrahedron (Veksler et al., 1998). Because F-phlogopite is much more stable than the natural OH-analogue, the F-doped system presents an extreme case of maximum phlogopite stability and its maximum effect on the melt evolution. We examined the effects of F-OH substitution by the study of the Diopside - Phlogopite (KMg₃AlSi₃O₁₀(OH)₂) and Diopside - Kalsilite (KAlSiO₄) - H₂O joins at 0.2 GPa. The preliminary results show that despite significant topological differences with the analogue F-doped join, the larnite-normative trend is observed in the liquids of the Diopside - Kalsilite - H₂O join.

CO₂/H₂O values are another important characteristic of natural melts which may have important consequences for the larnite-normative trend. Melilitolites are often found in close association with carbonatites and both rock types demonstrate extreme enrichment in CaO. Data on melt inclusions (Nielsen et al., 1997) suggest that carbonatitic melts may be derived from carbonated melilitic liquids by liquid immiscibility. It seems that the nepheline+diopside (ijolitic) association is an important branching point in the magmatic evolution and a subtle balance of total pressure, CO₂/H₂O and other conditions determine the evolution towards melilitolite and minor immiscible carbonatite, or to residual carbonatitic liquids associated with melilite-free ijolitic associations.

Nielsen TFD, Solovova IP & Veksler IV, Contrib. Mineral. Petrol., 126, 331-344, (1997).

Veksler IV, Fedorchuk YM & Nielsen TFD, Contrib. Mineral. Petrol, 131, 347-363, (1998).

O08 : 4B/33 : H2

The Use of Iron Partitioning between Plagioclase and Melt as an Oxygen Barometer for Hydrous Magmatic Systems

Max Wilke

(max.wilke@mineralogie.uni-hannover.de) &

Harald Behrens

Institut für Mineralogie, Universität Hannover, Hannover, Germany

Oxygen fugacity (f(O₂)) may have strong influence on the petrogenesis of magmatic rocks. Redox conditions in natural magmatic systems are mostly bracketed with the help of coexisting Fe-Ti oxides found in the rocks and experimental determination of phase relations (e.g. Rutherford & Devine 1996, Martel et al., 1998). The aim of this study is to add a further possible constraint on the f(O₂) by the use the f(O₂) dependant partitioning behaviour of Fe between plagioclase and melt. Crystallization experiments were performed using synthetic tonalitic glasses at 500 MPa water pressure and 750/850°C with varying f(O₂). The obtained partition coefficients (D_Fe) for iron vary from 0.54±0.12 for oxidizing conditions (Cu-Cu₂O) to 0.085±0.013 for reducing conditions (WM - 0.1). At 750°C the partition coefficients increase at constant f(O₂). A model is developed to describe the dependance of the partition coefficients on f(O₂), which is based on the redox equilibrium of ferric and ferrous melt species and describes the apparent partition coefficient as a linear combination of the two hypothetical endmember-partition coefficients of the ferric and ferrous cation. By fitting the experimental data to the model the parameters D(Fe³⁺), D(Fe²⁺) and K_m can be determined, where K_m denotes the equilibrium constant of the iron-redox equilibrium in the melt. Since the endmember-partition coefficients do not vary significantly in the investigated T intervall the variation of the partition coefficient is essentially controlled by the T dependance of the redox equilibrium in the melt. The T dependance of the equilibrium constant is derived from the T variation of the Fe partitioning and from Moessbauer spectroscopic determination of ferric-ferrous ratios in quenched melts. Thus a petrogenetic grid for f(O₂) determination in water saturated systems in the T range 750°-900°C can be developed. Of course, extrapolation to systems with largely different compositions is uncertain. Therefore the applicability of the oxygen barometer was tested by measuring of the D_Fe between plagioclase and glass (inclusion & matrix) of natural volcanic samples from Mt. Pelee and Mt. Pinatubo where the preeruptive conditions are bracketed by other methods (Evans & Scaillet, 1997; Martel et al.; 1998). Comparison of the determined f(O₂)-T conditions shows perfect agreement for the preeruptive conditions of Mt. Pinatubo (Evans & Scaillet, 1997). For the Mt. Pelee a significantly higher f(O₂) (1.1 - 1.5 log units) for the T intervall given by Martel et al. (1998) is obtained. D_Fe may be influenced not only by T and f(O₂) but also by the compositions of melt and crystal and the water activity in the system. As both natural examples do not largely differ in composition, the other parameters have to account for the observed discrepancy.

Evans BW & Scaillet B, American Mineralogist, 82, 625-629, (1997).

Martel C, Pichavant M, Bourdier JL, Traineau H, Holtz F, Scaillet B, Earth Planet. Sci. Letters, 156, 89-99, (1998).

Rutherford MJ & Devine JD, Fire and Mud: eruptions and lahars of Mt. Pinatubo, PHIVOLCS and Univ. of Washington Press, Seattle, 751-766, (1996).

O08 : 4B/34 : H2

Two Types of Garnets in the Metamorphic Rocks of Sub-Polar Urals

Alexander Pystin (reg@geo.komi.ru)

Institute of Geology, 54, Pervomayskaya st., Syktyvkar, 167000, Russia

The garnetiferous metamorphic rocks of Sub-Polar Urals are found in two structural levels: Lower and Upper Pre-Cambrian. The study of the chemical composition of garnets shows that all of them refer to grossular-pirope-almandine series and are represented mainly by zonal (in chemical composition) crystals. By the type of zonation garnets are subdivided into two groups.

Garnets with progressive type of zonation where the content of magnesium increases and the content of manganese decreases from the central to peripheral parts of the grains predominate. They are known throughout the studied section.

The other less abundant group is represented by garnets with regressive type of zonation: the grains centers are enriched in magnesium and the margins - in calcium. It is known that garnets with calcium enriched outer rims are very typical of the rocks which have been multiply metamorphosed uder higher pressures and lower temperatures. Such garnets have been revealed only in the lower Early Pre-Cambrian part of the section and are absent in the overlying Riphean successions.

Zonation revealed in the garnets of the second group cannotbe connected with regressive branch of the monocycle metamorphism. If the latter is the case the grains margins have enriched manganese contents with the same amount of calcium as in crystal nuclei. The stages of metamorphism must have occurred in succession. The second stage, dealing with related to crystallization of the outer parts of grains, was reguessive in terms of temperature and progressive in pressure.

On the basis of these two types of garnets with different chemical zonation and their distribution within the section of Pre-Cambrian metamorphic rocks of Sub-Polar Urals, the following conclusions can be made:

1. Lower Pre-Cambrian rocks of the area are multiply metamorphosed: they under went not less than two stages of metamorphism in P-T stability regime of garnets of grossular-pirope-almandine composition. Upper Pre-Cambrian rocks were subjected to one stage of metamorphism in the same P-T stability regime ofgarnets.

2. The latest of these two stages of metamorphism was common for both Lower and Upper Pre-Cambrian rocks. The crystallization of garnets with progressive zonation and growth of calcium rims over the already formed garnets with the development of regressive zonation is associated with this stage.

O08 : 4B/35 : H2

Super-Silicic Garnet Microstructures from an Orogenic Garnet Peridotite; Evidence for an Ultra-Deep (>7 GPa) Origin

Herman van Roermund (hermanvr@geo.uu.nl),

Martyn Drury,

Auke Barnhoorn &

Almar de Ronde

Geogynamic Research Institut, Faculty of Earth Sciences, Utrecht University, 3508 TA, Utrecht, The Netherlands

Relicts of super-silicic garnets have been discovered for the first time within orogenic garnet peridotites, Otrøy, western gneiss region, Norway. The evidence consist of two pyroxene exsolution from garnet. Super-silicic garnets are only stable at depths greater than 150 km. Estimates of the initial composition of the super-silicic garnets imply pressures of 6-7 GPa indicating that the Otrøy garnet peridotite were derived from depths > 185 km. The Otrøy garnet peridotites consist of interlayered fertile and depleted compositions with ~ 50% lherzolite and 50% harzburgite plus dunite. Three types of garnets were recognised: 1) normal matrix type, grain-size < 4 mm, 2) isolated large single crystals (grainsize < 4 cm) and 3) isolated garnet nodules up to 10 cm in size. Garnet reveals two characteristic microstructures: i) all garnets are heavily fractured and ii) nodular garnets contain two-pyroxene exsolution microstructures. The deformation-induced garnet microstructure pre-dates the well-known two pyroxene-spinel coronas as fractures are truncated by coronas. In addition garnet plasticity is indicated by occurences of high densities of naturally decorated dislocations. Decoration is (thought to be) due to spinel precipitation. Large nodular garnets consists of mm-scale (2-8 mm) garnet grains with interstitial orthopyroxene. In the larger garnet cores two-pyroxene exsolution microstructures occur. Pyroxene exsolution is absent in an exsolution-free zone within 2 mm of garnet grain boundaries. The two-pyroxene exsolution microstructure pre-dates garnet fracturing as pyroxene needles become off-set along fractures. In addition at such microstructural sites exsolved pyroxene becomes replaced by later alteration products. Estimates of the PT conditions for mineral chemical equilibration were obtained from geo-thermometry and barometry. The mineral compositions equilibrated at mantle conditions around 805 ± 40°C and 3.2 ± 0.2 GPa. Available geochronological data suggest a multi-stage, multi-orogenic exhumation history.

O08 : 4B/36 : H2

The Alpine Metamorphic Pattern in the Danubian Window, South Carpathians (Romania)

Magda Ciulavu (ciulavu@ns.igr.ro)¹ &

Rafael Ferreiro Mählmann (ferreiro@ubaclu.unibas.ch)²

¹ Institutul Geologic al Romaniei, 1 Caransebes str., RO 78344, Bucharest 32, Romania

² Mineralogisch-Petrographisches Institut, Bernoullistrasse 30, CH-4051, Basel, Switzerland

The central part of the Southern Carpathians comprises three main nappes systems. From the lowest to the upper tectonic unit they are: 1) Lower and Upper Danubian, 2) Severin-Cosustea, and 3) Getic. The Danubian nappes are made of continental basement and Mesozoic cover, while the Severin nappe is an ophiolite-flysch unit and the Cosustea nappe is a tectonic mélange. The Danubian and Severin units have been exhumed below a low angle detachment (Getic detachment) during Eocene to Oligocene.

The samples analyzed come form the metapelitic Jurassic and marly Cretaceous rocks of the Danubian and from Cretaceous pelagic and flysch formations of the Severin and Cosustea nappes. The samples were analyzed by XRD, optical microscopy and microprobe. The XRD studies were performed in order to determine the clay mineral association and the illite crystallinity (IC). The rock maturity was determined by optical microscopy using the vitrinite reflectance method. On the basis of optical microscopy, followed by microprobe analyses, P-T conditions of the metamorphism were calculated using chlorite thermometry and phengite barometry.

In the case of the Cretaceous samples, the mineralogy of the clay fraction is made of illite + chlorite + quartz ± mica/paragonite interstratification ± hematite. The Jurassic formation shows a similar mineralogy in the south-western part of the area, while in the NE a more complex mineral association has been identified: illite + paragonite + chloritoid + pyrophyllite + quartz ± chlorite ± mica/paragonite interstratification. In this last case, because of the interference of the peaks from several minerals in the 10 Å region, the IC was determined by deconvolution or/and calculated from the width of the 2 Å peak. The illite crystallinity values show obvious trends, namely an increase of illite aggradation from SW to NE, and from S to N. In the eastern part of the area, vitrinite reflectance shows a rank of maturity of the anthracite and meta-anthracite stage; close to the Getic detachment the values increase to the semi-graphite stage. Lower maturity values are characteristic for the western part. From chemical analyses of chlorite and mica/phengite, temperatures of 340-400°C and minimum pressure of about 3 kbar have been deduced for the NE. Higher minimum pressure conditions (~5 kbar in the Danubian and 4.5 in the Severin) are inferred for an earlier (pre-detachment) mica/phengite generation.

The results indicate an increase of metamorphic grade from SW to NE and from S to N. The SW-NE trend can be explained by the exhumation of a metamorphic core complex given by the Getic detachment. The S to N gradient may be inherited from pre-detachment top to S nappe stacking.

O08 : 4B/37 : H2

Corona Formation During Prograde Metamorphism of Gabbronorites

Tatiana Larikova (tata@igem.msk.su)

IGEM, Staromonetny per, 35, Moscow, Russia

Cpx-Hbl-Grt coronas in metamorphosed labrodorite-gabbronorites of the PR massif Tolstik in N.Karelia were studied. In minimal altered rocks were found the following corona structures between primary Opx and Pl: Opx/Cpx+Hbl+Qtz/Grt+Qtz/Pl. Garnet porphiroblastes are also present in this assemblage. Two main paragenesises have been distinguished: Cpx+Grt+Qtz belongs to the corona formation, and the Hbl is secondary. The Grt porphiroblastes show particularly pronounced prograde zonation. From its core to rim, Mg/Mg+Fe significantly increases and Ca content decreases. In the central part of garnet layer there is a peak in the Ca content, and Mg/Mg+Fe increases slightly from the inner to the outer part of the corona. This similarity in the composition and the structure of the Grt grains and coronas suggests that the formation of the coronas started in the prograde stage of the metamorphic event, approximately at 670°C and 5-6 kbar. The growth of Cpx and Grt coronas in the boundary between primary Opx and Pl started according to the reaction Opx+Pl=Cpx+Grt+Qtz by mechanism of diffusion metasomatism with opposing diffusion of Ca from Pl, and Mg and Fe from Opx. The diffusion rate of Ca controls the growth and the amount of Cpx. Mg and Fe controls the Grt. Si in this process is quite inert, the diffusion rate of Al is very slow and does not cause the formation of any phase. Composition of the coronas is determined only by the chemical potentials of Fe, Ca, and Mg. The calculated Onzager coefficients from the model of steady-state diffusion are L_AlAl / L_MgMg =0.19, L_AlAl / L_FeFe =0.38, L_AlAl / L_CaCa =0.46.Hbl in the inner layer of the corona was formed during the later retrograde a mphibolization of gabbronorites according to the reactions: Opx+Cpx+Grt+Na₂O+H₂O=Hbl+Qtz; Cpx+Grt+Na₂O+H₂O=Hbl+Pl. At this stage (550°C and 4-5 kbar) corona structures were destroyed, the Grt coronas were crystallized into the separate grains with retrograde zonation; and the gabbronorites were transformed into the garnet amphibolites.

O08 : 4B/38 : H2

Scapolite Formation During Desiccation of Lower Crustal Shear Zone Rocks

Kåre Kullerud (kaarek@ibg.uit.no)

Dept. of Geology, Univ. of Tromsø, N-9037 Tromsø, Norway

Fluid-rock interaction resulted in the formation of Cl-enriched biotite (Kullerud, 1995) and Cl-enriched amphibole (Kullerud, 1996) during shear zone formation in a noritic gabbro in Lofoten, northern Norway. Metamorphic salt has also been described from the rock (Markl and Bucher, 1998, Markl et al. in press). This paper presents the occurrence of scapolite in the shear zone rock. Complex metamorphic coronas composed of amphibole, quartz, plagioclase and garnet formed along the contacts between igneous mafic minerals (CPX, OPX, BT and ILM) and igneous plagioclase. Igneous plagioclase (An_50-60) reacted to polygranular aggregates of plagioclase with a lower An-content (An_20-55). Abundant small grains (<0.1 mm in diameter) of amphibole, biotite, epidote, magnetite and kyanite formed within the aggregates of plagioclase. Corundum, quartz and Ba-rich muscovite occur as tiny intergrowths within the altered igneous plagioclase. The Cl-bearing minerals show large compositional variations. The corona forming amphibole shows Cl contents in the range 1-3 wt%, while amphibole occuring in the plagioclase matrix shows Cl contents in the range 3-5 wt%. The Cl content of scapolite ranges between 1.3 wt% and 3.8 wt% corresponding to a content between 42% and 75% of the marialite endmember.Textural evidence shows that scapolite formed during reactions between an externally derived Cl-bearing fluid and the igneous plagioclase. The sodium content of scapolite and plagioclase in contact, however, is poorly correlated. Clearly, scapolite composition was neither controlled by the composition of the igneous plagioclase that was replaced, nor by the composition of the secondary plagioclase which now occurs in contact with scapolite. It is inferred that scapolite formed in equilibrium with the fluid phase. Plagioclase, however, did generally not attain equilibrium due to the sluggishness of the cation exchange reactions between fluid and plagioclase. Al³⁺ was apparently nearly insoluble in the fluid phase, and resulted in the formation of kyanite and occasionally corundum within the plagioclase matrix during the breakdown of the An-component of the igneous plagioclase. The distribution of kyanite and corundum in the rock can be related to variations in the activity ratio a_Si4+/a_Al3+ of the local environments. The Cl-contents of amphibole and scapolite in contact are clearly correlated. This suggests that the compositions of the minerals were principally controlled by the activity of Cl of the equilibrium fluid during mineral growth. The compositional variations of scapolite and amphibole can be explained to be the result of successive desiccation of an externally derived Cl-bearing fluid phase. The first amphiboles that formed extracted preferentially H₂O from the fluid phase, leading to a successive Cl-enrichment of the fluid phase. Cl-rich amphibole and scapolite formed later in equilibrium with an evolved, Cl-enriched fluid phase.

Kullerud K, Contrib. Mineral. Petrol, 120, 42-59, (1995).

Kullerud K, Eur. J. Mineral, 8, 355-370, (1996).

Markl G, Bucher K, Nature, 391, 781-783, (1998).

Markl G, Ferry J, Bucher K, Am. J. Sci, (in press).

Session O08:4P

O08 : 4P/01 : PO

Lujavrites from the Ilimaussaq Intrusive Complex in South Greenland: Crystallization Conditions and Composition of the Late-Magmatic Fluid Phase

Holger Sommer (markl@ruf.uni-freiburg.de) &

Gregor Markl

Institut für Mineralogie, Albertstrasse 23 B, 79104-Freiburg, Germany

Geological and geochemical evidence suggests that the Ilimaussaq intrusion in the Precambrian Gardar rift province of South Greenland was emplaced as three main pulses - nepheline-augite syenite, quartz-bearing granite and agpaitic rocks. The agpaitic rocks in the Ilimaussaq Intrusion have been dated to ~1168 Ma (Rb-Sr w.r., Blaxland et al., 1976). The third, and main, pulse of peralkaline, silica undersaturated magma lead to the formation of foid-bearing syenitic rocks (sodalite foyaite, naujaite, kakortokite) and extremely fluid-enriched residual melts, the lujavrites. Field and geochemical observations indicate the establishment of two or more, largely isolated sub-magma chambers of lujavrite. Changes from green ägirin lujavrite to black arfvedsonite lujavrite record changes in fO₂, fH₂O and possibly in fHF and fHCl. Interestingly, textural relationships within various samples suggest alternation from aegirin to arvedsonite and back to aegirine stability indicating oscillating intensive parameters of fluid and/or liquid. The final stages of magmatism are marked by the emplacement of arfvedsonite lujavrite and lujavrite extremely rich in villiaumite (NaF) and naujakasite. Reaction textures at naujaite-lujavrite interfaces and the occurrence of hydrothermal veins with abundant analcime, aegirine or arfvedsonite record the expulsion of late-magmatic, relatively low-temperature fluids in the analcime stability field from the lujavrites. These textures involve formation of abundant needle-shaped crystals of arfvedsonite and/or aegirin in veins and at the interfaces growing into the naujaite. On the other hand, aegirine and arfvedsonite in the lujavrites are in many places converted into each other in areas close to naujaite xenoliths. This indicates involvement of fluids originally stored in the naujaite and it may indicate that the lujavrite intruded while the naujaite was not yet fully crystallized (despite sharp borders of naujaite xenoliths in lujavrite). In order to understand the compositional and intrinsic variability in the lujavrites and especially their fluids, new data on F-amphibole and other halogen-bearing minerals are combined with microthermometry results from fluid inclusions and geothermometry based on various amphibole-feldspar-nepheline equilibria. The halogen-rich nature of the fluids is demonstrated by the occurrence Cl-rich ussingite, villiaumite, abundant sodalite and extremely saline fluids with up to 60 wt.% NaCl equivalent.

Blaxland et al., Lithos, 9, 31-38, (1976).

O08 : 4P/02 : PO

Crystallization Conditions of the Augite Syenite, Ilimaussaq Intrusion, Greenland

Michael Marks (markl@ruf.uni-freiburg.de) &

Gregor Markl

Institut für Mineralogie, Albertstrasse 23 B, 79104 Freiburg, Germany

The Ilimaussaq intrusion is part of the late Precambrian Gardar rift province of South Greenland. The Gardar intrusive complexes fall into two categories: those involving silica-saturated rocks, such as quartz syenite and granite and those involving nepheline-bearing syenitic rocks. Only in the Ilimaussaq intrusion both rock types occur together. The main rock types of the complex are nepheline-bearing augite syenite, quartz-bearing alkali granite and nepheline-bearing syenitic rocks (agpaitic rocks named naujaite, sodalithe foyaite, lujavrite and kakortokite). Geological and geochemical evidence suggest that the intrusion was emplaced as three main pulses: 1) augite syenite(dated by Sm-Nd mineral isochrone to 1.13±0.05 Ga), 2) alkali granitic rocks, and 3) agpaitic rocks. In order to constrain the early evolution of the complex and to be able to compare (and relate?) the various rocks to each other, the augite syenite was examined in detail at various localities. The augite syenite consists of alkali feldspar, olivin, clinopyroxene, amphibole, biotite, nepheline, apatite and magnetite. It forms a shell around the southeastern, southern and western margin of the Ilimaussaq intrusion. This shell is between 10 and 500 m thick and it is commonly separated from the inner agpaitic rocks by an agpaitic border pegmatite. Xenoliths of the augite syenite are found in all other rock units of the intrusion. In addition, the augite syenite forms a horizontal sheet in the roof zone northeast of Lake Taseq. The rock is chilled against the country rocks which consist of Julianehåb granite and Gardar sandstones and volcanics. It shows increase in grain size towards its inner contacts within the complex. This textural trend is accompanied by chemical trends: From the outer to the inner part, the Fe and Mn content in olivine and clinopyroxene increases systematically from Fa₈₁ to Fa₉₄ and from Ac₄Di₅₀Hd₄₆ to Ac₄Di₄₁Hd₅₅ indicating decreasing temperatures with continuous fractional crystallization. Nepheline becomes more abundant with increasing distance from the outer contact indicating decreasing SiO₂ activity. Detailed phase equilibrium studies using the QUILF technique combined with microthermometry of fluid inclusion are used to discuss the intensive parameters (P, T, fO₂, aSiO₂) during crystallization of the augite syenite and its relation to the agpaitic rocks within the Ilimaussaq complex.

O08 : 4P/03 : PO

Geochemical Behaviour of F and Cl During Crystallization of the Nepheline-Syenitic Units of the Ilimaussaq Intrusion, Greenland

Gregor Schwinn (markl@ruf.uni-freiburg.de) &

Gregor Markl

Institut für Mineralogie, Albertstrasse 23 B, 79104-Freiburg, Germany

The Ilimaussaq Intrusion belongs to the alkaline, probably rift-related intrusions in the Gardar Province of South Greenland. The approximately 1.18 Ga old intrusion exhibits the greatest diversity of different rock types of all Gardar intrusions and consists of principally three rock suites: nepheline-bearing augite syenite, quartz-bearing alkali granite and a wide variety of foid-bearing syenitic rocks. These three main rock suites are considered to represent three pulses of different kinds of magmas. The alkaline granite is found in the highest portions of the intrusive complex. It intrudes the augite syenite which forms an early shell into which - partly separated from the interior by a border pegmatite - the later foid-syenitic to agpaitic magma intruded. This central part consists of a sequence of different agpaitic foid syenites from sodalite-foyaite at the top, via naujaite, lujavrite to kakortokite at the bottom. The genetic relationship among these various rock types is still not understood. They are considered to represent flotation cumulates (sodalite foyaites, naujaites with up to 80 vol.% sodalite), bottom cumulates (kakortokites, but unclear of which magma, maybe of the naujaitic magma?) and very fluid-rich residual, highly differentiated melt (lujavrites). Early anhydrous mineral assemblages (pyroxene, olivine, feldspars, nepheline) in the foyaite develop towards hydrous assemblages with F-bearing amphibole and with aenigmatite in foyaite, kakortokite and naujaite, and finally to the extremely volatile (F and Cl)-rich lujavrites. The early changes in melt composition are indicated by the chemical changes from low-Zr to high-Zr aegirin augite that is overgrown by F-amphibole which in turn is again overgrown by Zr-free aegirine. These various phase assemblages reflect variability in the composition of both the coexisting fluids and the coexisting melt. The early stages of the crystallization history of the central portion of the Ilimaussaq intrusive complex is deduced from phase equilibria among the early, anhydrous minerals. Fluid compositional changes in terms of fO₂, fH₂O, fHF and fHCl are critically important for the development of agpaitic rocks. Further insight into the variations in these parameters is gained by the combination of these calculations with fluid inclusion microthermometry and with calculations on halogen-bearing minerals (mainly amphiboles) involving aegirine. We present systematic data from all units of the central part of the Ilimaussaq intrusion complex.

O08 : 4P/04 : PO

The Bjerkreim-Sokndal Layered Intrusion; Norway: Study of a Sulphide-Bearing Interval at the Boundary between two Megacyclic Units

Kristine Krogh Jensen (kristine@geo.aau.dk.) &

J. Richard Wilson (jrw@geo.aau.dk)

C.F.Moellers Alle, bygn. 110, DK-8000 Aarhus C, Denmark

The Bjerkreim-Sokndal layered intrusion is situated in southwest Norway. It belongs to the Rogaland intrusive complex which was emplaced in late Proterozoic times during the Sveconorwegian orogen. The Rogaland intrusive complex consists of massif-type anorthosites, smaller charnockitic intrusions, jotunite dyke swarms and the Bjerkreim-Sokndal layered intrusion.

The intrusion consists of three lobes: the Mydland, Sokndal and Bjerkreim lobes. Only work on the Bjerkreim lobe is involved in this project. The magma chamber had an initial saucer-shape, which has been deformed into a syncline whose fold axis plunges 20-40° S. Lithologically the intrusion consists of rock-types ranging from anorthosite, leuconorite, troctolite, norite, gabbronorite, jotunite, mangerite, quartz mangerite and charnockite.

The lower part of the Bjerkreim lobe comprises a 7000 m thick Layered Series which has been divided into six mega-cyclic units (MCU 0-MCU IV). The MCU's crystallised from separate batches of magma. Within each MCU the rocks gradually become more evolved upwards, followed by a compositional reversal at the base of the overlying MCU. The Layered Series is overlain by more evolved, largely unlayered rocks (mangerites-charnokites).

This project is concerned with the boundary between MCU II and III. Massive ilmenite norites in the upper part of MCU II become modally layered upwards before sporadically developed olivine-bearing norites occur in the lower part of MCU III (zone b). The modally layered interval commonly has a pyroxenite (up to 3 m thick) developed at the base. A rust zone is usually present in the vicinity of the pyroxenite. The rust zone is a result of the weathering of an interval with a high concentration of disseminated sulphides. The pyroxenite occurs in discontinuous bodies and where the pyroxenite is lacking the sulphides are situated in the norites.

Fieldwork was carried out in July '98, and a series of sample profiles have been collected across the MCU II-III boundary, with emphasis on the sulphide-bearing rocks and the pyroxenite. The aim of the project is to identify the sulphide phases, to explain how the increased concentrations of the sulphides were formed, and to obtain an understanding of the magma chamber processes responsible for crystallisation of the pyroxenite and sulphides near the boundary between two MCU's.

O08 : 4P/05 : PO

Extreme Assimilation in a Crustal Magma Chamber: Sr and Nd Isotopic Constraints from the Hasvik Layered Intrusion, Norway

Christian Tegner (christian.tegner@geo.aau.dk)¹ &

Brian Robins (brian.robins@geol.uib.no)²

¹ Dep. Earth Sciences, University of Aarhus, DK-8000 Aarhus C, Denmark

² Dep. of Geology, University of Bergen, N-5007 Bergen, Norway

Petrographic, mineralogical, whole-rock major- and trace-element compositions, and Sr-Nd isotopic data for mafic cumulates, chilled margins, and country rocks of the ~700 Ma Hasvik Layered Intrusion, North Norwegian Caledonides, constrain coupled crustal assimilation and fractional crystallisation (AFC) of mantle-derived tholeiitic basalt emplaced into metasediments at a deep crustal level (6-8 kbar, 4-600^oC). Correlated variations of whole-rock ⁸⁷Sr/⁸⁶Sr (0.7038-0.7089), <epsilon>Nd (+4.8 to -3.3) and mineral compositions (e.g. X_An = 0.72-0.52, and mg#_opx = 0.77-0.39) suggest steady-state AFC with a constant ratio, r, for the rate of assimilation to the rate of crystallisation of ~0.32 for ~1100 m of cumulates demonstrating a continuous tholeiitic fractionation trend (olivine-melt reaction, Fe-Ti oxides in, apatite and pigeonite in), following a ~335 m thick cumulate sequence with a reverse cryptic variation recording continuous magma emplacement at the chamber floor. This value of r is close to the upper limit permitted by the heat budget, placing the Hasvik Layered Intrusion among the most contaminated layered intrusions presently known. Slabs of highly-modified metasediment xenoliths are common in the cumulates, particularly in the upper part of the Layered Series, and are the remnants of the material assimilated.

O08 : 4P/06 : PO

Petrological, Mineralogical and Geochemical Outlines of the Late Permian to Triassic Karavanke Granitic Belt (Slovenia)

Meta Bole (meta.bole@uni-lj.si)¹,

Tadej Dolenec (tadej.dolenec@ntfgeo.uni-lj.si)¹,

Nina Zupancic (nina.zupancic@uni-lj.si)¹ &

Breda Cinc-Juhant (breda.cinc-juhant@uni-lj.si)²

¹ University of Ljubljana, Department of Geology, Askerceva 12, 1000 Ljubljana, Slovenia

² Slovene Museum of Natural History, Precernova 20, 1001 Ljubljana, Slovenia

The rocks of the Karavanke Granitic Belt belong to a bimodal magmatic association consisting of predominant syenogranite and syenite and contemporaneous mafic and intermediate rocks of alkaline character. Mafic rocks consist of olivine-bearing gabbro and monzogabbro, wich represent about 20% of the whole massif, and range in size from decimetric microgranular enclaves to large decametric bodies. The intermediate rock types are monzodiorite and monzonite wich show field, textural and chemical features suggest that they formed as a result of the interaction between felsic and mafic magmas. The porphiry syenite with rapakivi texture, which occurs in close spatial association with mafic enclaves, is interpreted as a piece of evidence for such interaction.

Field, petrographic and geochemical evidence indicate that the rocks of Karavanke Granitic Belt do not represent the result of a crystal fractionation process, nor of a simple bulk mixing process. Only some of the more mafic and, at the other side some of the more felsic rocks could be consistently linked by crystal fractionation. Rocks of intermediate composition are the results of a different degree of interaction between the two end-member magmas.

Chemical and mineralogical compositions indicate that the basic rocks represent the mantle derived magma. Rising and fractional crystallization of basic magma probably produced the heat necessary to trigger the melting of crustal material, which caused the formation of the felsic magma, as suggested by initial Sr isotopic data (Dolenec, 1994).

The highly contrasted acid-basic relationships and the alcaline character of rocks fit an anorogenic to post orogenic tectonic setting (Pitcher, 1993) and is consistent with the regime of incipient rifting suggested by Bonin et al. (1987) for the Western Mediterranean Magmatic Province, thus indicating that the late Permian to Triassic Karavanke Granite Belt is also part of the same magmatic province.

Bonin B, The enclaves of alkaline anorogenic granites: an overview. in Enclaves and Granite Petrology, J. Didier & B. Barbarin, Elsevier, Amsterdam, 179-188, (1991).

Dolenec T, Novi izotopski in radiometricni podatki o pohorskih magmatskih kamninah, Rudarsko-metalurgki zbornik, 41, 147-152, (1994).

Pitcher WS, The Nature and Origin of Granite. Blackie A & P, London, 321, (1993).

O08 : 4P/07 : PO

Geochemistry of Polycyclical Metamorphic Rocks of the Bragança Massif (Variscan Chain, NE Portugal)

José F. Santos (jfsantos@geo.ua.pt)¹ &

José M. Munhá (jmunha@fc.ul.pt)²

¹ Departamento de Geociências, Universidade de Aveiro, 3810 Aveiro, Portugal

² Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, 1700 Lisboa, Portugal

The most striking geological feature of NE Portugal is the presence of several exotic tectonic units emplaced upon the Central-Iberian autochthon. The upper allochthonous thrust complex crops out in the Bragança Massif, where four main rock types can be recognized: (1) high-pressure granulites; (2) eclogites (enclosed by paragneisses); (3) plutonic gabbroic bodies; (4) ultramafic rocks. Previous studies have shown that the first two lithotypes underwent a complex polycyclical tectonometamorphic evolution. Recently, in a type (1) rock, the HP/HT metamorphic event was dated as Grenvillian (1.0 to 1.1 Ga), but, most commonly, HP granulites are strongly affected by Hercynian metamorphism, giving rise to the Palaeozoic isotopic ages usually obtained, even for the polycyclical lithoptypes (granulites and eclogites).

The compositions of the Grenvillian high-pressure granulites seem to reflect fractionation processes under high-grade metamorphic conditions; redistribution of highly incompatible trace elements would have occurred, at least locally. The trace element contents of different types of granulites (from mafic to felsic), when compared to the compositions usually assumed to represent the lower crust, display LREE-depleted patterns, suggesting that, besides being remobilized, the most incompatible elements have suffered removal, probably via a silicate liquid phase.

Three main compositional groups of eclogites were defined, using immobile hygromagmatophile trace elements: A) rocks with NMORB-like compositions; B) subalkaline (tholeiitic to calc-alkaline) rocks displaying geochemical signatures typical of supra-subduction settings; C) rocks related to alkaline basalt protoliths. The diversity revealed by the eclogites together with the geochemical features of the enclosing paragneisses (characteristic of sediments from basins related to volcanic arcs) suggest that the protoliths of the metaigneous and metasedimentary rocks were formed in a back-arc marginal basin, operating during Precambrian times, most probably in the Cadomian tectonic cycle.

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Gran Canaria: The Subvolcanics of the Roque Nublo-Cycle; Geochemical and Petrological Relations to the Extrusives

Eric B. Barnert

(barnert@hrz1.hrz.tu-darmstadt.de)¹,

Gerhard P. Brey (brey@em.uni-frankfurt.de)² &

Wolfgang F. Müller

(wmueller@hrz1.hrz.tu-darmstadt.de)¹

¹ Technische Universität, Institut für Mineralogie, Schnittspahnstr. 9, 64287 Darmstadt, Germany

² J. W. Goethe-Universität, Institut für Mineralogie, Senckenberganlage 28, 60054 Frankfurt, Germany

Alcaline subvolcanic intrusives and hauyne bearing extrusives in the centre of Gran Canaria are the main components of mono- and polymict breccia-sheets. The max. 20 m thick sheets are dominated by the hauyne-bearing extrusives with a tephri-phonolitic to phonolitic bulk composition. These rocks are higher differentiated than the subvolcanics with a tephritic to phono-tephritic character. The breccias represent a separate stratigraphic unit between the underlying Los Listos Formation and the overlying large debris avalanche deposits of the Ayacata Formation. They can be used as marker for the reconstruction of the pre-Ayacata Formation paleomorphology. A new mapping in the Roque Nublo area revealed more subvolcanics as described before. They occur as fragments in the debris avalanche deposits and therefore are no longer interpreted as subvolcanics in a primary intrusive position (Brey und Schmincke, 1980). In the monomict sheets the matrix and the components have the same phase composition as shown by powder diffraction. The secondary transport from a higher position is an argument for the theory that Gran Canaria had a higher altitude during the pliocene volcanic cycle. The recent subvolcanic deposits are between 1230 and 1720 m high - the highest point is 1949 m above sea-level. Another argument is the plutonic grain size of the subvolcanics (essexites, theralithes). This indicates that the subvolcanics were overlain by several 100 m of other volcanic material. RFA-data of 8 subvolcanic rocks and 162 extrusive rocks out of the Roque Nublo-Cycle were compared using spider diagrams following the plotting order for oceanic crust suggested by Hoffmann (1988). The subvolcanics have nearly the same composition as lavas of the upper Mesa de Junquillo- to the Los Listos Formation. The only exception is the subvolcanic rock from Las Lagunetas with a basaltic composition. It is equivalent to lavas of the lower Mesa de Junquillo Formation (lower part of the pliocene Roque Nublo cycle) and does not belong to the miocene cycle. Volatile transport and alteration causes differences in the concentration of mobile elements between the subvolcanics and their corresponding lavas. Our studies have shown that the large debris avalanche deposits of the Ayacata Formation were not formed by a single event. Many striations in a small area with different directions and between the generations of debris avalanches are an argument for a longer interval of inactivity and erosion between the events.

Brey G & Schmincke HU, Bull. Volcanol., 43-1, 15-33, (1980).

Hoffmann AW, Earth Planet Sci. Lett, 90, 297-314, (1988).

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Geochemistry and Liquid Line Descent of the Tarrafal Nephelinitic and Basanitic Lavas, Santo Antão, Cape Verde Islands

Birgitte Printz Christensen (cb260874@geo.geol.ku.dk) &

Paul Martin Holm (paulmh@geo.geol.ku.dk)

Department of Petrology, Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 København, Danmark

Cape Verde Islands are situated in the Atlantic Ocean about 500 km west of Africa between 15° and 17°N and 22° and 26°W. The archipelago consists of ten major islands and several smaller islets and can be grouped into a northern and a southern array, where Santo Antão is the westernmost located island in the northern array. The islands are related to an active hotspot below the slowly moving African plate. The island of Santo Antão offers very good opportunity to study the evolution of nephelinitic and basanitic melts, as the entire 779 km² island is made up from suites of rocks derived from such magmas. From the village of Tarrafal in the western part of Santo Antão a deeply eroded valley trends SW-NE. In this valley a stratigraphic section of 500 m of accessible volcanics are exposed. The volcanics consist mainly of pahoehoe type lavas. Very few pyroclastic deposits of lapilli and bombs and a sequence of two ignimbrites are also observed. The rocks are all silica undersaturated and can be grouped into picrobasalts, basanites, tephrites, melanephelinites and nephelinites. The major phenocryst phases in the lavas are olivine, clinopyroxene and Fe-Ti oxides, whereas plagioclase and haüyne occur only in minor amounts. Haüyne occurs only in the most evolved tephrites with a MgO content of around 5 wt%. Some of the lavas are highly porphyritic with up to 40 vol% phenocrysts. With decreasing MgO content from 18.6 to 4.7 wt%; SiO₂, TiO₂, Al₂O₃, FeO_total, K₂O, P₂O₅ and MnO increase, while CaO decrease slightly. The magmatic evolution is unrelated to stratigraphic position. A simple general differentiation is indicated involving early fractionation of olivine ±chromite, which is joined by clinopyroxene at 10 wt% MgO. The olivine fractionation is evident from the negative correlation of MgO vs. Ni, over the whole range of MgO contents. Moreover Cr and Co contens decrease with decreasing Ni. Sc and CaO contents decrease for a Ni content of less than 200 ppm, which indicate the onset of clinopyroxene fractionation. Fractionation to more evolved phonolitic compositions as observed in other areas of Santo Antão is not evident from the volcanics of this profile. The variations of MgO, Ni and Cr vs. stratigraphic height are similar and several samples with high MgO, Ni and Cr content occur up through the profile. Variation diagrams, especially the ones concerning incompatible elements show more than one fractionation trend with MgO, which indicate that not all lavas are comagmatic. The primitive character of the volcanics, the variation diagrams and the stratigraphic variation of MgO, Ni and Cr could suggest an evolution controlled by repeated injections of primitve magma into a fractionating magma chamber.

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Geochemical Data from Two Lava Sequences, Santo Antão, the Cape Verde Islands

K. Malene Hein (hm021271@geo.geol.ku.dk) &

Paul Martin Holm (paulmh@geo.geol.ku.dk)

Department of Petrology, Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Københanv, Danmark

The Cape Verde Islands are situated in the E. Atlantic Ocean between 15°-17°N and 22°-25°W. There are nine major islands arranged in a northern and southern archipelago. They overlie the uplifted ocean floor of the Cape Verde Rise. Active volcanoes are found on the two westernmost islands in the southern archipelago. The formation of the Cape Verde Islands is believed to be related to an underlying mantle plume. Santo Antno is the western most island in the northern archipelago. It has an area of approxi-mately 780 km² and a rugged topography reaching about 1930 m asl. The volcanics on the island are silica-undersaturated which is a characteristic feature of all the Cape Verde Islands.

We present data from two profiles on Santo Antno; the Agua Nova profile in the northwestern part and the Escabecada profile in the southwest. The profiles consist chiefly of lava flows with few dykes. The rock types range from alkaline basalts (ankaramites) through tephrite to haüyne or nepheline porphyritic phonolitic tephrite and tephriphonolite.

Magmatic differentiation due to crystal fractionation is reflected in the geochemistry of the lavas. Ni decreases as MgO decreases from 18 to 2 wt%. This is evidence of fractional crystallisation of olivine through the entire sequence. Sc and CaO decrease markedly from around 9 wt% MgO reflecting fractionation of clinopyroxene. From 6 to 2 wt% MgO, the SiO₂ content increases significantly while TiO₂ and to a lesser degree P₂O₅ decrease demonstrating the removal of Fe-Ti oxide and apatite. The content of Al₂O₃ increases with decreasing MgO indicating that Al-bearing phases like plagioclase, amphibole and feldspatoids have no significance as fractionating phases.

Generally, the Agua Nova samples have lower SiO₂, Al₂O₃ and Ni content and higher content of TiO₂, P₂O₅ and incompatible elements compared to the Escabecada samples. This is evidence that different magmas were erupted. The higher content of incompatible elements point to a smaller degree of melting of the Agua Nova source or a source that is more enriched. Additionally, the range in composition for a given MgO content in either of the profiles excludes the possibility of the volcanic products within a single profile being comagmatic.

In the Escabecada profile the stratigraphic variation of MgO seems to reveal two pulses of primitive magma with differing contents of SiO₂. With these two primitive magmas the diversity in SiO₂ of the more primitive lavas may be explained. A development towards more evolved compositions is seen towards the top. In the Agua Nova profile no clear development is seen with stratigraphic height.

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Petrology and Geochronology of Santo Antão, Cape Verde Islands ­ A Preliminary Report

S. Plesner (SUSANNE@geo1.aau.dk) &

J. R. Wilson (JRW@geo1.aau.dk)

Department of Earth Sciences, Geological Institute, University of Aarhus, 8000 Århus C, Denmark

The Cape Verde Islands are located in the Atlantic, 15-17°N and 500 km west of Senegal. Santo Antão (780 km²), the westernmost island of the northern chain, has not been volcanically active in historic times. The ages below were obtained in 1998 from ⁴⁰Ar/³⁹Ar incremental heating experiments at Oregon State University.

A ca. 800 m-thick sequence (Chã de Morte Series) has been sampled in the central part of the island (Fig.1). The lowest exposed lavas are basalts/picrobasalts with an age of 5.61±0.13 Ma. These are overlain by ca. 220 m of more evolved lavas. These varied lavas are intercalated with (picro)basalts through the next ca.320 m. Towards the top of the Series the lavas become more evolved, reaching phonolites at 1400 m. Tephrites at the very top of the profile have an age of 1.20±0.01 Ma. The Chã de Morte Series is cut by numerous dykes. The variations in lava-type and MgO-contents indicate repeated events of magma chamber development.

Lithology, whole rock MgO% and ⁴⁰Ar-³⁹Ar ages of the Chã de Morte Series in the central part of Santo Antao. Rock names are based on a TAS-diagram. Heights are in meters above sea level.

Much of the island is covered by lapilli deposits and minor lava flows erupted from locally well-preserved volcanic cones. These deposits are post-dated by a phonolitic pumice (the Cão Grande pumice) that drapes over much of the present topography. The vent from which it was erupted has been buried by lapilli deposits and minor lava flows in the Tope de Coroa area (0.20±0.01 Ma). The Cão Grande pumice has an age between 0.43 and 0.20 Ma. The youngest volcanic activity is represented by nephelinitic lavas near Porto Novo in the south of the island (0.08±0.01 Ma).

The youngest volcanic activity on Maio, the oldest island in the southern chain, took place 7 Ma years ago. If the islands become progressively younger towards the west, the presence of >5.61±0.13 Ma old lavas on Santo Antão does not leave much time for development of the intermediate islands unless some of them were formed simultaneously.

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Dykes and Sills of Santo Antão, Cape Verde Islands

Lisbeth Hansen (lisbeth@geo.aau.dk) &

J. Richard Wilson (jrw@geo.aau.dk)

Department of Earth Sciences, University of Aarhus, 8000 Aarhus C, Denmark

Santo Antão belongs to the Cape Verde archipelago that comprises a group of volcanic islands situated in the Atlantic Ocean 15-17° north of the equator and 300-600 kilometres west of Senegal.

Dykes and sills are abundant on Santo Antão; the purpose of the present project is to study their orientations, relative ages and distribution in different parts of the island and establish their petrographic and geochemical variation. The proportion of dykes compared to sills is large and in some areas sills are rare. Large parts of the island are covered by recent pyroclastic deposits, which hide the older lava sequences and dykes/sills. Most dykes are 40-200 cm wide and they commonly have chilled margins against the host lavas. Since they usually are more resistant to weathering than the lavas, many of the dykes stand out like isolated walls in the landscape. Some dykes have been observed to have acted as feeders to local volcanoes preserved on the tops of recently eroded steep mountainsides.

In some areas the abundance of dykes is much greater than elsewhere, indicating that dyke activity was most intensive there. The abundance of dykes in these areas implies considerable dilation; a rough estimate based on the abundance and width of the dykes is 10-20%. On the basis of orientations, crosscutting relationships and macroscopic compositions at least five generations of dykes have been recognised in the field.

There are large, systematic variations in the dominant strike of the dykes between the regions, but chemical analyses do not reveal significant geochemical differences. The majority of the dykes have 40-45% SiO₂ (volatile-free basis) and 3-7% Na₂O + K₂O, plotting as nephelinites and basanites/tephrites in a TAS diagram. The samples that fall in these fields are classified according to the "norm ne versus norm ab classification" suggested by Le Bas (1989). This results in more than half of the dykes that are basanites/tephrites according to the TAS-diagram being classified as either nephelinites or melanephelinites. Some dykes have more evolved compositions, plotting as phonotephrites and tephriphonolites, whereas a few are less alkaline and evolve towards trachytes. The most primitive dykes (7-16% MgO) plot on an olivine control line, whereas the more evolved ones (1-7% MgO) have decreasing CaO and TiO₂, consistent with the fractionation of clinopyroxene and Fe-Ti oxide. Other minor phases may also be involved. These compositions are consistent with those of the lava sequences and younger volcanics, in keeping with the dykes having acted as feeders for these extrusives.

Le Bas MJ, J. Petrology, 30(5), 1299-1312, (1989).

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Recent Volcanism on Santo Antão, Cape Verde Islands

Rikke Pedersen (rikkep@geo.aau.dk),

Anette K. Mortensen &

J. Richard Wilson

Hasle Centervej 239,3.tv, 8210 Århus V, Denmark

The Cape Verde Islands are situated 500 km W of Senegal between 15-17°N of the equator. The Cape Verde Islands consist of two chains, where Santo Antão (SA) is the youngest and most westerly island of the northern chain. This abstract presents a short description of two recent volcanic events on Santo Antão, the Cão Grande phonolitic pumice deposit and the Porto Novo nephelinitic lava flows.

The pumice occurs as a white, layer, which is patchilly preserved over the entire island, which has an area of 780 km². The deposit is thickest (up to 8 m) and most widespread in the southwestern and central parts of SA. Over much of SA the Cão Grande pumice represents the youngest volcanic deposit and elsewhere it provides a valuable marker horizon. In the central and western parts of SA it is locally covered by lapilli deposits and minor lava flows. In the SE part of the island it is overlain by the Porto Novo nephelinitic lava flows which represent the youngest volcanic activity on the island.

These two volcanic events represent end-members of SiO₂-undersaturated alkaline magmatism. The phonolitic pumice represents an evolved magma resulting from extensive fractionation in a magma chamber, whereas the primitive, nephelinitic lava, which contains numerous mantle nodules, has come more or less directly from a mantle source.

The vent from which the Cão Grande pumice was erupted has not been found; it appears to have been buried by younger lapilli deposits. The approximate location has, however, been estimated by mapping the variations in thickness of the deposit and the size distribution of lithic- and pumice fragments. Isopleth and isopach maps indicate that the volcanic vent must have been in the western part of SA.

The Porto Novo lava was extruded from several vents in the central part of SA and flowed to the SE, reaching the coast; the flows cover an area of ca. 40 km². The sequence consists of at least 7 flows separated by either thin scoria deposits or lapilli layers. The absence of laterite horizons indicates that the flows were extruded within a short time span.

Based on ⁴⁰Ar-³⁹Ar dating carried out by Susanne Plesner, it is now possible to determine that the eruption of the Cão Grande pumice must have taken place at some time between 430 and 200 Ky ago and the Porto Novo lavas were erupted about 80 Ky ago.

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Eruptive History of Mt. Erciyes Stratovolcano (Central Anatolia, Turkey)

Erdal Sen (sen@jeo.hun.edu.tr),

Biltan Kurkcuoglu (biltan@jeo.hun.edu.tr) &

Erkan Aydar (erkan@jeo.hun.edu.tr)

Hacettepe University, Dept. of Geological Engineering, Beytepe- Ankara, Turkey

Mount Erciyes is the most voluminous volcano (3917 m) in the Central Anatolia. The evolutionary stage of Mt. Erciyes is divided into two stages: Koç Dag stage; The volcano was mainly built up by the lava pile. The known first lavas correspond to the alkali basaltic flows. The dominant nature of lavas is characterised by andesitic lava pile. The monogenetic vents as cinder cones are basaltic andesitic in composition that represent the weak explosive activities. The lava generation was followed by pyroclastic eruptions in two sequences which led to the caldera collapse (18 x 14 km) 2.8 Ma ago. The first sequence witnessed a large episode of plinian phases with four eruption units and pumice flows which were over 30 km from the caldera. The emplacement of Valibaba Tepe ignimbrite occurences was defined as secondary. It mostly represents well-welded rheology with 3700 km² of areal distribution and is the LAR type ignimbrite. This stage was terminated by andesitic and dasitic dome extrusions which used the caldera boundary.

Recent Erciyes stage; Two phases were distinguished in its evolutionary history. First phase is characterised by effusive and weak extrusives activities which are andesitic lava flows, dasitic dome flows, basaltic andesitic lava flows, dasitic domes and andesitic cones. Second phase witnessed the dasitic explosive activities, which were well concentrated on the summit area, producing the block and ash flows, and the rhyodacitic activities that were related to dome emplacements of Dikkartin and Perikartin. Prior to these dome extrusions the plinian air fall products, surges and pumice flows were produced during this period. The last volcanic activity is represented by the debris avalanches deposits related to the summit of the volcano forming an avalanche caldera.

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Volcanological Evolution of Kula Volcanics, Western Anatolia, Turkey

Erdal Sen (sen@jeo.hun.edu.tr)¹,

Erkan Aydar (erkan@jeo.hun.edu.tr)¹,

Hasan Bayhan (bayhan@jeo.hun.edu.tr)² &

Alain Gourgaud

(gourgaud@opgc.univ-bpclermont.fr)²

¹ Hacettepe University, Dept. of Geological Engineering, Beytepe- Ankara, Turkey

² Universite Blaise Pascal, Laboratoire de Geologie, 5, rue Kessler, 63038 Clermont-Ferrand Cedex, France

Detailed geologic mapping of Kula alkali basalts show that at least 70 seperate eruptions have occured from 1.1 Ma to historical times covering about 1000 km² in a tectonic depression. Areal coverage, physical properties and age of the products led to us divide the basaltic volcanism into three generations: First, second, and the third (FG, SG, and TG). Their eruptions were accomponied by formation of NE-SW and NW-SE oriented grabens under extentional stress in Western Anatolia. Some eruptive vents were offset from the graben axis.

FG basalts (1.1 Ma) probably erupted from an elliptical fracture zone. They crop out to eastern side of study area and form lava plateaus with a very well devoloped columnlar joint structures. In contrast to FG; SG (0.3 Ma) and TG (10000 years) alkali basalts resulted in primarily monogenetic features: cinder cones, spatter cones, and maars with lava flows and base surge deposits. Explosive activitiys of maars were replaced by dry eruptions changing vent position to cinder cones on the rim of the crater. The products of individual eruptive centers vary in distribution from 1 km² to 17 km². While FG and SG lava flows are massive, TG basalts have a porous and black appearence having the flow features of aa lavas.

Although petrological data indicate similarities between different vents, their phenocrysts assemblage can be different such as for FG: Leucite (Lc)-olivine (Ol)-clinopyroxene (Cpx) and for SG, TG: Lc-Ol-Cpx-hornblende (Hb); Ol-Cpx-Hb; and Cpx-Hb. SG and TG basalts are rich in enclaves, which are harsburgite, pyroxene-hornblendite, hornblend-pyroxenite, gabbro, and metamorphic rocks. Hornblendites and pyroxenites (5-30 cm in size) have vesicules and glass including microlites. Olivine bearing xenolithes are only present in the maar deposits. Textural and mineralogical evidences show that enclaves were related to a zoned magma chamber and were erupted in a short time.

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Characterization of The Miocene Acidic Sequence of The Pire Mahuida Volcanic Field, Extraandean Patagonia, Argentina

Flavia Salani (fms@gl.fcen.uba.ar)

CONICET-Dto.Cs. Geologicas. Universidad de Buenos Aires, Pab. II. Ciudad Universitaria, Buenos Aires, Argentina

The effusion of a Miocene acidic volcanism is exposed at the Pire Mahuida Sierra (68° 30' W Long.; 42° S Lat.) located in the southern part of Argentina, in the Extraandean Patagonia region. It belongs to the Pire Mahuida Volcanic Field, a complex of 800 km² and a volume of 100 km³, constituted by rhyolitic and dacitic lavas, and pyroclastics of rhyolitic composition, related to lesser amount of basaltic flows. The suite represents a back-arck assemblage developed in an extensional regime. The objetive of this work is focused on the characterization of the acidic rocks through stratigraphic, morphological and petrogenetic analysis. The rhyolitic lavas occur as eight aligned domes distributed with a subcircular pattern, defining the western boundary of the volcanic field, and represent the first eruptive event. They show flow banding, vesiculation textures, litophysaes, and often, large gas cavities. In the central area of the sierra the rhyolites crop out as less viscous flows, and they correspond to a second eruptive stage. Most lavas are aphyric or poorly porphyritic, with sanidine, and quartz as phenocryst, and as intergrow in the groundmass. The dome lavas contain biotite, while the flows of central region show little amount of arfvedsonite. Trydimite is the common mineral of the vapor phase crystallization. The dacites occur in the southwestern part of the field as porphyric hornblenditic lava domes. A pyroclastic sequence dominates the northern part of the region. It corresponds to the main explosive event represented by ignimbrite sheets, surges and pyroclastic flows associated to rhyolitic domes. They show high proportion of vitroclasts, and subordinated crystals of sanidine, quartz, and biotite, and volcanic lithoclasts of jurassic andesites. Small volume of younger pyroclastic rocks, are related to lava-pyroclastic cones, and crops out in the central area. They contain vitroclasts, sanidine crystals, alkaline pyroxenes, and in some cases olivine and scarce lithoclasts. The petrographic and chemical features indicate two evolutive patterns: 1) a calcakaline high silica rhyolite-dacite sequence of lava domes, and pyroclastics of northern area; 2) a rhyolitic suite (lava and pyroclastic flows) with alkaline affinities belonging to the central region. The proposed model for the generation of the acidic magmatism is partial crust melting, where the heat for the fusion is supplied by the intrusion of basaltic magmas in an extensional regime. In the further evolution of the liquids, participate different processes such as fractional crystallization and volatile accomplishment.

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Upper Mantle and Lower Crustal Phenomena During Intraplate Magmatism at the Onset of the Variscan Cycle: Geochemical Evidence from the Bragança Massif (Galicia-Trás-Os-Montes Zone, NE Portugal)

José F. Santos (jfsantos@geo.ua.pt)¹,

José M. Munhá (jmunha@fc.ul.pt)²,

Fernando O. Marques (gfmogm@fc.ul.pt)² &

Graeme Rogers (grogers@geology.gla.ac.uk)³

¹ Departamento de Geociências, Universidade de Aveiro, 3810 Aveiro, Portugal

² Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, 1700 Lisboa, Portugal

³ Isotope Geosciences Unit, SURRC, East Kilbride, Glasgow, G75 0QF, UK

The Galicia - Trás-os-Montes Zone is composed of several exotic tectonic units emplaced upon the Central-Iberian autochthon. In the Bragança Massif, the upper allochthonous thrust complex is represented by several lithologies of dominantly mafic and ultramafic compositions, including HP granulites and eclogites recording Precambrian orogenic events, but also (i) plutonic bodies, essentially composed of gabbroic rocks, and (ii) ultramafic rocks. These last two lithotypes were affected by metamorphism and deformation exclusively during the Variscan tectonic cycle.

Trace-element and isotope compositions of gabbroic rocks (whose emplacement would have occurred during Upper Precambrian and/or Lower Cambrian times) indicate that they crystallized from continental tholeiitic magmas; the <epsilon>_Nd vs. <epsilon>_Sr features reveal interaction with, older, lower-crustal metamorphic rocks. Micro-domainal partial recrystallization and incomplete isotopic equilibration are typical features of the gabbroic rocks, reflecting long-term magma underplating during continental breakup.

Ultramafic rocks represent a portion of subcontinental lithospheric upper mantle, located in an intraplate tectonic setting in the Lower Palaeozoic. Intimately associated peridotite and pyroxenite layers are probably testimonies of the penetration of magmas (from which pyroxenes were segregated) through rocks (of harzburgitic or similar compositions) previously submitted to melt removal. Some pyroxenite bodies are well individualized and may contain garnet; this type of pyroxenites has trace-element patterns characteristic of their derivation from alkaline magmas; the ascent of strongly LREE-enriched melts through upper mantle rocks, besides being responsible for the generation of those bodies, have also caused cryptic metasomatism preferentially affecting the refractory lithologies.

It is concluded that both the ultramafics and the gabbroic rocks record deep manifestations of the igneous phenomena that accompanied the extensional period marking the beginning of the Variscan tectonic cycle.

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Intraplate Magmatism of the Kola Province, NE Fennoscandinavia

Tamara Bayanova (tamara@geo.kolasc.net.ru) &

Felix Mitrofanov (felix@geo.kolasc.net.ru)

Gelogical Institute of the Kola Research Sentre, RAS, 14, Fersman st., 184200, Russia

In the northern and southern parts of Fennoscandia there are two Archaean cratons: Murmansk and Karelian-Finnish. Between them is the Early Proterozoic Kola Collision Structure with an ensemble of obducted lower-middle crustal domains, Archaean greenstone belts, and Early Proterozoic belts: Pechenga, Imandra-Varzuga and others. In Late Proterozoic time this area acquired a continental cover, and later, Devonian alkaline intrusions (about fifty massifs) were formed.

The Archaean history of the region finished with magmatism of a stable regime and the formation of extensive alkaline granites in Keivy (2.75 Ga), rapakivi-like granites (2.76 Ga), basic dyke swarms (2.74 Ga), and carbonatite associations (2.58 Ga).

Processes of intraplate magmatism and high-pressure metamorphism in the Kola province of Early Proterozoic time started 2.5 Ga ago with the formation of layered intrusions containing Cr and PGE deposits. The Pana Intrusion with PGE-bearing reefs is one of the largest. A multi-phase interval of its magmatic crystallization lasted at least 40 Ma: from 2491±1.5 (gabbronorite) to 2447±12 Ma (anorthosite). The underplating source with a negative ENd (-3 to -1), that produced layered intrusions of the Stillwater type (Pana, Moncha, Generalskaya, Imandra), was functioning for at least 100 Ma: from 2.5 to 2.4 Ga.

In the period from 2.4 to 1.98 Ga this source was replaced by localized magmatic reservoirs with a positive (+2) <epsilon>_Nd(T), which contained magmas of the Pechenga-rift type and produced Cu/Ni/Co deposits.

Starting from the epoch 2.0 Ga ago, mantle intraplate processes were concentrated in discontinuous linear zones. They resulted in the formation of alkaline intrusions with or without carbonatites, and kimberlites-lamproites. Their origin can be related to hot-spots whose age has been determined as 1950-1900, 1700 and 1650 Ma.

The most active hot-spot processes accompanied by various alkaline intrusions with economic mineral deposits took place in the Kola alkaline province in Devonian time, from 410 to 360 Ma ago. We have determined that this time interval is 50 million years, rather than 20 Ma (380 to 360 Ma) according to (Kramm et al., 1993).

Kramm et al., Lithos, 30, 33-44, (1993)

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Hydrothermal History of the Hellenic Volcanic Arc

S. P. Varnavas (S.P.Varnavas@upatras.gr)¹,

P. Halbach²,

M. Halbach²,

D. Panagiotaras¹,

E. Rahder² &

A. Hubner²

¹ Dept. Of Geology University of Patras, Patras, Greece

² Free University of Berlin, Berlin, Germany

Sediment cores studied from six major submarine hydrothermal fields along the Hellenic Volcanic Arc led to important implications regarding the recent hydrothermal history of the Arc. The sediment cores were recovered by scuba diving above submarine hydrothermal vents offshore Methana Peninsula, from Palaechori Bay, Voudia Bay and Adamas Bay, Milos island, offshore Kephalos Bay, Kos island and offshore Yali island.

The lithologic description and the chemical analysis of the cores showed the presence of distinct sediment horizons in the subsurface sediments which are indicative of hydrothermal fluxes which took place in the recent past. In all sediment cores studied the concentrations of hydrothermal elements such as Mn and Fe are higher in subsurface horizons compared to their values in the surface sediments. It is therefore implied that the hydrothermal events which took place in the recent past were stronger than the present day hydrothermal event.

The presence of hydrothermally influenced sediment horizons in the subsurface sediments in most of the hydrothermal fields investigated suggests that similar hydrothermal events took place along the Hellenic Volcanic Arc. Moreover, overall along the Arc there is a tendency for the degree of the hydrothermal activity to decrease towards the present.

This work has been funded by EU under MAST programme contract No MAS-2-CT940101

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Petrological and Geochemical Investigation of Kepecitepe (Isparta - Turkey) Volcanics Related to Their Genesis

Mustafa Kumral (kumral@itu.edu.tr)¹,

Atsever Gedikoglu (atsever@itu.edu.tr)¹,

Ahmet Celenli (celenli@itu.edu.tr)¹,

Murat Budakoglu (budakoglu@itu.edu.tr)¹,

Hakan Coban (coban@sdu.edu.tr)² &

Fikret Suner (suner@itu.edu.tr)¹

¹ Istanbul Technical University Faculty of Mines Geology Department 80626 Maslak Istanbul Turkey

² Suleyman Demirel University Geology Dept. Isparta Turkey

The study area is located in the north of Gonen (Isparta) town. From bottom to top, lithostratigraphical sequence consists of 1) The consecution of Eocene sandstone-shale-marl-microconglomerate-clayey limestone having thin strata in yellowish-redish color. 2) Oligocene poligenic conglomerate including limestone-sandstone-radiolarite-chert pebbles. 3) Middle-Late Miocene aged micritic limestone, an allocton unit, in creamy white color. 4) Late Miocene discordant poligenic conglomerate including limestone-serpentinite-radiolarite-chert detritics. 5) Fair grey colored trachyandesite in porphyritic texture, which had cut the whole sequence and emplaced in Pliocene. 6) Recent alluvium detritics.

Mineralogical studies indicated that the volcanics include large sanidine crystals and enriched in terms of albite-biotite and well-crystallized hornblende minerals. Chemical data of the volcanics were plotted on various rectangular and triangler diagrams such as Na₂O+K₂O-SiO₂, SiO₂-Zr/TiO₂, FeO-Na₂O+K₂O-MgO and Zr/117-Th-Nb/16. These diagrams obviously showed that mentioned volcanics are of trachyte-trachyandesite type rocks and have calcalkalen character. Furthermore, the diagrams pointed out that they had been derived from upper crust and located at the border of depleted margin.

Spider diagram of Kepecitepe volcanics

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Petrography and Petrochemistry of The Upper Cretaceous Volcanic and Subvolcanic Rocks In Olucak Region (Gumushane-Torul/NE Turkey) and Their Genetic Relation to Torul Granitoide

Hakan Coban (coban@sdu.edu.tr)

Suleyman Demirel University Geology Dept. Isparta Turkey

The study area includes the Olucak (Gumushane-Torul/NE Turkey) and its surrounding areas located at between the northern and southern zones of the East Pontide. Upper Cretaceous outcrops wide spreadly in the region and consist of magmatic rocks (Karaburun dacite-Torul granitoide) and volcano-sedimentary rocks (Kermutdere-Tepekoy formations). Two phases were determined in the Upper Cretaceous volcanism. The first phase (Early phase) includes the volcanics Kermutdere Formation. In this phase, rocks have basaltic-trachyandesite/tranhyandesite and phonolitic composition and indicate silica-saturated and silica-undersaturated magmas. The bottom level of the phase consist of the rocks having plagioclase (neutral) + sanidine + hornblende + clinopyroxene (augite). The upper part of the phase is composed of the rocks having plagioclase (acid-neutral) + sanidine + nepheline + sodalite (nosean) + clinoamphibole (arfvedsonite) and olivine. The second phase (late stage) is chracterized by a silica over-saturated magma and indicates rhyolitic to dacitic rocks in composition (Tepekoy formation). The rhyolites in the second phase have a perlitic texture and are a rapidly cooled magma origin. Some textural features, observing in Upper Cretaceous volcanites, such as melting inclusions in plagioclases, occurrence of needle-shaped apatites, ocellar texture (maphic mineral inclusion bearing quartz) are the evidences of the same aged homogene magma-mixing between both maphic and felsic magmas. The petrochemical analysis of volcanics showed that the volcanics of Kermutdere have alkaline (shoshonitic)-calc-alkaline magma character and the Karaburun dacite-Tepekoy formation volcanics have calc-alkaline magma character. These characters are associated with their mineralogic composition. These petrographic-petrochemical aspects and especially petrogenetic trace elements parameters of Upper Cretaceous volcanics indicate that volcanics are derived from different magma origines and Kermutdere formation volcanics have hybridic magma genesis related to partial melting of upper mantle and partially crustal material. Tepekoy formation and Karaburun dacite have dominantly crustal genetic magma and are interpreted that as Karaburun dacite and Tepekoy formation volcanics are the marginal sub-volcanic and volcanic extrusion facies of the Torul granitoide, which intruded in the Upper Cretaceous age.

O08 : 4P/22 : PO

Did Ultrabasites from the Hercynian Maures Massif (France) Originate in an Arc Context? Petrographical and Geochemical Evidences

JP Bellot (Fax +33 467 54 7362)¹,

C Laverne² &

G Bronner²

¹ Géochronologie, Géochimie et Pétrologie (UMR 5567), Univ. Montpellier II, Place Bataillon, 34095 Montpellier Cedex 05, France

² Faculté Sciences St-Jérôme, case 441, 13397 Marseille Cedex 20, France

Among the various ultrabasites which underline the sutures of the Variscan belt, the ultrabasites of the Maures massif have not been studied in details until recently (Laverne et al., 1997; Bouloton et al., 1998). This massif, located in the southern part of the west European Variscan belt, is its easternmost part, unaffected by the alpine orogenesis. Therefore, it is of peculiar interest, but its geodynamical reconstitution remains difficult.

We have mapped and sampled 21 sites of serpentinites. They are typically north-south elongated lenses, in tectonic contact with the host-leptyno-amphibolic complex. The main results of our petrographical and geochemical study are given below.

Only 12% of the 220 collected samples contain relics of the primary paragenesis, which has been obliterated by successive metamorphisms. Modal compositions indicate that protolithes are dunites, orthopyroxenites, clinopyroxene-dunites and plagioclase-dunites. The most ultrabasic rocks are located in the southern part of the massif, whereas slightly more basic rocks occur in the North.

We observed a mineralogical layering in only one site (Levant), which is composed of a succession of beds of various ultrabasites (dominant dunite, amphibole-bearing dunite, pyroxenite, and metagabbro). Primary textures are rarely preserved. Nevertheless, we could observe heteradcumulates and mesocumulates. This is the first time that primary layering and cumulate textures are described in this massif. The lack of under-structures in olivine and orthopyroxene, and of any tectonite texture favors a magmatic origin for these rocks.

Major, trace, and rare earth elements of bulk rocks discriminate two sets of samples: dunites and pyroxenites (Al₂O₃<2%), which are clustered; Al₂O₃-rich (3%<Al₂O₃<9%) plagioclase-dunites and the gabbro (Al₂O₃=20%), which are distributed along a straight line, showing an increase of SiO₂ and CaO contents, and a decrease of MgO, FeOt, Cr, Ni, Co and V contents with increasing Al₂O₃. This indicates a fractionated crystallization process and shows that the plagioclase-dunites and the gabbro are cogenetic. Geochimical investigations attest for a cumulate origin of dunites, pyroxenites and plagioclase-dunites.

The Ti/V ratios distinguish the southern sites from the northern ones, respectively assigned to an arc context and an extensional context. The chemical compositions of the chromian-spinels also confirm this distinction: dunites and pyroxenites (southern sites) originated from a high-Mg arc magma, whereas plagioclase-dunites and some other dunites (northern sites) originated from an oceanic affinity tholeiitic magma. This mixed origin is confirmed by isotopic ratios on Sm/Nd (Bouloton et al., 1998) into several gabbros associated with ultramafites from the Maures massif.

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Mesozoic Noritic Dykes in the Baldissero Peridotite Body (Iurea-Verbano Zone, NW Italy

Maurizio Mazzucchelli (mazzuc@unimo.it)¹,

Giorgio Rivalenti (riva@ unimo.it)¹,

Alberto Zanetti (zanetti @crystal.unipv.it)² &

Giancarlo Cavazzini (gianca@epidote.dmp.unipd.it)³

¹ Dipartimento di Scienze della Terra, Universita' di Modena, Italy

² CNR, Centro di Studio per Ia Cristallochimica e Ia Cristallografia, Pavia, Italy

³ CNR, Centro di Studio per Ia Geodinamica Alpina, Padova, Italy

Noritic dikes in the Baldissero mantle peridotite body (located in the south-westernmost part of the lvrea-Verbano Zone) have been recently recognized. They are fine-grained, from centimetric to decimetric in thickness and sharply cross-cut the Iherzolite country rocks. The mineral assemblage is constituted by dominant plagioclase and orthopyroxene, with subordinate clinopyroxene, and interstitial amphibole and spinel. The major element whole rock geochemistry shows a calc-alkaline affinity. The dykes have REE patterns from weakly LREE-depleted to weakly LREE-enriched (La_n/Yb_n = 0.5-2.4), with REE concentration (Primitive Mantle normalized) ranging from 0.6 to 4. A small positive Eu anomaly is always present (Eu/Eu* = 1.3-1.5). Extended trace element patterns show a weak negative Zr anomaly (Zr/Zr* = 0.8-0.9), positive Sr anomaly (Sr/Sr* = 5.6-10.4) and Ba_n/Nb_n = 1.5-3.5 and La_n/Nb_n = 1.2-2.0. The peridotite country rocks have strongly LREE depleted patterns (La_n/Yb_n = 0.003-0.017). LREE depletion decreases in the contact region with the dikes (La_n/Nb_n = 0.10-0.26). Trace element mineral analyses show almost coinciding REE patterns for clinopyroxenes and amphibole, with mildly convex-upward profiles (La_n/Yb_n around 1 and Sm_n/Yb_n = 2.3), being amphibole weakly LREE-enriched with respect to the clinopyroxene. Interestingly, these phases present HFSE-enriched compositions: clinopyroxene has no Ti anomaly (e.g. Ti = 10500 ppm) and only minor Zr and Nb negative anomalies (e.g. Nb = 1.8 ppm, Zr = 90 ppm), whereas amphibole has large Ti and Nb positive anomalies (e.g. Ti = 25000 ppm, Nb = 62 ppm). Plagioclase shows LREE enriched pattern with a strong Eu positive anomaly (Eu/Eu* = 15.5). Orthopyroxene has a LREE depleted pattern with Ce_n/Yb_n = 0.28. A two point mineral (plagioclase + clinopyroxene) best fit calculated from Sm-Nd isotopic data yields a slope corresponding to an age of 180±26Ma, with a Nd_i = 0.51280 and <epsilon>nd_i = 7.8 (Obermiller, 1994). Other isotopic determinations are in progress. More indications of Mesozoic ages, obtained in other parts of the Ivrea-Verbano Zone, are reported in Lu et al. (1997). If the minimum age of the Baldissero dyke intrusion will be confirmed, this would support the hypothesis of a remarkable episode of Mesozoic magmatism in the lvrea-Verbano Zone. If this hypothesis is true the lvrea-Verbano zone could represent a key-area for understanding the middle Triassic calc-alkaline to shoshonitic magmatism of the central and eastern Southern Alps.

Lu M, Hofmann AW, Mazzucchelli M, Rivalenti G, Chem. Geol. 140, 223-235, (1997).

Obermiller WA, Unpub. PhD thesis, J. Gutenberg-Universität Mainz, 192, (1994).

O08 : 4P/24 : PO

Experimental Phase Relationships in Natural Phonolitic Systems on the Example of the Laacher See Volcano, Germany

Jasper Berndt

(J.Berndt@mineralogie.uni-hannover.de)¹,

Francois Holtz,

Jürgen Koepke &

Gerhard Wörner²

¹ Institut für Mineralgie, Welfengarten 1, D - 30167 Hannover, Germany

² Geochemisches Institut, Goldschmidtstr.1, D - 37077 Göttingen, Germany

Stability fields and compositions of the mineral phases occurring in natural phonolitic systems have been determined experimentally as a function of p, T, f(O)₂ and aH₂O. A comparison between the experimental products and the natural occurring minerals allows to constrain the pre-eruptive conditions in the Laacher See magma chamber (LSM). The three investigated samples were phonolitic pumice collected from the middle (MLST) and the upper (ULST) layers of the pyroclastic Laacher See Tephra (LST) deposits. They correspond to the lower and the middle part of the mineralogically and chemically zoned Laacher See magma chamber prior to eruption [1]. Crystallization experiments were performed with starting materials composed of water and initially crystal-free and homogeneous dry glasses, obtained by heating the natural samples at 1600°C at 1 atm. The added water contents were 4 wt% and 8 wt% H₂O. Experiments were conducted at 200 and 300 MPa for 4 days in the temperature range 760°C-880°C. One part of the experiments were carried out in Ni-NiO buffered water-pressurized cold-seal vessels (log f(O₂)=NNO) and the other part in cold-seal vessels pressurized with argon (log f(O₂)=NNO+2,3). Besides glass, following mineral phases have been identified and analysed with microprobe: pyroxene, plagioclase, alkali feldspar, amphibole, biotite, titanite, magnetite and a sodalite (hauyne-like) phase. p,T, f(O₂) and water content of the charge influence the stability and composition of the crystalline phases. Pyroxene and especially biotite are Mg-richer in the most oxydizing conditions. Both the Mg-rich and Fe-rich biotites obtained at high and low f(O₂), respectively, have natural equivalents in the LST. Plagioclase compositions similar to those observed in the natural assemblage (for the corresponding composition) could only be reproduced at water undersaturated conditions (experiments with 4 wt% H₂O added). Sodalite was only obtained at water undersaturated and oxidizing conditions. Amphibole is not observed at 200 MPa but few amounts were observed in the 300 MPa products. The results indicate that the pressure of 200 MPa usually assumed for the LSM is probably underestimated and that the main phases crystallized at water-undersaturated conditions. The prevailing f(O₂) conditions were more oxydizing than those fixed by the Ni-NiO-buffer. However, the biotite compositions suggest that significant local variations of f(O₂) in the magma chamber may have occurred prior to eruption.

Wörner G, Schmincke H-U, Journal of Petrology, 25, 805-835, (1984).

O08 : 4P/25 : PO

Differentiation of Ferro-Basaltic Magmas in Systems Closed to Oxygen: A New/Old Experimental Approach

Dominique Lattard

(dlattard@classic.min.uni-heidelberg.de)¹,

Georg M. Partzsch

(partzsch@classic.min.uni-heidelberg.de)¹ &

Michael J. Toplis (mtoplis@crpg.cnrs-nancy.fr)²

¹ Mineralogisches Istitut, Universität Heidelberg, D-69120 Heidelberg, Germany

² CRPG, 15 rue Notre Dame des Pauvres, F-54501 Vandoeuvre-les Nancy, France

Oxygen fugacity (fO₂) is a critical parameter during the differentiation of ferrobasaltic magmas because of its influence on the stability of crystalline Fe-Ti oxides, which in turn greatly affect the evolution of the iron content of the residual melt. In experimental studies of basaltic differentiation, oxygen fugacity is generally controlled either by a solid oxygen buffer or by gas mixtures. In both cases, the system is open to oxygen (i.e. the sample may exchange oxygen with the external buffer) and thus during heating or cooling the sample will follow a T-fO₂ path parallel to that of an oxygen buffer such as FMQ or NNO (e.g. Toplis & Carroll, 1995).

In several layered intrusions and in some volcanic suites, however, the oxygen fugacity does not appear to follow a buffer curve with decreasing temperature. Instead, it increases before Fe-Ti oxide saturation and subsequently decreases (e.g. Snyder et al., 1993). Experimental work in simple systems (e.g. Presnall, 1966) and model calculations in complex systems (Toplis & Carroll, 1996) show that this evolution of oxygen fugacity may take place if the differentiation occurs under conditions closed to oxygen. As these model calculations bear large uncertainties, there is a need for experimental data in closed systems, which, up to now, were considered to be impossible.

In order to perform crystallisation experiments under conditions closed to oxygen, we have revived and developed the technique of performing experiments in evacuated silica glass tubes. This technique has been extensively used for sulfide synthesis, as well as for the study of subsolidus equilibria in oxide systems (e.g. Lattard, 1995).

The starting materials are synthetic ferrobasaltic glasses melted at 1400°C in air, quenched and ground to fine powders. The experiments are conducted in two steps: (1) Small charges of the starting materials are either pressed onto a loop of thin Pt-wire or into a small AgPd capsule (both pre-treated to minimise iron-loss), and equilibrated at 1200°C (above the liquidus) in equilibrium with a CO/CO₂ gas mixture to fix initial fO₂, in the range FMQ -2 to FMQ +2. (2) The equilibrated charges are placed in evacuated silica glass capsules and re-equilibrated under sub-liquidus conditions (1050-1170°C). At the end of the experiments the silica glass capsules are quenched in water.

Preliminary experiments show that run products consist of euhedral crystals which are generally homogeneous in composition, suggesting an approach to equilibrium. Phases present include plagioclase, olivine, clinopyroxene, ilmenite and magnetite, depending on the temperature and fO₂ fixed during the first experimental step. The phase equilibria and residual melt compositions will be compared with those determined for crystallisation under conditions open to oxygen, in order to assess the importance of distinguishing differentiation under conditions open and closed to oxygen.

Lattard D, Am. Mineralogist, 80, 968-981, (1995).

Presnall DC, Am. J. Sci, 264, 753-809, (1966).

Snyder D, Carmichael ISE, Wiebe RA, Contrib. Mineral. Petrol, 113, 73-86, (1993).

Toplis MJ & Carroll MR, J. Petrol, 36, 1137-1170, (1995).

Toplis MJ & Carroll MR, J. Petrol, 37, 837-858, (1996).

O08 : 4P/26 : PO

Viscosity and Electrical Conductivity of Basaltic Melts

Markus Hauser (mhauser@chemie.fu-berlin.de)¹,

Jörg Arndt &

Frank R. Schilling²

¹ Institut für Mineralogie, Freie Universität Berlin, Takustr.6, 14195 Berlin, Germany

² Geo Forschungs Zentrum Potsdam (GFZ), Telegrafenberg, 14473 Potsdam, Germany

The viscosity and conductivity measurements were carried out on 15 different melts in the system Anorthite-Diopside-Forsterite. Viscosities were obtained in the range of 10^-8 to 10^-13 dPas using a micropenetration method between 700 and 900°C (Hummel and Arndt, 1985). Electrical conductivity measurements were performed using impedance spectroscopy in the frequency range between 1 Hz and 1 MHz (Partzsch, 1997).

The measured samples show the well known correlation between viscositiy and the relation of NBO (nonbridging-oxygens) and tetrahedrally coordinated cations (T= Al, Si). Hence the viscosity decreases with decreasing Anorthite content and increasing Forsterite content. All samples in the measured temperature range show only minor deviation from an Arrhenian temperature dependence of viscosity.

Activation energies of viscous flow show a general increase with increasing Forsterite content and a decrease with increasing Anorthite content with an exception for the samples on the binary Anorthite-Diopside join. They exhibit a local minimum in activation energy for An/An+Di values near 0.4.

First analyses of the conductivity data at constant temperature show no constant increase of the electrical conductivity with increasing degree of depolymerisation or the concentration of Mg⁺⁺ and Ca⁺⁺. This might be due to the non-linear mixing effect of the charge carrying cations Mg⁺⁺ and Ca⁺⁺. Since the thermal histories of the glasses are similar, variation of the ac conductivity due to variable liquid structures in equilibrium with the fictive temperature are improbable.

Hummel W & Arndt J, Contrib. Mineral. Petrol., 90, 83-92, (1985).

Partzsch GM, Dissertation, FU-Berlin, (1997).

O08 : 4P/27 : PO

Intrusion 4 in the Honningsvåg Intrusive Suite, Northern Norway

Kasper L. Lundgaard (leth@geo1.aau.dk)¹,

Brian Robins (brian.robins@geol.uib.no)²,

J. Richard Wilson (jrw@geo.aau.dk)¹ &

Christian Tegner (christian.tegner@geo.aau.dk)

¹ Department of Earth Sciences, Arhus University, 8000 Århus C, Denmark

² Department of Geology, Alle´gaten 41, 5007 University of Bergen, Norway

Fieldwork in Intrusion 4 (I.4) in the Honningsvåg Intrusive Suite (HIS), Magerøy, northern Norway (71° N, 26° E), was carried out in the summer of 1998. The HIS is emplaced in Silurian flysch-type sediments, which are part of the Upper Allochthon of the Scandinavian Caledonides. Graptolites of Middle Llandoverian age are preserved in the contact aureole, yielding a maximum age for emplacement of the HIS of ~ 425 Ma.

I.4 covers an area of ~3x1.5 km. The rocks in the 1160 m-thick Layered Series are generally medium to coarse grained and mafic to ultramafic, and consist of feldspathic peridotite (olivine-chromite cumulate), melatroctolite/troctolite (olivine-plagioclase-chromite cumulate) and olivine gabbro (plagioclase-Ca-rich pyroxene-olivine cumulate). The intrusion is intruded by later, generally coarse-grained, olivine gabbro dykes/sills which commonly exhibit modal layering. The peridotite, melatroctolite/troctolite and olivine gabbro cumulates are repeated in 9 cyclic units. Each cyclic unit is believed to represent fractional crystallization of a new batch of magma. A 248 m thick basal peridotite represents the initiation of the magma chamber. Melatroctolite is the most abundant cumulate in I.4. Melatroctolite varies from massive and very olivine-rich to cumulates with a strong plagioclase lamination. There is a continuous gradation between these two extremes. The abundance of cumulus olivine suggests that the melatroctolites are non-cotectic cumulates and are possibly hybrids generated by magmas of different compositions. The contacts between the different rock units in I.4 are generally planar and parallel to the modal layering or lamination. However, contacts between peridotite and melatroctolite can be highly irregular, with fingers and pipes of replacive peridotite penetrating stratigraphically upwards into melatroctolite.

Samples have been collected to cover the entire stratigraphic sequence (1160 m) of I.4. Additional samples have been collected through one of the macrocyclic units in order to investigate possible lateral variations in mineral chemistry. Geochemical analyses (microprobe and whole rock XRF) of the samples are in progress.

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The Composition of the Depleted Mantle End-Member of the Cape Verde Volcanics: Is DMM an Important Source Component at the Cape Verde Islands?

Paul Martin Holm (paulmh@geo.geol.ku.dk)¹,

Thomas Find Kokflt (tkokfel@ugcvax.dnet.gwdg.de)² &

Søren Lund Hansen (hs010573@geo.geol.ku.dk)¹

¹ Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 København, Danmark

² Geochemical Institute, University of Goettingen, Goldschmidtstrasse 1, D-37077 Göttingen, Germany

The isotopic variation in Cape Verde volcanics have been explained by mixing of HIMU, EM1 and DMM type components with contributions from in either the lithospheric mantle or an inhomogeneous plume (Gerlach et al., 1988; Kokfelt et al., 1998). In the northern Cape Verde island of Santo Antão (779 km²) the presence of an old relative depleted component is particularly clear. We have reviewed existing data for basaltic rocks from the Mid Atlantic Ridge between 10^oN and 30^oN and discuss them in the light of the three component mixing model proposed for the Cape Verde Island magmas. Our new isotopic and incompatible element data from primitive nephelinitic and basanitic volcanics (MgO > 7 wt%) of Santo Antão demonstrate that the main variation of the Santo Antão volcanics is not related to mixing between HIMU and DMM type sources but rather to non-DMM components. Besides being an isotopic, the 14^oN anomaly of the MAR is also characterised by relatively low K/Nb < 300 and high Rb/Sr > 0.03. This is comparable to the low ²⁰⁶Pb/²⁰⁴Pb end of the Santo Antão compositional field with K/Nb > 200 and Rb/Sr = 0.03-0.06. Models of mantle melting and the elemental and Sr, Nd and Pb isotopic variation of Santo Antão primitive volcanics demonstrate that the source of the 14^oN anomaly at the MAR could probably be the relative depleted end-member component of the island and that mixing with DMM or very depleted MAR basalts would not explain the Santo Antão trend.

The 14^oN geochemical anomaly thus seems to have at least the age of the oldest Santo Antão samples, c. 5 Ma. If the depleted component in the Cape Verde volcanics was derived from the oceanic mantle lithosphere, the anomaly was also present in the Jurassic. If the component is sublithospheric, a widespread occurence of at least 2000 km for this component is indicated

Gerlach DC, Cliff RA, Davies GR, Norry M & Hodgson N, Geochim. Cosmochim. Acta, 52, 2979-2992, (1988).

Kokfelt TF, Holm PM, Hawkesworth CJ & Peate DW, J. Petrol., (submitted).

Session O08:5A

O08 : 5A/01 : H2

Ca-Al-Silicates as Retrograde Crystallization in Hercynian Granitoids

Regina Freiberger

(reginaf@cregu.cnrs-nancy.fr)¹,

Lutz Hecht (hecht@cregu.cnrs-nancy.fr)¹,

Michel Cuney (cuney@cregu.cnrs-nancy.fr)¹ &

Denis Virlogeux²

¹ CREGU, B.P. 23, Vandoeuvre-lès-Nancy, France

² ANDRA, 1-7, Rue J.Monet, 92298 Châtenay Malabry Cedex, France

Several Ca-Al-silicates have been identified as retrograde crystallization within intermediate to basic plutonic rocks of the Mid-European Hercynides from two localities in Germany and France. The plutonic complex of the Fichtelgebirge (NW Bohemian Massif, Germany) consists of grani-tes associated with granodiorites, diorites and gabbros (locally called "Redwitzites"). The plutonic complex of Charroux-Civray (Massif Central, France), covered by a 150 m thick sedimentary pile, is mainly composed of tonalites, monzonites, and monzogabbros rarely associated with granites. The intermediate to basic plutonic rocks of both localities always contain biotite as a major com-ponent. Most of these rocks can be classified as low- to medium-K calcalkaline to shoshonitic plutonites (after Peccerillo & Taylor 1976). Epidote, probably of magmatic origin, is another par-ticular component of both plutonic complexes. The primary biotite is the host of several retrograde Ca-Al-silicates like garnet, pumpellyite, prehnite, and epidote which occur as lenses parallel to the main cleavage of biotite. Actinolite is also present in some Fichtelgebirge samples. Textural relationship shows that prehnite and pum-pellyite have been formed after garnet. Garnet contains up to 12% hydrogarnet component and presents intermediate composition between andradite and grossular. The Ca-Al-silicates are rela-tively rich in Fe, e.g. up to 20% FeO in pumpellyite and up to 18.5% FeO in epidote. The absence of penetrative postmagmatic deformation suggests that hydrogarnet, prehnite, pum-pellyite, and epidote have been formed during retrograde cooling of the plutonic rocks. Probable magmatic epidote indicates a pressure of > 3 kbar during final pluton crystallization. Hydrogarnet occurs as one of the first retrograde subsolidus Ca-Al-silicates most probably above 350°C (Liou et al. 1983). Finally the prehnite-pumpellyite paragenesis points to P-T conditions of 200 to 300°C and 2 to 3 kbar (Frey et al. 1991). The preferable crystallization of the Ca-Al-silicates within biotite may be due to catalytic effects as described by Nijland et al. 1994.

Frey M, de Capitani C & Liou JG, J Metamorph Geol, 9, 497-509, (1991).

Liou JG, Kim HS & Maruyama S, J Petrol, 24, 321-342, (1983).

Nijland TG, Verschure RH & Maijer C, CRAS, 318, 501-506, (1994).

Peccerillo & Taylor, Contrib Mineral Petrol, 58, 63-81, (1976).

O08 : 5A/02 : H2

Metamorphism of Carbonaceous Material (Bündnerschiefer, Switzerland): XRD and Thermal Analyses Data

Tatjana V. Petrova (petrova@ubaclu.unibas.ch),

Willhelm B. Stern (sternwb@ubaclu.unibas.ch &

Martin Frey (freym@ubaclu.unibas.ch)

Mineralogisches-Petrographisches-Institut, der Universität Basel, Bernoullistrasse 30 CH-4056 Basel, Switzerland

Metamorphism of carbonaceous material results in graphitisation (reorganisation of the carbon atoms into better organised structure) which is believed to be an irreversible process and may be a useful indicator of the maximum metamorphic grade attained. The metamorphic profile in the Bündnerschiefer of eastern Switzerland reflects an increase in grade from the anchizone to the lower amphibolite facies. This metamorphic span is broad enough to observe different stages of graphitisation. Lithologically the collected samples refer to the deep oceanic facies and are assumed to be rich in organics of algae origin. The total organic content (TOC) of the samples varies from 0.1 to 1.5 wt.%. The mineralogy of the samples does not vary much, containing quartz, calcite, illite/muscovite, paragonite, chlorite, albite, rarely dolomite, pyrite, plus garnet in few samples of the lower amphibolite facies.

XRD data of carbonaceous material from 40 samples (mostly metapelites) show that with increase of metamorphism, the d-values and peak shapes of the graphite-like phases (poorly and not yet-well-ordered graphite) vary largerly. In general, d-values decrease and the peaks are getting narrower. However, the reflections of well-ordered graphite are still rare in the samples of higher metamorphic grade.

Carbonaceous material has been further studied with thermal (thermal gravimetrical and differential-thermal) and combustion analyses under oxygen and nitrogen atmospheres. The material produces exothermic peaks of combustion under oxygen atmosphere. We have observed that, with increase of metamorphism, the maximum combustion temperatures of the carbonaceous material shift to higher values (from 400 to nearly 700°C) but still do not reach 800°C, i.e. the combustion temperature of well-ordered graphite. This temperature shift may be correlated with the graphitisation process, i.e. structurisation of the carbon atoms with increasing metamorphic grade. Besides, with increase of metamorphism, the relative CO₂ and H₂O values of carbonaceous material (obtained by combustion analysis) vary as well that may be related to the loss of H atoms with graphitisation of the carbonaceous material.

Our XRD and thermal analyses data indicate to heterogeneity of carbonaceous material in the samples, i.e. each sample may contain different carbon-bearing phases or substances (kerogen, coal macerals, poorly-to-well-ordered graphite). Thus, in diffractograms of carbonaceous material various peaks of graphite-like phases as well as humps may be present. The results of thermal analyses show that the combustion of carbonaceous material produces various CO₂ and H₂O peaks at different temperatures in one sample. It is suggested that different types of carbonaceous material react differently during the graphitisation process. Moreover, the graphitisation process itself may be influenced by deformation of the samples.

O08 : 5A/03 : H2

Mineralogy and Geochemistry of the Apatite Bearing Marbles from Eppawala, Sri Lanka

Amarasooriya Pitawala

(paleo@mpch-mainz.mpg.de)¹,

Manfred Schidlowski

(paleo@mpch-mainz.mpg.de)¹,

Wolfgang Todt¹ &

W. Hofmeister²

¹ Max-Plank Institute für Chemie, Abteilung Biogeochemie, 3060, D-55020 Mainz, Germany

² Johannes Gutenberg-Universität, Institut für Geowissenschaften, Becherweg 21,D-55099 Mainz, Germany

The Eppawala apatite marbles, on which little petrological, mineralogical and geochemical information had been as yet available, occur in the northern part of central Sri Lanka as scattered individual bodies within a Precambrian high-grade terrain. They differ distinctly from other Sri Lankan marbles in their mode of occurrence, mineral assemblage and textural features. Calcite, dolomite, apatite, magnetite and ilmenite are the major rock forming minerals, whereas forsterite, tremolite-actinolite, pyrite, phlogopite, magnesite, enstatite, talc, spinel, baddeleyite and rutile occur as minor or accessory constituents. Most non-carbonate minerals show wide variation in grain size (few mm to several dm) and shape (subrounded to euhedral).

The Eppawala marbles are characterised by a low Si-content (maximum 1.15%), high P (up to 21.73%) and significant amounts of selected incompatible trace elements (e.g., Sr: 2700 - 6819 ppm; Ce: 124 - 1026 ppm; Nd: 33 - 493 ppm; La: 51 - 574 ppm) as compared to the other (metasedimentary) marbles that occur in the larger Eppawala area. Although they are impoverished in most trace elements as compared to typical carbonatites, the general distribution pattern of the broader element spectrum strongly suggests that the carbonates are genetically related to a primary magmatic source. Enrichment of light Rare Earth Elements (LREE) with respect to heavy REE (HREE) in these rocks is pronounced, and La/Yb ratios vary between 14 and 43. As for single minerals, both apatite and calcite show markedly elevated Sr-levels (~0.6%). The Sr/Mn and Ce/La ratios in the apatite are ~40 and ~2 respectively. These chemical data are decidedly consistent with a carbonatitic derivation of the apatite crystals. Generally, the main REE bearing minerals of the paragenesis are apatite, monazite and baddeleyite. Observed ranges of ¹³C_PDB (-2.2 to -3.4‰) and ¹⁸O_SMOW (7.7 - 16.4‰) also differ distinctly from the values recorded for metasedimentary marbles from the surrounding terrain.

In sum, petrographic observations and geochemical data indicate that the apatite bearing marbles are related to carbonatitic source. Based on their mineralogy, the rocks can be classified as apatite-calcite carbonatites. Since most of the primary igneous minerals are still preserved basically unchanged, it can be inferred that the carbonatitic magma had intruded into the high-grade terrain after the peak metamorphism.

O08 : 5A/04 : H2

Highly Metamorphosed Stromatolithes? ­ A Case Study on a High Grade Impure Marble of NW-Arizona

Tobias Moerz (tmoerz@geomar.de)¹,

Peter Michael Sachs (psachs@geomar.de)² &

Martina B. Moerz (tmoerz@geomar.de)¹

¹ Kiebitzredder 2c, 24235 Laboe, Germany

² Wischhofstr. 1-3, 24148 Kiel, Germany

The Cerbat Mountains in NW-Arizona bear a rich and diverse assemblage of Proterozoic high temperature and low pressure metamorphic rocks. The last metamorphic maximum reached temperatures of more than 700°C and pressures of approximately 3.5 kbar and was associated with intense tectonism. The metamorphic series can be divided into an older predominantly supracrustal and a younger intrusive granodioritic to granitic rock suite. The supracrustal suite has a minimum age of 1.73 Ga. (Duebendorfer & Chamberlain, 1994) and is dominated by E-MORB-type amphibolites with relic pillow structures in association with garnet-biotite and quartz-cordierte gneisses.

Olivine-diopside marbles are volumetrically one of the smallest rock units. Nevertheless their distinct occurrence as thin (0,3 m to 3 m) layers "on top" of the amphibolites and as extra-pillow cavity fillings combined with their characteristic internal banding may yield important evidence on the formation and environmental setting of the difficult to interpret supracrustal unit.

A detailed petrographic study supported by intense electron microprobe analyses was conducted on a sample of the banded impure marbles. All major mineral phases have been evaluated regarding their composition and their relative time of formation. Special emphasis was given to further constrain the last PT maximum conditions. These conditions are used to establish minimum requirements for a possible protolith of the rock unit. Finally this information combined with hole rock RFA analyses and petrography of non metamorphosed stromatolithes leads to a possible metamorphic and structural formation mechanism of this rock unit.

Preliminary results of our study support a possible stromatolithic protolith for the banded oivine-diopside marble of the Cerbat Mountains, NW-Arizona.

Duebendorfer EM & Chamberlain K, AwP, GSA Meeting, 12, (1994).

O08 : 5A/05 : H2

Experimentally Determined K-Rb-Cs Exchange Coefficients between Phlogopites, Richterites and Aqueous Chloride Solutions at 200 and 1800 MPa: The Role of Amphiboles and Micas in Controlling the K-Rb-Cs Budget of Rocks and Fluids

Stefan Melzer (smelzer@gfz-potsdam.de)

GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

The large-ion-lithophile (LIL) elements K, Rb and Cs have been suggested as important geochemical tracers. Relative element ratios of the alkalis may serve as indicators for fluid-rock ratios in metamorphic and metasomatic processes. In these processes, OH-bearing minerals, such as the amphiboles or micas, act as source or sink for free water. Because Rb and Cs may replace K in these minerals, they may significantly fractionate these elements between fluid and rock. For an exact quantitative understanding of K-Rb-Cs variations, it is a prerequisite to determine the partitioning of these elements between amphiboles, micas and aqueous solutions. Experimental results, however, for pressures up to 200 MPa are rare (Beswick, 1973; Volfinger, 1976; Melzer et al. 1998) and absent for higher pressures.Therefore, the distribution of Rb-K between phlogopites and 2 molal (K-Rb) aqueous chloride solutions has been investigated at 200 MPa and 800°C and at 1800 MPa and 800°C. Derived Rb-K exchange coefficients Kd for Phl-Fluid of 1.6 at 200 MPa and about 3 at 1800 MPa shows that Rb strongly fractionates into Phl. Initial experiments for K and Cs at 200 MPa give evidence that Cs preferentially partitions into the fluid. The Rb-K exchange coefficient for 200 MPa and 800°C differ from those reported by Beswick (1973) and Volfinger (1976), which are 1.28 and 1.89, respectively. Additionally, the Rb-K distribution between richterites and aqueous chloride solution was examined at 800°C and 1800 MPa and a Kd of 0.19 was evaluated. This value is slightly higher, than the Rb-K Kd (richterite-fluid) obtained at 800°C and 200 MPa. The results help to explain the alkali budget of fluids during dehydration and hydration processes. The formation or breakdown of either amphibole or mica during metamorphic processes will alter the K-Rb-Cs ratios of rocks and fluids. A pyroxene-bearing dyke behaves as a chromatographic column during the hydration of pyroxene to amphibole . The development of a K/Rb exchange front will mainly depend on the amount of fluid and is controlled by the K-Rb fractionation of the amphiboles. First fluids produced during subduction processes due to the dehydration of micas may show relatively high K/Rb-ratios according to the derived exchange coefficient >3 at higher pressures. During the ongoing dehydration process the K/Rb-ratio of fluid will then continuously decrease until phlogopite is completely used up, if we assume an "open system" behavior.

Beswick AE, Geochim. Cosmochim. Acta, 37, 183-208, (1973).

Melzer S, Gottschalk M & Heinrich W, Contr. Mineral Petrol, in press

Volfinger M, Geochim. Cosmochim. Acta, 40, 267-282, (1976).

O08 : 5A/06 : H2

Grain Sizes of Minerals from Igneous and Meta-Morphic Rocks ­ A View into the Microscope

Ludwig Biermanns

Inst. Geol. Paläontol. Univ. Tübingen, Sigwart-straße 10, 72076 Tübingen, Germany; E.U.

Microstructural studies on igneous and metamorphic rocks are common for both the basic research and the applied geology. For investigations of these rocks, also studies on grain sizes with the microscope are frequently of major significance. Up to now, however, no common grain-size classification has been defined for these rock groups which could be applied to the research under the standard petrological T.L. microscope. The grain-size attribution used in the field, however, cannot be applied to investigations with a standard microscope. From a grain size of < 1 mm, identification in the field begins to be very difficult. From this grain size, studies with the petrological microscope is indispensable. A grain-size classification should be defined which is suitable for investigations of igneous and metamorphic rocks under the microscope with minor total magnification, between ca. 30 and 150 x, because under major total magnification, it is hardly, or not possible at all to get a general impression on the grain sizes of a rock in the thin section.

Proposed grain-size classification for igneous and metamorphic rocks under the petrological microscope.

Interval of grain-size diameter Classification

> 3200 µm very coarse-grained

800 - 3200 µm coarse-grained

200 - 800 µm medium-grained

50 - 200 µm fine-grained

12.5 - 50 µm very fine-grained

<12.5 µm extremely fine-grained

glassy (hyaline) no mineral grains

A table on grain-size delimitations from > 3 200 µm (very coarse-grained) to < 12.5 µm (extremely fine-grained) which is subdivided into several grain-size classes (tab. 1), seems to be most suitable. For more detailed mineral investigations, in general, grain sizes of > 50 µm are necessary. With decreasing grain sizes, it gets increasingly difficult to determine a mineral. From < 10 µm, it is hardly, or even not possible at all to determine an unknown mineral grain, unless identification is possible by means of absolutely typical features, like crystal habit, twinning, refractive index, opake behavior, mineral or interference colours.

Session O08:5P

O08 : 5P/01 : PO

Rotliegendes Metavolcanic Rocks (NW Poland): Evidence for Very Low Grade Metamorphism

Elzbieta Dubinska (dubinska@geo.uw.edu.pl)¹,

Boguslaw Baginski (bobag@geo.uw.edu.pl)¹,

Grzegorz Kapron (gkapron@geo.uw.edu.pl)¹,

Pawel Bylina (bylina@twarda.pan.edu.pl)²,

Andrzej Kozlowski¹ &

Cyprian Kulicki (kulicki@twarda.pan.pl)³

¹ Institute of Geochemistry, Mineralogy and Petrography, Faculty of Geology, Warsaw University, al. Zwirki i Wigury 93, 02-089 Warsaw, Poland

² Institute Of Geological Sciences, Polish Academy of Sciences, ul. Twarda 51/55, 00-818 Warsaw, Poland

³ Institute of Paleobiology, Polish Academy of Sciences, ul. Twarda 51/55, 00-818 Warsaw, Poland

Rotliegendes metavolcanic rocks in the Gorzow Wielkopolski region (Fore Sudetic Monocline) overlie Paleozoic basement and are covered by Zechstein evaporitic sequence as well as Mesozoic sequence. The metavolcanic basic rock typically no relict minerals; scarce relict clinopyroxene, ilmenite, and Cr-spinel were found only in vesicle poor varietes. The metamorphic assemblage includes abundant pumpellyite, laumontite, corrensite-type mineral, quartz and chalcedony, albite, calcite, and minor anhydrite, potassium feldspar, titanite, hematite, pyrite, as well as accesory chalcopyrite and prehnite. This assemblage occurs in bulk rock, amygdales, veinlets, and phenocryst pseudomorphic replacements. The phenocrysts of olivine and pyroxene did not survive alteration and were completely obliterated by corrensite and pumpellyite. Laths of magmatic plagioclase are totally replaced by albite+pumpellyite±K-feldspar (adularia) assemblage. The groundmass contain irregularly distributed corrensite, pumpellyite, laumontite, chalcedony, quartz, calcite, hematite, and titanite; despite intense alteration the groundmass still shows ophitic texture. Hydraulic fracturing commonly dismembered metavolcanic rocks into fragments of highly altered fragments cemented by younger anhydrite, chalcedony, quartz, anfd calcite, producing breccia-type species. Moreover, the amygdales and veilets typically contain abundant albertite-type bitumens concomittant wit Al-pumpellyite. The pumpellyite is often imhomogeneous; e.g. fan-shaped aggregates are zoned with Fe-rich base and Al-rich apex. Different types of pumpellyite include zig-zag aggregates at the corrensite background formed at the expense of mafic mineral phenocrysts. Clay minerals were identified by means of X-ray diffraction; their distribution and charge were estimated on the basis of SEM observation combined with microanalytical measurements in situ using technique proposed by Schiffman and Southard (1996). Detailed petrographic study combined with extensive XRD, SEM, and microprobe analysis revealed typical mineral sequence:Fe-pumpellyite + laumontite + Mg-rich clay minerals albite + anhydrite * Al-pumpellyite + Fe-rich clay minerals. There are several episodes of SiO₂ group and calcite formation. Homogenization temperatures of fluid inclusions in quartz, anhydrite, and calcite were diversified and they varied horizontally in the studied rock sequence; typical temperatures of quatz inclusion homogenization fall in the range 160-170°C, wheraes anhydrite bears inclusions that regularly homogenized in 140-160°C. The VLG metamorphism of Rotliegended volcanic rocks can be attributed to penetration of sea water through an evaporite sequence; however, albertite seems to be a product upwelling fluids released during metamorphism of sedementary rock series underlying extrusive Permian unit. The source of metamorphic heat is ambiguous; it can be tentatively assigned to the exothermal reactions of magmatic mineral alteration processes and/or Mesozoic upwelling of continental mantle diapir.

Schiffman P & Southard RJ, Clays and Clay Minerals, 44, 624-634, (1996).

O08 : 5P/02 : PO

Mineral Growth During Uplift of High- and Ultrahigh-Pressure Eclogites from the Tauern, Austria and from the Dabie Shan, China

Alexander Proyer (alexander.proyer@sbg.ac.at) &

Edgar Dachs (edgar.dachs@sbg.ac.at)

Department of Mineralogy, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria

Ultrahigh-pressure eclogites containing coesite-pseudomorphs from the Shima area, Dabie Shan, China, record peak metamorphic conditions of 31 ± 3 kbar and 800-900°C. While no relics of prograde metamorphism are preserved, several stages of mineral growth during uplift are recorded in the cores of eclogite lenses embedded in an amphibole-bearing biotite-epidote gneiss. The peak metamorphic assemblage consists of garnet + omphacite + coesite + phengite + talc + kyanite + rutile. Barroisitic amphibole is the first product of pressure decrease and/or temperature increase, growing in a dehydration reaction that involves mainly talc and omphacite. Talc inclusions in barroisite decompose to anthophyllite + quartz at conditions of 22-29 kbar and around 800°C, thus constraining barroisite formation to relatively high pressures.Paragonite is formed by subsequent minor hydration from kyanite + omphacite at around 19 ± 2 kbar and 680-760°C. Coarse-grained poikiloblastic epidote and mostly barroisitic amphibole are frequently found also in other UHP-eclogites of the Dabie mountains as post-peak phases overgrowing the UHP-mineral assemblage. The reactions leading to the terminal breakdown of four important eclogite facies minerals (paragonite, epidote, amphibole and talc) and - apart from talc - to their reintroduction during decompressional rehydration, can be used to subdivide the large PT-field of high- and ultrahigh-pressure metamorphism into sensible subfacies fields.

Glaucophane and paragonite bearing eclogites from the Glockner Region (Hohe Tauern, Eastern Alps) were not subducted deep enough to transform to talc- and/or kyanite-eclogites like the ones from Dabie Shan. They were uplifted and tectonically emplaced into metabasites and calcareous micaschists of the Bündnerschiefer series, suffering severe retrograde hydration under amphibolite facies conditions during this last metamorphic event. In some cases an earlier hydration has produced a patchwork of coarse grained amphibole (tremolitic hornblende or barroisite), consuming mainly glaucophane and epidote. Further continuous reaction in some samples becomes apparent from the zoning trends of these new amphiboles and uses up most if not all of the omphacite. Transition into the amphibolite facies is signalled by the production of plagioclase and yet another type of amphibole from any remaining omphacite, paragonite and the coarse grained amphibole variety, resulting in a relatively coarse grained type of symplectitic plagioclase-amphibole-intergrowth. Only the final stage of metamorphism can be considered coeval with the present country rocks. It becomes evident as differing degrees of hydration, ranging from an a(H₂O)-buffering assemblage/reaction of: garnet + amphibole + paragonite + H₂O = epidote + plagioclase + chlorite + quartz to various types of schists where one or more of the phases garnet, amphibole or paragonite are missing.

O08 : 5P/03 : PO

Al-Amphibole Growth in Retrograde Eclogites: Microtextural, Compositional and Crystalchemical Constraints

B. Messiga (messiga@crystal.unipv.it),

R. Grassi (grassi@crystal.unipv.it) &

L. Ungaretti (ungaretti@crystal.unipv.it)

Dipartimento di Sc. della Terra, v. Ferrata 1, 27100 Pavia

Microtextural and microprobe analyses carried out on amphiboles developed as reaction rims on garnets, during early stages of the retrogression of mafic eclogites, demonstrate that amphiboles have always analogous compositions unresponsive of the type of eclogite-facies terrains.

Mineral chemistry data on amphiboles are presented and compared in order to set some of the petrologic and crystal chemical constraints controlling the appearance and composition of amphiboles.

Amphibole zoning is characterised by strong depletion of Al₂O₃ and FeO with a concomitant gain of SiO₂, MgO, and Na₂O going from the garnet boundary towards omphacite. In terms of major elements garnet-omphacite coronas show a strong asymmetric zoning. Moving from garnet to pyroxene Si, Ca and Na increase while Al and Fe decrease.

The major oxides plotted against Al₂O₃ reveal that amphiboles grow at the expense of garnet more than omphacite. When facing garnet, the amphibole shows a composition very close to it. A possible explanation should be related to a strong disequilibrium reaction occurring between garnet and omphacite, during the retrograde path of the host eclogite.

Growth process of amphibole may be explained by "steady-state" diffusion reaction induced by chemical potential gradients.

O08 : 5P/04 : PO

The Collisional Regional Metamorphism in the Maures and Tanneron (South of France) Area. A Critical Review

François Buscail (buscail@dstu.univ-montp2.fr) &

André Leyreloup

(leyreloup@dstu.univ-montp2.fr)

UM2 Place E. Bataillon, UMR5567, ISTEEM, c/c 058, 34095 Montpellier, France

The collisional regional metamorphism of the Maures-Tanneron area has been studied from more 2000 greso-pelitic rocks, georeferenced in the petro-structural database (MAUTA, Buscail, 1997). Critical parageneses, phase relationships and mineral reactions at T max have been analysed in the KFMASH model system, reduced to the AFM projection by quartz, water, muscovite and plagioclase, assumed of constant composition. From W to E, the Maures metamorphism cannot be considered, yet, as continuous and the isograds pattern is not related to a simple normal progressive metamorphic episode from low to high grade. The successive isometamorphic zones encountered in the occidental and central Maures are ZCld-Chl, ZGrt-Chl, ZSt-Bt (+ and Seyler, 1977), ZKy-Bt, ZAnd-Bt, ZSil-Bt, ZSil-Bt-Crd anatectic, ZAnd-Bt and ZKy-Bt under the carboniferous Plan de La Tour (PDT) basin, close to the sinistral PDT wrench fault. In the Oriental Maures and Tanneron, on the other side of the PDT fault, anatexis is widespread. Everywhere, anatexis is reached in the muscovite stability field and granulitic Sil-Kfs parageneses lack. As the andalousite zone is clearly under the Ky-Bt zone so, a part of this cross section is obviously inverted (foliation and/or isograds). Futhermore, there is a marked T discontinity between the Kyanite zone and the Andalusite zone (Holland et Powell, 1991). The metamorphic gradient is discontinuous (intermediate pressure type until the Ky-Bt zone of the occidental Maures and low pressure type in the central and easten part of the orogen). The succession of the metamorphic zones presented above shows that the Andalusite/Kyanite isoreaction-grade is folded or refolded as a synform (with a core of Ky-Bt zone) and then, close to the stephanian basin, as a thermal antiformal dome with a core of anatectic and slightly porphyric leucogranitic rocks (Argens-Hermitan axial zone at 338 Ma). This complex paleo-thermal metamorphic pattern may be interpreted as an syntectonic inverted isograd giga-fold with allochtonous anatexis zone, quite similar to those describe in the Zanskar area (Searle et al., 1989) synchronous with the former westwards nappe emplacement (iron metamorphism in the parautochtonous terrains), but here, refolded eastwards during the back-thrusting episode (Bard and Caruba, 1981). The low pressure anatexis dome may be also interpreted as linked to the latte exhumation process and in this case, the anatexis may be younger than in the former model. In this latter case the two parts of the metamorphic belt could be separed by a normal detachement fault, top to the W-NW. We have statigraphic (Bronner, personnal comm.), and tectonic evidences (Arthaud et Matte, 1966) to show that the occidental Maures series have been tectonically inverted. The actual isograd observed pattern must be related, at least, to a polyphased or to a polymetamorphic event. Related to the nappe emplacement to the west followed by east-back-thrusting and exhumation of the central block of the belt. The P-T intermediate conditions of this collisional regional metamorphism have thus changed during the emplacement of submeridian thermal anomalies. The emplacement in relation with successive syn- to post-deformational injections of granitic magmas at different moments of the history of this belt since 345 Ma (Bormes les Mimosas syntectonic granite) to 324 Ma (Plan de La Tour post-tectonic granite). These intrusions are synchronous with the eastward back-thrusting and the syn- to tardi-tectonic exhumation.

Arthaud F & Matte P, C. R. Acad. Sci. Paris, 262, 436-439, (1966).

Bard JP & Caruba C, C. R. Acad. Sci. Paris, 294, 103-106, (1981).

Searle MP & Rex AJ, J. metamorphic geol, 7, 127-134, (1989).

O08 : 5P/05 : PO

Alpine P-T and Fluid Evolution of Micaschists from the Sopron Area (E-Alps, W-Hungary)

Kálmán Török (torokklm@ludens.elte.hu)

Dept. of Geophysics, Eötvös University, Budapest, Ludovika tér 2, Hungary

The Sopron area is built up by medium grade micaschists and orthogneisses which belong to the Grobgneis series of the Lower Austroalpine Unit. The micaschists suffered pre-Alpine medium grade, low-to medium pressure metamorphism which is shown by andalusite-sillimanite-biotite schist occurring in a very restricted area. Alpine metamorphism produced a different mineral assemblage with quartz, muscovite, biotite, garnet, kyanite, staurolite, chloritoid, chlorite, plagioclase in the first type of micaschists and quartz, muscovite, chlorite±biotite±garnet±plagioclase in the second type of micaschists. These schists and the related gneisses suffered high pressure Alpine metamorphism with several fluid migration events from the peak metamorphic fluids to the late retrograde fluids. The pressure peak may have been at 1100-1250 MPa at about 550-600^oC. The temperature peak of the metamorphism occurred at 575-630^oC at about 900-1200 MPa. The first observable fluid event may have occurred between the pressure and the temperature peak. This was a strong Mg-metasomatism along shear zones, which transformed the micaschists and gneisses into kyanite-bearing Mg-chlorite-muscovite-quartz schists in the close vicinity of the shear zone and caused Mg enrichment in garnet, biotite and muscovite and dissolution of plagioclase further away. Another less intense fluid movement caused precipitation of quartz-tourmaline veins. Geothermo-barometry on garnets, intergrown with tourmaline gives 560-610^oC temperature and 950-1230 MPa pressure, which shows that this fluid migration event may have occurred also between the pressure and temperature peak. Retrograde metamorphic conditions are shown by the second generation of garnet, biotite, plagioclase and muscovite (730-860 MPa and 570-610^oC). Late CO₂-bearing hypersaline fluids in quartz veins caused retrograde metamorphic mineral reactions and a small scale REE-phosphate and Th mineralization. Fluid inclusion data show, that the minimum forming conditions were at about 400 MPa and 400^oC. Pervasive migration of this fluid may have been caused the different mineralogy in the second type of micaschists. This work was supported by OTKA (National Scientific and Research Fund) grant, number: F0158860 and OTKA postdoc grant, number D23918

O08 : 5P/06 : PO

Static Low Pressure Thermal Metamorphism in the Chiavenna Unit (Central Alps)

Caterina Talerico (caterina@erdw.ethz.ch)¹,

Volkmar Trommsdorff (vtro@erdw.ethz.ch)¹,

Igor M. Villa (igor@mpi.unibe.ch)² &

Joerg Hermann (joerg.hermann@anu.edu.au)³

¹ IMP ETH-Zentrum, Sonneggstrasse 5, 8092 Zuerich, Switzerland

² Institut Isotopengeologie, Erlachstrasse 9a, 3012 Bern, Switzerland

³ Research S. Earth S., Australian National University, Mills Road Acton, Canberra ACT 0200 Australia

The ophiolitic Chiavenna Unit is exposed between the middle Penninic Tambo nappe to the north and the migmatitic Gruf Unit to the south. It comprises metaultramafic rocks, amphibolites with tholeiitic affinity and metacarbonates. In the Chiavenna Unit two Alpine metamorphic events can be distinguished: First, a synkinematic, greenschist facies to amphibolite facies metamorphism in the north and secondly, a postkinematic low pressure thermal overprint. The latter starts at 2 km distance from the Gruf migmatites and gradually increases up to pyroxene amphibolite facies conditions towards the contact with the Gruf migmatites. Isograds of this thermal metamorphism (Schmutz, 1976) run approximately E-W and are parallel to the contact between the Chiavenna Unit and the Gruf migmatite. The overprint relationships are microscopically visible from the epidote-out isograd on in mafic rocks and from the antigorite-out isograd on in ultramafic rocks. This corresponds to temperatures of 500-520°C at 0.2 GPa. At the contact between the Chiavenna and Gruf Units maximum temperatures of about 650°C are recorded by the paragenesis Di+Grs+Wo+Qtz+Cal+An in the metacarbonates. In the, approximately isochemical, mafic rocks a continuous increase in temperature is documented by a systematic increase of the edenite component in amphibole at a constantly low Na (M4) content. This indicates pressure conditions even lower than for the eastern Bregaglia contact metamorphism. In the ultramafic rocks the prograde character of this metamorphism is documented by zoned spinels with Al/Al+Cr=0.71-0.78 in their cores and Al/Al+Cr=0.85-0.89 in their rims.The thermal overprint of the Chiavenna Unit is interpreted as due to the emplacement of the hot migmatitic Gruf Unit. ³⁹Ar/⁴⁰Ar dating of amphiboles in the Chiavenna Unit gives two different ages. Hornblendes in amphibolite rocks 2 km north of the Gruf contact yield ages of 45 Ma, whereas edenitic hornblendes in amphibolites close to the contact give ages around 31-32 Ma. The edenitic composition suggests that the younger ages represent a new crystallization event and thus record the emplacement of the Gruf Unit. This appears to be almost contemporaneous to the intrusion of the Bregaglia tonalite.

Schmutz H, Beitr. Geol. Karte Schweiz, 149, (1976).

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Eclogitic Residues from beneath the Sierra Nevada Batholith; Similarities between the Extraction of Cordilleran and Archean Granitoids

Mihai Ducea (ducea@gps.caltech.edu),

Jason Saleeby &

Hugh Taylor Jr.

Caltech, MS 100-23, Pasadena, CA 91125, USA

Garnet pyroxenites are the most common deep-crustal and upper-mantle xenolith assemblages found in Miocene volcanic rocks erupted through the central part of the Cretaceous Sierra Nevada batholith (SNB) (Ducea and Saleeby, 1996). We propose that the SNB granitoids and the pyroxenite xenoliths are respectively the melt and unmelted residues/cumulates from partial melting of a high-Mg basaltic protolith at depths exceeding 35-40 km. This protolith included 20-30% of material with a pre-existing near-surface hydrothermal and/or weathering history ¹⁸O =6.5 to 9 in both xenoliths and SNB). We will review many chemical similarities between the SNB granitoids and Archean TTGs, and between the Sierra pyroxenites and eclogitic xenoliths in kimberlites. Modelling shows that if the end-member with elevated ¹⁸O is removed, the major and trace element compositions of silicic magmas from modern convergent arcs would be virtually indistinguishable from the Achean TTGs. We suggest that the central Sierra Nevada and other segments of the Cordilleran batholiths have originated by petrologic processes similar to those which led to the extraction of the voluminous granitoids that make up the cratonic nucleii of the continents.

Ducea MN, Saleeby JB, J. Geophys. Res., 101, 8229-8244, (1996).

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Partial Melting and Rheology of Pelite/Quartzite Interlayers in the Little Cottonwood Stock Contact Aureole, Salt Lake City

Lukas P. Baumgartner

(lukas@mail.uni-mainz.de)¹,

Tatyana M. Pak²,

Benita Putlitz³ &

Thomas Wenzel¹

¹ Inst. Geowissenschaften, J. Gutenberg-Universiät, D-55099 Mainz, Germany

² Dept. Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA

³ Inst. of Earth Sciences, Hebrew University, Jerusalem, 91904, Israel

The Tertiary quartz-monzonitic Little Cottonwood Stock (LC) intruded the Proterozoic Big Cottonwood formation, which consists of interlayers of quartzites and pelites (centimeter to decameter range). Basin and Range faulting resulted in 5 km to 11 km exhumation and a rotation of approx. 20 degrees (Kowallis et al., 1990; John, 1989). This, together with extensive topography yields exposures of the upper part of the aureole in the East, while deeper parts are exposed in the West. The study focused on the well exposed western, northern, and eastern contact aureole.

The difference in exposure level is mirrored by the metamorphic sequence observed in the pelites. The mineral zones mapped in the East, in order of increasing metamorphic grade, are: biotite(bi) zone, andalusite(and) or cordierite(cd) zone, cd+and zone. Towards the West metamorphic grade increases at the host rock-monzonite contact. Starting with the assemblage muscovite(mu)+quartz(qtz)+and+cd+bi in the East one encounters the mu+qtz * and+k-feldspar (kfs) reaction at the contact, followed by the appearance of sillimanite(sil)+ kfs towards the West. Migmatites occur within the sil+kfs zone in the West.

Little deformation is observed in host rocks in the East and North, while intense flattening and shearing is obvious within the migmatites at the western contact. There the quartzitic layers flowed into and around schollens of pelites. Small pockets of pegmatite like material formed preferentially in pressure shadows (e.g. boudin necks). In lower grade migmatites, distant from the contact, pelitic layers are surrounded by centimeter scale zones consisting of millimeter sized, round quartz grains. They are surrounded by alkali feldspar on grain boundaries. The amount of feldspar increases towards the individual pelite layer. These zones increase in size towards the intrusive contact, until most quartzites show this texture. These observations suggest that melt, derived from partial melting of pelites and/or LC magma, pervaisivly infiltrated the quartzite. Several mechanisms seem possible, including tectonic driven infiltration during deformation of the migmatites and/or capillary effects due to wetting behaviour differences between melt/quartzite and melt/pelite. Apparently, small amounts (<10%) of granitic melt on grain boundaries reduce the viscosity of the quartzites enough to obtain a crystal mush like viscosities.

John DA, Econ Geol, 84, 386-409, (1989).

Kowallis BJ, Ferguson J & Jorgensen GJ, Nuclear tracks and radiation measurements, 17, 325-329, (1990).

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Low-Grade Metamorphism and Hydrothermal Alteration of the Keweenawan Portage Lake Volcanics, Michigan

Ulrich Pueschner (pueschner@ubaclu.unibas.ch) &

Susanne Th. Schmidt (schmidts@ubaclu.unibas.ch)

Mineralogisch-Petrographisches, Insitut Bernoullistrasse 30, CH-4056 Basel, Switzerland

The Keweenawan (Precambrian) Portage Lake Volcanics on the Keweenaw Peninsula, Michigan host the famous native copper deposits with the Locus Typicus of the low-grade metamorphic mineral pumpellyite. The ca. 3000 m thick sequence of mainly basaltic lava flows experienced a complex hydrothermal alteration-low-grade metamorphic history. Due to the mining activities in the thirties and fifties 600 000 feet of well documented diamond drill core are still accessible. The sequence dips 30^o to the South-East which allows to study a vertical cross section through the whole stratigraphic sequence as well as to look for lateral mineralogical variations along SW-NE strike on a distance over 60 km. Interlayered sediments can be used as marker horizons to correlate the stratigraphic zones in the various drill sites.

The lava flows which are amygdaloidal at the top and massive in the flow interior have been studied for their secondary mineral assemblage. The chemical composition of these minerals was analyzed to detect possible changes reflecting metamorphic-hydrothermal event(s). The main alteration minerals are pumpellyite, prehnite, epidote, quartz, calcite, laumontite, datolite and wairakite indicating zeolite and prehnite-pumpellyite facies. The zeolite assemblage is restricted to the stratigraphically highest part, and wairakite occurs in one massive flow interior. In the lower part of the sequence prehnite-pumpellyite assemblages become locally dominant but zeolite assemblages are present throughout the whole vertical profiles. Along strike, a temperature gradient is observed with lower temperature conditions in the East and higher ones to the West.

Based on the first results it is postulated that the Portage Lake Volcanics probably experienced a multiple alteration history. (i) Since wairakite occurs only in the massive flow interior and not in the flow top it is assumed that an early hydrothermal event lead to the deposition of the mineral. (ii) The abundant laumontite-chlorite assemblage in most of the rocks is probably the result of a younger zeolite metamorphism during burial. (iii) A later hydrothermal phase is responsible for the deposition of the pumpellyite-prehnite-epidote assemblage. This event occurs only locally but when present is very dominant.

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The Composition of the Depleted Mantle End-Member of the Cape Verde Volcanics: Is DMM an Important Source Component at the Cape Verde Islands?

Paul Martin Holm (paulmh@geo.geol.ku.dk)¹,

Thomas Find Kokflt (tkokfel@ugcvax.dnet.gwdg.de)² &

Søren Lund Hansen (hs010573@geo.geol.ku.dk)¹

¹ Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 København, Danmark

² Geochemical Institute, University of Goettingen, Goldschmidtstrasse 1, D-37077 Göttingen, Germany

The isotopic variation in Cape Verde volcanics have been explained by mixing of HIMU, EM1 and DMM type components with contributions from in either the lithospheric mantle or an inhomogeneous plume (Gerlach et al., 1988; Kokfelt et al., 1998). In the northern Cape Verde island of Santo Antão (779 km²) the presence of an old relative depleted component is particularly clear. We have reviewed existing data for basaltic rocks from the Mid Atlantic Ridge between 10^oN and 30^oN and discuss them in the light of the three component mixing model proposed for the Cape Verde Island magmas. Our new isotopic and incompatible element data from primitive nephelinitic and basanitic volcanics (MgO > 7 wt%) of Santo Antão demonstrate that the main variation of the Santo Antão volcanics is not related to mixing between HIMU and DMM type sources but rather to non-DMM components. Besides being an isotopic, the 14^oN anomaly of the MAR is also characterised by relatively low K/Nb < 300 and high Rb/Sr > 0.03. This is comparable to the low ²⁰⁶Pb/²⁰⁴Pb end of the Santo Antão compositional field with K/Nb > 200 and Rb/Sr = 0.03-0.06. Models of mantle melting and the elemental and Sr, Nd and Pb isotopic variation of Santo Antão primitive volcanics demonstrate that the source of the 14^oN anomaly at the MAR could probably be the relative depleted end-member component of the island and that mixing with DMM or very depleted MAR basalts would not explain the Santo Antão trend.

The 14^oN geochemical anomaly thus seems to have at least the age of the oldest Santo Antão samples, c. 5 Ma. If the depleted component in the Cape Verde volcanics was derived from the oceanic mantle lithosphere, the anomaly was also present in the Jurassic. If the component is sublithospheric, a widespread occurence of at least 2000 km for this component is indicated

Gerlach DC, Cliff RA, Davies GR, Norry M & Hodgson N, Geochim.Cosmochim.Acta, 52, 2979-2992, (1988).

Kokfelt TF, Holm PM, Hawkesworth CJ & Peate DW, J. Petrol., (submitted).

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Metabasic Rocks from the Eastern Marginal Part of the Madan-Davidkovo Structure, Central Rhodope (Bulgaria)

Stefka Pristavova (stprist@geology.bas.bg) &

Lilan Daieva (ldaieva@geology.bas.bg)

Geological Institute, G.Bonchevstr.Bl.24, Sofia 1113, Bulgaria

The Madan-Davidkovo Structutre (MDS) is situated in the central part of the polymetamorphic Rhodopian complex. The latter is build up of hidh-grade metamorphic rocks (gneisses, shists, amphibolites, marbles etc.), divided stratigraphically in two main supergroups - Prarhodopian and Rhodopian. Magmatic relics of basic rocks occur as several small bodies in the eastern periphery of the MDS within amphibolites of the Vishnevo Formation (Prarhodopian Supergroup). These metabasites are of lens-like boudin shape extended conformably to the foliation of the host metamorphites and of well preserved igneous texture and mineral composition in their central parts. Magmatic minerals and textures gradually decrease and disappear towards their periphery. The magmatic assemblage is represented by Mg-rich olivine, plagioclase (An 76-40), ortho- and clinopyroxene, biotite (high-Ti phlogopite) and spinel. All these minerals with garnet (pyrope type) form typical corona textures. Magmatic minerals, textures and geochemical parameters defining the rocks as gabbro, gabbronorites, norites and troctolites. Magmatic temperature of about 1160-1170°C/7 kbar has been determined. Garnet (probably indicating initial eclogitization), amphibole (hastingsite), plagioclase (andesine), titanite and chlorite replaced the primary mineral assemblage during a subsequent metamorphism at 550-570°C/6 kbar determined in the host amphibolites.

The high MgO (vary of 24.3 to 13.4%), Ni (671-336 ppm) and Cr (1479-212 ppm) abundances at intermediate SiO₂ and low Ti (0.32-0.29%), Zr (28-15 ppm), and HREE are the most striking features of these rocks. Comparison of the MDS metabasites with petrographically and chemically analogous rocks from other regions shows their complicated geochemical character. The content of major, some trace elements and REE of the rocks from MDS, their CaO/Al₂O₃, CaO/TiO₂ and Al₂O₃/TiO₂ ratios, the negative Nb and Ti anomalies in the spidergrams are typical of modern boninitic lavas whereas the Ti/Zr, Ti/Y and Zr/Y ratios are typical of island arc tholeiites (IAT). The geochemical characteristics of the MDS metagabbros define them as transitional between modern boninite rocks and island-arc tholeiites and show their marked affinitiy to primitive island-arc rocks. Several authors have pointed out that analogous transitional compositions are typical of the Early Proterozoic high-magnesian series from a number of Precambrian terrains. The most distinctive features of the Precambrian high-magnesian series are their isotopic characteristics. Further isotopic analyses of the MDS metabasic rocks can determine their probable relation to the Precambrian magmatic activity.

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Metamorphic Evolution of the Early to Medium Proterozoic Granulite Facies Rocks of the Central Segment of the Brasiliano-Panafrican Ribeira Belt, Southeastern Brazil

Beatriz Paschoal Duarte (biapasch@uerj.br) &

Monica Heilbron (heilbron@uerj.br)

Faculdade de Geologia, Universidade do Estado do Rio de Janeiro (UERJ), Brasil

The study area is located within the central segment of the Brasiliano-Panafrican Ribeira Belt which lies along the SSE border of the São Francisco Craton. Three NW-directed, thrust-driven tectonic domains with cratonward vergence were identified (from bottom to top): Andrelândia Tectonic Domain; Juiz de Fora Tectonic Domain; and Paraíba do Sul Tectonic Domain.

The Juiz de Fora Tectonic Domain, aim of the present work, includes tectonically bounded metasedimentary rocks (partially correlated to the Medium Proterozoic Andrelândia Depositional Cycle) overthrusted by Early Proterozoic granulitic orthogneisses of the Juiz de Fora Complex. Deformation was caused by extreme crustal shortening during the Brasiliano-Panafrican Orogeny. The main phase of deformation (D₁ + D₂) is characterised by structural stacking of tectonic domains, tight to isoclinal folding associated to penetrative foliation, shear zones associated to the development of mylonitic rocks and unfrequent mineral lineation. A late phase of deformation is related to folding and bending of the penetrative foliation as well as normal faulting and ductile-brittle shear zones.

Two metamorphic events were identified within the Juiz de Fora Tectonic Domain. The late one (M₁) was associated to the main phase of deformation. This is characterised by the growth of its mineral paragenesis within the penetrative foliation. M₁ has evolved under intermediate pressure conditions, reaching thermal peak during the development of D₂. There are evidence for temperature and/or fluid pressure gradients. For instance, the underlying thrust sheets of the metasedimentary rocks record dehydration reactions, whereas retrogressive hydration reactions along the D₂ foliation and shear zones appear within the overlying thrust stacks of the Juiz de Fora Complex granulites. Hardly did the metapelitic gneisses (s.s.) of this domain develop metamorphic facies diagnostic parageneses. Nevertheless, the relatively more immature pelitic gneisses show M₁ orthopyroxene-bearing mineral parageneses and evidence of the evolution of vapor-absent partial melting process. Field, petrographic and geothermobarometric data show that M₁ evolved under the following conditions: Temperature > 700 - 750°C, pressure between 6 and 7 Kb and gradients of fluid (H₂O) pressure. A "hot iron" model is proposed for the evolution of M₁ metamorphic event.

The early metamorphic event (M₀) is recorded on all of the Juiz de Fora Complex lithotypes, including leucossomes and paleossomes of migmatitic gneisses. Such rocks show a granoblastic mineral fabric and granulite facies parageneses are clearly prior to the Brasiliano-Panafrican-related penetrative foliation. Thermometry data indicate M₀ thermal peak conditions between 800 - 895°C. Chemical composition of granulitic facies amphiboles point to M₀ low to intermediate pressure conditions. Overall, a M₀ passive metamorphic model seems to have resulted from the heat and CO₂ fluids released during an extensional event by underplated basic magma. An anti-clockwise P-T-t path of M₀ metamorphism is proposed on the basis of those observations.

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The New Metamorphic Map of the Alps

Martin Frey (freym@ubaclu.unibas.ch)¹,

Jacqueline Desmons² &

Franz Neubauer (franz.neubauer@sbg.ac.at)³

¹ Mineralogisch-Petrographisches, Institut der Universität Bernoullistrasse, 30, CH-4056 Basel, Switzerland

² 3, rue de Houdemont, F-54500 Vandoeuvre-les-Nancy, France

³ Institut für Geologie und Paléontologie der Universität, A-5020 Salzburg, Austria

After 16 years of compilation, the New Metamorphic Map of the Alps is now completed and will be on sale. More than 50 geologists from Austria, France, Italy and Switzerland contributed to this project. The map consists of an Alpine metamorphic map (scale 1:500,000) and a pre-Alpine metamorphic map (scale 1:1,000,000) of the Alps. The following metamorphic facies were distinguished: "very low-grade metamorphism", greenschist facies, amphibolite facies, granulite facies, blueschist facies (subdivided into Lws+Ab+Chl facies, Gln+Lws+Jd facies, Gln+Ep facies), and eclogite facies. Some isograds, isotherms and isobars are shown as well as the occurrence of selected index minerals.

On the Alpine metamorphic map, metamorphic associations of Cretaceous peak are distinguished from associations of Tertiary peak using different colours. In addition, for basement units the intensity of pre-Alpine metamorphism is shown with overlay signatures to outline the polymetamorphic character of these terrains. Furthermore, information on the distribution of Alpine and Variscan granitoids and Mesozoic ophiolites is given.

On the pre-Alpine metamorphic map, three main periods of metamorphism are distinguished: older than Variscan, Variscan, and Permian. In addition, various age groups of granitoids are separated, i.e., >360 Ma, 360-320 Ma, and 320-260 Ma.

The New Metamorphic Map of the Alps may be purchased for SFr. 30 or an equivalent amount in foreign currencies.

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Retrograde Metamorphism of UHP Garnet Peridotites (La Charme, Vosges, France): Indications for Exhumation and Uplift During the Variscan Orogenesis

Sonja Nicolaisen

(snicolai@classic.min.uni-heidelberg.de) &

Rainer Altherr

(raltherr@classic.min.uni-heidelberg.de)

Mineralogisches Institut, Universität Heidelberg, Germany

Lenses of (ultra)high-pressure peridotites occur in the internal parts of most collision belts worldwide. During the course of continental collision, exhumation of these rocks often results in variable degrees of retrograde overprinting, often in the presence of considerable amounts of fluids. Information on newly formed and often multiply overprinted phases as well as the composition of the involved fluids can help to characterize the processes of exhumation and orogenic evolution.

In this context our objective is to understand how peridotites came to be exhumed and uplifted and why they are so heavily overprinted while their host rocks show less signs of overprinting. We try to reconstruct the way the peridotites went to the surface by examining the retrograde assemblages formed in various stages by interaction with fluids. By this we have the opportunity to reconstruct a detailed metamorphic exhumation path.

The garnet peridotites of La Charme (Vosges mountains, France) equilibrated within the diamond stability field at about 1150°C and 5.8 GPa (Altherr & Kalt 1996). Mineral zoning patterns show that exhumation was rapid and nearly isothermal. Beginning with the transition to the spinel-garnet peridotite stability field and after hot (still about 1000°C) emplacement into the crust, which is made up by gneisses and granulites, several stages of retrograde metamorphism can be traced which are evidenced by multiple secondary fluid-bearing minerals, distributed within the rock or associated in veins pervading the massif. The breakdown of garnet was followed by the infiltration of alkali-rich fluids or melts, resulting in the formation of pargasite, rare phlogopite(I) and apatite in the developing garnet-pseudomorphs. This stage was probably related to emplacement of the peridotite in the crust. Formation of phlogopite(II) ± pargasite in the matrix, can be related to infiltrating alkali- and Ba-rich fluids indicating a change in fluid composition. Low Si-contents and K/Na ratios in the phlogopites suggests formation at medium PT-conditions. The formation of tremolite veins below 800-900°C was related to deformation of the rock unit. This was due to rapid uplift of the rock unit into higher crustal levels. At the same time or shortly afterwards, formation of anthophyllite and talk at 600-700°C and 0.2-0.5 GPa was due to infiltration of a more Si- and Ca-bearing fluid phase (probably derived from the surrounding crustal rocks). The formation of dolomite further indicates a change in fluid composition. At even lower temperatures serpentinization of the peridotite was followed by the formation of serpentine veins ± chlorite indicating migration of Mg- and H₂O-rich fluids. We conclude that tectonic transport of the peridotites from the mantle into higher crustal levels was strongly related to the interaction with fluids emphasizing the importance of studying retrograde phases to constrain the exhumation paths in orogenic belts.

Altherr R, Kalt A, Chem. Geol, 134, 27-47, (1996).

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Extreme Grain Size Development in Graphite-Bearing Calcitic Marbles from the Karibib Formation, Namibia

Christian J. Gross (cgross@gwdg.de),

Klaus Weber (kweber@gwdg.de),

Axel Vollbrecht (avollb@gwdg.de) &

Siegfried Siegesmund (ssieges@gwdg.de)

IGDL, University of Goettingen, Goldschmidtstr. 3, 37077 Goettingen, Germany

The generation of coarsely crystalline rock fabrics leading in some cases to the development of extreme crystal growth (spathization) is a widely occurring phenomenon in metacarbonate rocks from the Damara Orogen, Namibia. High grade graphite-bearing calcitic marbles from the Karibib Formation exhibit grain-coarsened fabrics and the development of large euhedral sparry calcite crystals > 10 cm in size. The grain coarsening only arises in layers poor in calc-silicate minerals. Spathization begins locally in the coarse-grained marbles and spreads laterally outward cutting across ductile Damara structures suggesting a post-deformation origin. Exceptions to this occur in the study area near the Mesozoic reactivated Omaruru-Waterberg fault zone where the extremely coarse-grained marble has been overprinted by mylonitic and cataclastic deformation. The process of grain coarsening begins in a marble exhibiting a granoblastic fabric with equilibrium triple-point grain boundaries. In thin section, the graphite-bearing calcitic marbles exhibit serrated, periodically bulbous interpenetrating grain boundaries. Graphite (3-4 mm) occurs in nests and as thin segregated layers. Petrographic observations indicate that grain boundary migration (GBM) recrystallization plays a dominant role in the formation of these fabrics and the extreme calcite growth. Evidence for GBM in thin section are indicated by microstructures such as bulging, calcite grains moving into neighboring grains and transecting twin boundaries, and left-over grains. Direction of migration is indicated by the absence of substructures (e.g. twins, subgrains) behind the migrating boundary front. In addition, grain nucleation occurs by a secondary process of migration laterally away from coherent twin boundaries. Cathodoluminescence investigations reveal that the calcite grains and sparry calcite crystals exhibit homogeneous red colors. No crystal growth zonations are apparent. The goal of this specific research is the determination of the fabric-forming processes and in particular the mechanisms leading to grain-coarsening and extreme crystal growth in these metacarbonates. This investigation is part of the University of Goettingen Collaborative Research Project SFB 468: Wechselwirkungen an geologischen Grenzflaechen (Interactions on Geological Interfaces).

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Tosudite ­ A Product of Postmetamorphic Fluid-Related Alteration of Low-Grade Metamorphic Rocks (Iòaèovce-Krichevo Unit)

Adrian Biroò (biron@gu.bb.sanet.sk),

Ján Soták (sotak@gu.bb.sanet.sk) &

Ján Spisiak (spisiak@gu.bb.sanet.sk)

Geological Institute, Slovak Academy of Sciences, Severná 5, 974 01 Banská Bystrica, Slovak Republic

Tosudite, a regularly interstratified di-dioctahedral chlorite/dioctahedral smectite, has been identified in low-grade metasedimentary formations of the Iòaèovce-Krichevo Unit (pre-Neogene basement of the Eastern Slovakian Basin). It occurs in quartz-dolomite veins cutting hydrothermally altered metapelitic rocks in association with dickite, illite/smectite mixed-layers, siderite and (locally) pyrite, chalcopyrite, tetrahedrite and hematite. Tosudite forms unusually large (up to 10 µm in diameter) platy crystals arranged in fan-shaped aggregates, which infill the vein cavities. X-ray diffraction data reveal a basal spacing of 2.66 nm in Na-saturated, air-dried state that shifts at 3.10 nm after ethylene-glycol solvation. Mg-saturated and glycerol solvated tosudite forms both single- and two-layer glycerol complexes indicating some heterogeneity in charge distribution. Coefficients of variability calculated for air-dried and ethylene-glycol solvated tosudite are 0.03 and 0.15%. This is indicative of exceptionally well-ordered interstratification (R1) of chlorite and smectite component layers with almost ideal 50:50 ratio. A chemical composition of tosudite (mean of 38 quantitative EDS analyses) corresponds to the structural formula:

(Si_6.88Al_1.12)_8.00O₂₀(OH)₄(Al_5.98Mg_0.27Fe⁺²_0.06)_6.31 (OH)₆Ca_0.06Na_0.28K_0.09.

Assuming that the octahedral sheets in the 2:1 layers are occupied by Al and negative charge originates from the expandable layer, the structural formula for the chlorite layer is:

(Si_3.38Al_0.62)_4.00Al_2.00O₁₀(OH)₂(Al_1.98Mg_0.27Fe⁺²_0.06)_2.31(OH)₆,

and that for the expandable layer is:

(Si_3.50Al_0.50)_4.00Al_2.00O₁₀(OH)₂Ca_0.06Na_0.280.09.

The small amounts of Mg and Fe suggest that the chlorite layer belongs to donbassite rather than sudoite. However, increased amounts of Li in tosudite-bearing clay fractions (in comparison with tosudite-free equivalent samples) might be indicative of composition intermediate between donbassite and cookeite. The structural formula of expandable layer is compatible with a beidellite with a negative charge of -0.50/O₁₀(OH)₂. The textural relationships (i.e. exclusive occurrence in veins) prove that tosudite is not a part of metamorphic assemblages. Moreover, it was found in metasediments of all known stratigraphic formations (Upper Triassic - Middle Eocene) and metamorphic conditions ranging from higher anchizone to biotite-isograde rocks. It is unlikely, that this highly expandable mixed-layer phase could withstand the metamorphic temperatures as high as 400°C. Thus, its origin is most probably related to the post-metamorphic hydrothermal alteration of the Iòaèovce-Krichevo Unit, occurring probably during its uplift. This is evidenced by strong kaolinization of the host rock white-micas, which could released a plenty of material needed for tosudite precipitation.

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Very Low-Grade Metamorphism of Northern New Caledonia

Sébastien Potel (potel@ubaclu.unibas.ch) &

Martin Frey (freym@ubaclu.unibas.ch)

Mineralogisch-Petrographisches, Institüt, Bernoullistrasse, 30, CH-4056 Basel, Switzerland

In northern New Caledonia, two metamorphic sequences are distinguished: (i) A 175 km long and 35 km wide high-pressure schist belt of mid-Tertiary age (e.g. Brothers, 1974; Black, 1977; Black et al., 1993). It exhibits an increasing field gradient from SW to NE, ranging from pumpellyite(Pmp)-actinolite assemblages through blueschists and glaucophanitic greenschists into eclogites. (ii) Further to the SE, in the "Chaîne centrale", a second metamorphic sequence exists with an increasing field gradient from E to W or NNE to SSW and is believed to be of post-Jurassic and pre-Senonian age (Paris, 1981). The metamorphic progression of this apparently older metamorphism consists of a Pmp-prehnite(Prh) zone, followed by a lawsonite(Lws) zone and a glaucophane(Gln) zone (PARIS, 1981). Where the two metamorphic sequences overlap, the Lws and Gln isograds of the "Cretaceous" metamorphism are at right angles to the Tertiary Lws isograd (Paris, 1981). The aim of our project is to elucidate the petrologic significance of this peculiar arrangement of isograds.

A combination of X-ray powder diffraction methods, optical microscopy, and electron microprobe has been used. Our preliminary results are based on illite crystallinity (IC) data of 137 samples and electron microprobe analyses on seven metabasites and five metapelites. Our preliminary results include: (i) The lowest grade part of the Tertiary metamorphic belt shows, from SW to NE, an increase of metamorphic grade from the diagenetic zone (IC > 0.42 ° 2<theta>) to the epizone (IC > 0.25 ° 2<theta>). At the Lws isograd, upper anchizonal conditions (0.25¾IC¾0.30 ° 2<theta>) were reached. (ii) Senonian metapelites as well as other Mesozoic formations in the "Chaîne centrale" area indicate epizonal and upper anchizonal conditions but IC data show no specific regional trend.

For the Tertiary metamorphic sequence, our results confirm the conclusion reached by previous authors, i.e. a regular increase of metamorphic grade from SW to NE. In addition, we have extended the metamorphic zonation to below the Lws isograd. For the "Cretaceous" metamorphic sequence of the "Chaîne centrale", our results indicate that the Senonian formation reached similar metamorphic grade as other adjacent Mesozoic rocks. This contradicts Paris (1981) who regarded the Senonian formation as non-metamorphic and, therefore, concluded that the metamorphic event affecting the "Chaîne centrale" was "Pre-Senonian" (Cretaceous). Consequently, the age of this metamorphic event should be reconsidered.

Brothers RN, Contrib. Mineral. Petrol, 46, 109-127, (1974).

Black PM, Tectonophysics, 43, 89-107, (1977).

Black PM, Maurizot P, Ghent ED & Stout MZ, J. metamorphic Geol, 11, 455-460, (1993).

Paris JP, Bureau de Recherches Geologiques et Minières, Memoire, 113, 240, (1981).

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A Shear-Zone Related, Albite Porphyritic Mafic Breccia in Northern Sweden

Anders Wikström (andwik@sgu.se)¹ &

Björn Öhlander (bjorn.ohlander@sb.luth.se)²

¹ SGU, Box 670, SE 751 28 Uppsala, Sweden

² Div. of Applied Geology, Luleå Univ. of Technol., SE 971 87 Luleå, Sweden

The Baltic-Bothnian megashear (Berthelsen & Marker, 1986) is a major structure displayed above all in aeromagnetic maps of Northern Sweden where it can be followed from the Norwegian border in the north to the Bothnian Bay in the south to the same latitude as the town of Luleå. On available geohysical maps no continuation further to the south towards the Baltic sea can be recognized. See also Wikström et al (1996).

The Siknäs dyke situated within the shear zone is a c. 2 km wide and c.30 km long intrusion, geographically situated between the towns of Luleå and Kalix in northernmost Sweden. The extension, partly occurring under water, is mainly followed on aeromagnetic maps. Most outcrops display a heterogeneous albite porphyritic breccia with albite phenocrysts irregularly distributed both in fragments and matrix. Some albite grains have a sceletal appearance. Chlorite is the most common mafic mineral. On close inspection, the subdivision in fragments and matrix is not clear where the two components sometimes grade into each other and also reverse chemically; at places fragments are more mafic than the matrix, in other cases the opposite condition prevails. As indicated by the mineralogy, the Siknäs dyke is altered which has resulted in large scatter in the concentrations of elements such as Na and K. Both the contents of these elements as well as the Na/K ratio show large variations. In a SiO₂ vs. Zr/TiO₂ diagram, most samples plot in the andesite field but a few plot in the basalt and dacite fields. The SiO₂ content ranges from 50.1 to 64.6 wt.%.

The variation of chemical composition is as large on outcrop scale as it is between outcrops. Although matrix and fragments clearly has a similar composition and show the same total variation, there is generally a distinct difference in composition between a fragment and the immediately surrounding matrix. There are also significant differences in composition between adjacent fragments, and between different matrix samples from the same outcrop. However, the geochemical results show that matrix and fragments stem from the same magma system. It is suggested that the variations observed could be explained by a violent, surface-near magma-mixing/mingling process triggered by tectonic movements in the shear zone. Fragments have mechanically been transported from other parts of the magma system than that represented by the matrix now surrounding them.

Berthelsen A, Marker M, Tectonophysics, 128, 163-181, (1986).

Wikström A, Skiöld T, Öhlander B, Geol. Soc. Spec. Publ., 112, 249-259, (1994).

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The Significance of Zircon and Zirconolite in Metarodingites from South Penninic Ophiolite Remnants

Andy Stucki (stucki@erdw.ethz.ch)¹,

Volkmar Trommsdorff (vtro@erdw.ethz.ch)¹ &

Guenther Detlef (guenther@inorg.chem.ethz.ch)²

¹ Institut fuer Mineralogie und Petrographie, ETH Zentrum, 8092 Zuerich, Switzerland

² Laboratorium fuer Anorganische Chemie, ETH Zuerich, 8092 Zuerich, Switzerland

Zircon, zirconolite and baddeleyite have been found associated with metarodingite dikes in ultramafic lenses of the Zone of Bellinzona, a South Pennine unit of the Central Alps. The ultramafic rocks of this zone represent high grade (650°C; 6 to 7 kbars) metamorphic ophiolite remnants. Zircon was found to be abundant in several Zr-einriched rodingites as clear, euhedral grains up to 2 mm occurring together with fassaitic diopside, Mg-ilmenite and hornblende and in several dikes of pyroxene amphibole rock. Cathodoluminescence shows two generations of zircon growth: a core of fine scaled oscillatory zoning is overgrown by a homogenous rim. Both generations are enriched in HREE relative to MREE and LREE with higher LREE contents in the rim. Zirconolite (ideally CaTi₂ZrO₇) was found as fragmented and partially corroded grains up to 1 mm predating Alpine metamorphism. It is concentrated at the rodingite/blackwall contact associated with diopside, Mg-ilmenite, olivine, chlorite, hornblende and zircon. It has locally been replaced by zircon in presence of CO₂ -bearing fluids. Chondrite-normalized patterns of zirconolite show an enrichment of MREE relative to LREE and HREE. In the rocks studied, zirconolite shows REE-enrichment of generally 10000x to 40000x relative to chondrite values and therefore incorporates most of the available REE. Th- and U-contents are comparably low, not exceeding 0.5 oxide wt%. In terms of crystal chemistry, zirconolite shows extensive replacement of Ca by REE (including Y), occupying up to 60% of the Ca-site. Ferric iron replaces 30% of the Ti-sites. Thus, the coupled substitution REE³⁺Fe³⁺=CaTi accomodates the high amounts of REE. No substitutions have been observed for the Zr site apart from minor Hf.Baddeleyite was found as small (<50 µm) euhedral crystals in the blackwalls between metarodingite and metaperidotite. It occurs together with Mg-ilmenite, olivine, chlorite and zircon. It contains only 75% ZrO₂ and has high contents of TiO₂, SiO₂, Fe₂O₃ and Al₂O₃.The bulk chemistry of the rodingite dikes containing the minerals of interest is that of former, highly differentiated Fe-Ti-gabbros with the igneous texture in places still preserved. Additional metarodingites are Mg-rich, Fe- and Ti-poor rodingites and correspond to former Mg-gabbros. The presence of Zr minerals in the Fe-Ti-gabbros must be attributed to magmatic differentiation or to rodingitization with concomitant blackwall formation.

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Contrasting Fluid Flow Patterns in Scapolite-Bearing Metaturbidites of the Kuiseb Formation, Damara Orogen, Central Namibia

Anja Dombrowski

(kris040@mail.uni-wuerzburg.de)

Mineralogisches Institut, Am Hubland, D-97074 Würzburg, Germany

The Khomas Trough of the southern Damara Orogen of central Namibia comprises metaturbidites of the Upper Proterozoic Kuiseb Formation which underwent amphibolite facies regional metamorphism and a polyphase deformation history. In the northern part of the Khomas Trough two shear zone hosted scapolite-bearing sequences document contrasting mechanisms of fluid flow as can be monitored by the distribution of Cl-bearing minerals like scapolite, biotite, apatite and amphibole. Reconstruction of fluid evolution reveals a complex interaction between devolatilization reactions, permeability enhancement, fluid infiltration and internal buffering of fluid compositions. It can be shown that scapolite formation in the Kuiseb Formation is not due to the infiltration of externally derived Cl-bearing fluid but is incipiently the consequence of a carbonate-bearing, calc-pelitic protolith composition. In the course of prograde metamorphism Cl was concentrated in the fluid phase which permeated the Kuiseb rocks along fracture channelways. In calc-silicate rocks decarbonation reactions gave rise to a reaction-enhanced increase of permeability due to the release of large quantities of CO₂ which channelized fluid flow along these layers. Thus, Cl-bearing fluids derived from the dehydration of adjacent pelites preferentially migrated through these portions of the sequence. In addition, CO₂ release by decarbonation reactions diluted the infiltrating fluid to a value where scapolite became stable.

While in scapolite-bearing samples fluid flow is dominantly channelized in layers with reaction enhanced permeability, fluid flow in metapelites was highly channelized by fracture permeability. Cl-poor and Cl-enriched biotite occurring on a dm-scale indicate that equilibration with Cl-bearing fluids occurred along microfractures while adjacent areas remained unaffected.

It can be shown that in the fluid phase, at least partly, interdependent variations of H₂O, HCl, NaCl and CO₂ activities existed resulting from initial variations in protolith composition and subsequent modification during devolatilization reactions where permeability enhancement played a major role.

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Diffusion and Infiltration of Magmatic Brines into Marbles ­ Mapping Ancient Fluid-Flow and Element-Transport by Neutron-Induced Autoradiography

Rolf L. Romer (romer@gfz-potsdam.de)¹,

Wilhelm Heinrich (whsati@gfz-potsdam.de)¹ &

Carl-Otto Fischer (fischer-co@hmi.de)²

¹ GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

² Hahn-Meitner Institut, Glienicker Strasse 100, 14109 Berlin, Germany

High-salinity magmatic brines percolated at ca 1 kbar and ca 650°C along chert layers and induced decarbonation reactions with the wall-rock marble. Marbles were largely impermeable for these brines. CO₂ generated by the reaction Qtz+Cc = Wo+CO₂ unmixed from the brine and escaped as a CO₂-rich H₂O-CO₂ fluid into the marble. Elements originally solved in the fluid or hosted by the consumed chert and marble calcite are redistributed between the newly formed wollastonite and the two fluid phases. The spatial distribution of these elements that were introduced or redistributed by fluids and that (in part) were originally not contained in the rock sample corresponds directly to the pattern of fluid-flow. Anomalously high contents reflect scavenging processes by certain newly formed minerals. The distribution and relative amount of uranium are mapped using autoradiography, those of Al, Mn, Na, Fe, ZrHf, and LaCeEu are mapped by neutron-induced autoradiography. Spatial resolution mainly depends on the used film material and exposure time. Sensitivity depends, among others, on concentration, activation time, interfering elements, and time of exposure. The spatial distribution pattern is sensitive for some elements even at ppm level. It (1) demonstrates the position of the fluid channels; (2) distinguishes between fluid infiltration and fluid percolation by contrasting patterns. Furthermore, (3) anomalous concentrations of originally dissolved matter at reaction fronts illustrate scavenging processes for these elements; and (4) multiple fluid-percolation becomes recognizable in intersecting infiltration and diffusion patterns. Our data demonstrate that U, Rb, Sr is scavenged in residual meta-argillite layers at the aquifer margin. The contrasting contents of U at the upper and lower aquifer border demonstrate that considerably more fluid has left the aquifer through the upper aquifer border. U is also scavenged along the margins, especially at the upper margin of the consumed impure chert. Redistribution of U by aqueous fluids along late fractures resulted in secondary U-enrichment along the fractures and at the former marble-chert contact. Elements dominantly fractionating into the aqueous brine (e.g., Na, K, Al) are confined to the consumed chert layer. Diffusion haloes of these elements extend less than 5 mm from the reaction front into the marble.

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Transition Elements in Minerals of Garnet Websterite Xenoliths: Information on P-T-Evolution and Relative Element Diffusivities

Bettina Olker

(bolker@classic.min.uni-heidelberg.de) &

Rainer Altherr

(raltherr@classic.min.uni-heidelberg.de)

Mineralogisches Institut, Universität Heidelberg,, Im Neuenheimer Feld 236,, 69120 Heidelberg, Germany

We present preliminary results of a combined electron microprobe / ion microprobe study on mantle xenoliths. Samples investigated were garnet websterite xenoliths from the Quaternary Marsabit shield volcano (Kenya). They contain large garnets (up to 7 mm) with thin (~200 µm) kelyphitic rims and orthopyroxene (opx) and clinopyroxene (cpx) of variable grain size (up to 2 mm). Clinopyroxene often shows exsolution of orthopyroxene and vice versa.

All minerals display chemical zoning. In grt, Ti and X_Mg decrease while Fe and Mn increase from core to rim, with the largest grains having a constant core composition. The outermost rims (~ 200 µm) sometimes show an increase of X_Mg. Cpx is strongly zoned with Ca and X_Mg decreasing and Al and Na increasing from core to rim. In opx, Ca and Al increase and Mg decreases from core to rim. However, a constant core composition is apparent. In both opx and cpx diffusion profiles of Al are broader than those of Ca.

Ionprobe trace and minor element analyses were carried out along the same profiles. Cr, Sc, Ti and V show a strong chemical zoning across grt, with Cr, Sc and Mn increasing and Ti, V decreasing from core to rim. Cr and Sc have constant core compositions, with Sc zoning reaching further into the core than Cr. Co decreases only in the outermost rims, while Ni is more or less constant. This seems to indicate an increase of the diffusion rate from Cr to Sc to Ti/V which is not in acccordance with the change of their ionic radii. In cpx, Cr, V and Mn increase from core to rim, whereas Ni slightly decreases. Co and Sc seem to be constant; only the largest grains show an increase of Sc towards the rim. Ti first increases from core to rim, followed by a decrease in the outermost rims. In cpx, the diffusion rate seems to increase from Cr to V to Sc. Trace element zoning in opx is very minor to absent in grains of comparable size to the examined clinopyroxenes, which might indicate faster diffusion rates of transition elements in opx than in cpx, which is in accordance with the faster diffusion of Ca, Al, Fe and Mg in opx.

Considering the large grain size and slow diffusion rates in grt compared to pyroxenes we ascribe the broader zonations in grt to an older event and the zonations in pyroxene and in the outermost rims of grt to a younger event. According to this, the reconstructed P-T-history starts with a longer cooling period, followed by a relatively short heating event. Decompression started with or during the cooling and continued until the uptake in the host magma.

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Origin of Cr-Diopside - Olivine - Spinel - Glass Clusters in Mantle Xenoliths from Baarley, West Eifel, Germany

Cliff Shaw (cliff.shaw@uni-bayreuth.de)

Bayerisches Geoinstitut, Universitaet Bayreuth, 95440 Bayreuth, Germany

Mantle-derived xenoliths from the Baarley quarry in the Quaternary West Eifel Volcanic field range from anhydrous depleted peridotites to hydrous (metasomatised) peridotites, veined and brecciated peridotites, glimmerites, megacrysts and clinopyroxenites. Previous studies of the West Eifel xenoliths have identified three major events (see Shaw 1997): 1) Partial melting, responsible for depletion of peridotite; 2) an early metasomatic event involving formation of disseminated hydrous phases; 3) a late stage event associated with the early stages of West Eifel volcanism that involved formation of cross-cutting veins of phlogopite, pargasite and clinopyroxene, as well as clinopyroxenites, glimmerites and a range of megacrysts.

None of the previous studies have examined the origin of pargasite and phlogopite breakdown textures and their relationship to the entrainment and transport history of the xenoliths. Pargasite and phlogopite in these xenoliths commonly show evidence of melting and reaction with silicate melt derived from the host lava. Disseminated pargasite in peridotites is commonly broken down leaving clusters of Cr-diopside, olivine, spinel and vesicular glass. In many samples this breakdown is complete however, in a few samples there are relic, partly digested grains of pargasite. Phlogopite in or close to these regions also shows evidence of breakdown in the form of glass and microlite-rich fringes.

Veined and brecciated peridotites, glimmerites and clinopyroxenites also show evidence of pargasite breakdown. In this case the breakdown is limited to grain margins where the pargasite is in contact with infiltrated melt from the host lava. Phlogopite also shows evidence of breakdown where it is in contact with partly melted amphibole and infiltrated melt from the lava.

Temperature estimates using the Ca in opx geothermometer of Brey and Kohler (1990) give temperatures of around 900°C for the hydrous peridotites suggesting that the event which lead to pargasite melting was of too short duration to lead to re-equilibration of the main silicate phases. The distinct difference in the breakdown textures between hydrous peridotites and the veined and brecciated peridotite as well as clinopyroxenite suggests that the textures developed in different ways. Pargasite breakdown in hydrous peridotite was due to decompression melting with only minor reaction with infiltrating melts from the host lava. Veined and brecciated peridotites and clinopyroxenites may be been derived from shallower levels in the mantle and did not have time to undergo decompression melting during transportation. In these samples the main breakdown reaction is due to partial dissolution of pargasite and phlogopite by infiltrating melts from the host lava.

Brey, GP & Koehler T, J. Petrol, 31, 1353-1378, (1990).

Shaw, CSJ, Can. Min, 35, 1453-1463, (1997).

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Moessbauer Spectroscopic Determination of the Ferric-Ferrous Ratio of Interstitial Glass in Synthetic Ferro-Basalts ­ Method and First Results

Georg M. Partzsch

(partzsch@classic.min.uni-heidelberg.de)¹,

Catherine McCammon (catherine.mccammon@uni-bayreuth.de)² &

Dominique Lattard

(dlattard@classic.min.uni-heidelberg.de)¹

¹ Mineralogisches Institut, Universität Heidelberg, D-69120 Heidelberg, Germany

² Bayrisches Geoinstitut, Universitaet Bayreuth, D-95440 Bayreuth, Germany

In natural silicate melts the ratio of ferric to ferrous iron is controlled by pressure, temperature, bulk composition and oxygen fugacity. The relationship between the last three variables has been calibrated at 1 bar for a wide range of melt compositions, under different oxygen fugacities, in the temperature range 1200-1630°C (e.g. Kress & Carmichael, 1991). The resulting equation which correlates the ferric-ferrous ratio of quenched glasses and the oxygen fugacity of the corresponding silicate melts is frequently used to calculate the oxygen fugacity of natural melts or, vice versa, to retrieve the ferric-ferrous ratio of synthetic glasses equilibrated at known oxygen fugacity. It is, however, often used for largely crystallised synthetic samples and natural rocks equlibrated at temperatures well below 1200°C.

To test the validity of the Kress & Carmichael equation for residual melts at temperatures below 1200°C, we are using the Moessbauer milliprobe to investigate the ferric-ferrous ratio of interstitial glasses in run products equilibrated at known temperatures and oxygen fugacities.

The starting materials are synthetic glasses with ferro-basaltic compositions which were molten at 1400°C in air, quenched and ground to fine powders. For each experiment, a small charge of starting material is either pressed onto a loop of thin Pt-wire or into a small AgPd capsule (both pre-treated to minimise iron loss). The experiments are conducted in a gas-mixing furnace at different oxygen fugacities in the range FMQ -2 to FMQ +1 and at subliquidus temperatures between 1180 and 1100°C. The run products are coarse-grained (crystals up to 3 mm in length), with relatively large interstitial glass pods. Depending on temperature and oxygen fugacity, the crystalline phases are plagioclase, olivine, clinopyroxene, ilmenite and magnetite. Microprobe analyses show that most phases are homogeneous, except for some zoning in clinopyroxene and plagioclase.

The ferric-ferrous ratio is measured using the Moessbauer milliprobe, which allows spectra to be collected on spots with diameter of the order 100 microns (McCammon, 1994). Preliminary experiments have demonstrated that spectra can be sucessfully collected from the glassy areas of these samples, and show two broad absorption lines typical for glasses with these compositions. The ferric-ferrous ratio is largely determined by the difference in area between the absorption lines, and is hence relatively independant of the fitting model.

Kress VC & Carmichael ISE, Contrib. Mineral. Petrol., 83, 136-140, (1991).

McCammon CA, Hyperfine Inter., 92, 1235-1239, (1994).

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Calculating Crystalization of Calc-Alkaline Magmas at Dry Conditions

Renat Al'meev (almeev@chat.ru)

Vernadsky Institute, Kosygin St. 19, Moscow, Russia

The development of modern techniques of genetic interpretation of calc-alkaline suites needs a construction of computer models allowing petrologists to correctly calculate phase equilibria in the systems ranging from basalts to dacites.

The aim of the paper is to present results of the calibration of a new phase equilibria model calculating evolution of minerals and melt compositions corresponding to petrochemical trends of calc-alkaline series.

With this purpose we have calibrated a set of low- and high-pressure geothermometers describing mineral-melt equilibria in the systems ranging from high-alumina basalts to dacites. These equations have been derived from the INFOREX database (Ariskin et al., 1997) as result of thermodynamic processing of data from 14 experimental studies conducted at 1 atm pressure and unhydrous conditions (run duration for all of the experiments was more than 48 hours). A whole dataset includes 46 runs representing olivine-melt equilibrium (Ol-L), 54 for plagioclase (Pl-L), 18 for augite (AU-L), 15 for pigeonite (PG-L), and 41 for orthopyroxene (OP-L).

Comparison of the temperatures calculated using the geothermometers with those obtained in experiments indicate a good accuracy of 12°C for OL, 8°C for PL, 7°C for AU, PG, and OP. These geothermometers have been incorporated into the new version COMAGMAT phase equilibria model (Ariskin et al., 1993), what allowed us to calculate crystallization sequences in the melts similar to calc-alkaline magmas. To test the model, we used data of melting experiments on a basaltic andesite and andesite, comparing them with the results of calculations of equilibrium crystallization for the same initial melt composition. These comparisons allow us to conclude that the model developed well predicts both the onset crystallization temperatures and mineral compositions. The calculated liquid lines of descent are also in good agreement with experimental data.

Further development of the new COMAGMAT model will include the effect of H₂O on phase equilibria (Al'meev & Ariskin, 1996).

Ariskin AA., Meshalkin SS., Al'meev RR. et al., Petrology, 5, 28-36, (1997).

Ariskin AA, Frenkel MYa., Barmina GS., Nielsen RL, Computer & Geoscience, 19, 1155-1170, (1993).

Al'meev RR, & Ariskin AA, Geochem. Internat, 7, 624-636, (1996).

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Trace Element Analysis of Granitoid Minerals Using Laser-Ablation-ICP-MS

Antje Gebel (gebel@immr.tu-clausthal.de),

Hagen Stosnach (stosnach@immr.tu-clausthal.de),

Kurt Mengel (mengel@immr.tu-clausthal.de) &

Kai H. Schmidt (schmidt@immr.tu-clausthal.de)

Institut fuer Mineralogie, Adolph-Roemer-Str. 2A, 38678 Clausthal-Zellerfeld, Germany

Whole rock analysis of fresh and hydrothermaly alterated granitic to dioritic rocks from Äspö/Sweden have shown that the concentrations of REE and several other trace elements remain largely constant during alteration processes. Microanalytical methods (Electron-Microprobe, EMP and Laser-Ablation-ICP-MS, LA-ICP-MS) are used, to investigate the distribution of trace elements on a grain size scale. By this approach, it should be possible to calculate massbalances for trace elements in granitic rocks and to get information about the behaviour of selected trace elements during hydrothermal alteration. LA-ICP-MS analyses are performed with an excimer-laser based system working at 248 nm. Typical spot sizes are about 50 µm and give detection limits in the range of 10-100 ppb for most trace elements. At 30-40 ppm, accuracies are about ± 10%. Concentrations analysed in reference materials compare well to certified values. First results from granodioritic rocks show that nearly all bulk rock HREE (incl. Y) and about 80% of the bulk rock LREE are contained in titanite. Also, more than 70% of Ti, 50% of Th and about 40% of U are contained in this mineral which forms 1-2% of the whole rock. In the more granitic rocks, REE are mainly contained in allanite which was formed during the alteration process. Phosphat phase like monazite and xenotime which are known to be the dominating phases for REE in several granites (e.g. Bea 1996) are uncommon in the rocks studied here. Biotite dominates the balance of Rb and Cs in similar way as titanite does in the case of REE. In contrast, the amounts of Ba in biotite are similar to its amount in the whole rock. K-feldspar seems to be the dominating phase for Ba. Beside Rb and Cs, biotite also contains about 50% of the whole Mn and Co. The REE-concentrations in biotite are very low, only La (avg. = 0.1 ppm) and Ce (avg. = 0.2 ppm ) are above the detection limits in most analyses. Because some analyses show concentrations below the detection limits (0.04 ppm La and 0.05 ppm Ce), the higher average concentrations are suggested to be caused by micro inclusions. During the alteration process, biotite reacts to chlorite which seems to have significantly lower concentrations of Rb and Cs.

Bea, F., Journal of Petrology, 37, 521-552

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Interrelationship between Zircon Typology and Trace and REE Element Geochemistry

Giovanna Rizzo (giovanna@dmp.unipd.it)

Dept. Mineralogy and Petrology C.so Garibaldi 37, 35137 Padova, Italy

The relationship between zircon typology and geochemical variations of major, trace and REE elements have been determined on zircon populations from phyllitic complexes Austroalpine and Southalpine (Eastern and Southern Alps). The Zircon typology of 21 rock samples from the Austroalpine and 14 from Southalpine phyllitic complexes was ascertained according to the criteria and classification scheme of Pupin (1980). Zircon crystals from the Southalpine phyllites are generally rounded and display overgrowth, zoning, corrosion and yellow turbidity; some rounded crystals display pitted surfaces, which might be due to metamorphic processes (resorption), or related to metamictisation of U rich overgrown rims. Zircon grains from the Austroalpine phyllites are euhedral, palely coloured or colourless, displaying a typical igneous morphology. In many samples tiny inclusions were observed within the zircon, outlining probably a relic core. The most common zircon typologies are S₇, S₈ and S₉ in both Austroalpine and Southalpine phyllitic complexes. The geochemical variations of major, trace and REE elements within zircons of different typology has been determined by electron microprobe analysis (EMPA) using high acquisition times. The detection limit using counting times of 40 minutes for trace elements and 3 minutes for major elements is as low as some tens of ppm for REE. Over all, 350 spot analyses of Zr, P, Si, Hf, and 177 spot analysis of REE, Sc, Th, U along chemical traverses from rim to core of 32 zircon crystals show an oscillatory zoning in all trace elements. This variation is decoupled with variations in major element contents. Additionally, chemical traverses through the zircon crystals show that particular chemical behaviours can be associated with different zircon typologies: for example, rounded zircons show a systematic and sympathetic increase of Y, Er, Yb, U and Th towards the core, whereas Hf shows the opposite behaviour. Most of zircons with typology S9 show that Sc, Yb, Y, Er, Dy, Sc, Th and P decrease towards the core and Hf shows again the opposite behaviour. These features suggest that zircon typology and trace element patterns are strongly interrelated and may be controlled one by the other.

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Present-Day Paragonite-Smectite Mixed Layer Phyllosilicate from the TAG Active Hydrothermal Mound, Mid-Atlantic Ridge

Benoit Jacquemont (67084) &

José Honnorez

Centre de Géochimie de la Surface, UMR n°7517, CNRS-ULP, 1, rue Blessig,Strasbourg-Cedex, France

The TAG (Trans-Atlantic Geotraverse) active hydrothermal mound located on the mid-Atlantic Ridge axis, at 26°N, is generated by the episodic discharge of high temperature hydrothermal solutions. The basaltic basement under the TAG mound was and still is affected by hydrothermal alteration which varied through time and space: chloritization with minor quartz and pyrite was followed by silicification, pyritization and paragonitization. A paragonitic mica presently forms through the replacement of priorly chloritized basalts during the proximal hydrothermal alteration near the focused discharge of "black smoker'" solutions at 370°C. It also forms by direct precipitation form these solutions in the interstitial voids of the quartz+pyrite mound. The presence of paragonitic mica reflects the high Na/K ratio measured in the hydrothermal fluids. Various X-ray diffraction and transmission techniques, electron micro diffraction, electron microprobe analyses and Newmod modelling of 3 samples from drill core samples collected during ODP Leg 1958 indicate that the TAG mound paragonotic mica is a mixture between a pure paragonite and an ordered irregular paragonite/dioctaedral smectite mixed layer. This type of mixed layer phyllosilicate has never been identified before.

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Reverse Zoned High Ti-Phlogopite Phenocrysts of the Bekkestul Ultramafic Lamprophyre, Fen Province, Southern Norway: Evidence for Volatile Loss and Magma Mixing

Jacob L. Mey (jacob@geo.aau.dk)

Dept. of Earth Sciences, B110, Aarhus University, 8000 Aarhus C., Denmark

A newly discovered 1-2 m wide composite ultramafic lamprophyre dike crosscut the Central Telemark Supracrustals (1600-1200 Ma) at Bekkestul, S. Norway. The dike is probably a small satellite intrusion 50 km NV from the Lower Cambrian alkaline main complex of the Fen province (578 ± 24 Ma). The intrusion contains abundant phlogopite macrocrysts (~25%) and phenocrysts (3-5%), set in a fine-grained groundmass consisting of olivine and pyroxene (now pseudomorphed), Mn-calcite ocelli, atoll-spinels, acicular diopside, biotite, poikilitic sphene and apatite. The studied phlogopite phenocrysts consists of a discontinuous-zoned high-Ti (7-8 wt% TiO₂) core surrounded by a continuous-zoned less Ti-rich (3-5 wt% TiO₂) epitaxial mantle. The mg# is constant at 74-76 throughout the cores and increase in a smooth reversed trend to ~84 in the outer mantle. Pressure estimations, using BaO^phl/liq partitioning, suggest that the core phenocrysts crystallized at high pressures (30-25 kbar), whereas the phenocryst mantles formed at shallower crustal levels (10-5 kbar). Furthermore, the cores consists of two distinct phlogopite phases with slightly different compositions, implying that the host magma was a hybrid at upper-mantle levels. The tetraferriphlogopite-trend exhibited by the groundmass phlogopite, indicate relatively reducing conditions, probably caused by major phlogopite crystallization. It is therefore suggested, that the magma was stalled in the crust, perhaps due to rapid change in magma viscosity as a consequence of major volatile losses. The reverse zonation in the phlogopite was produced during the devolatilization and magma-volatile replenishment, causing perturbations (increase) in the liquidus temperature, as a result of latent heat released from the exsolving volatiles (Hort, 1998). The annite-trend of the later groundmass biotites was produced under relatively oxidizing conditions, perhaps due to interaction with meteoric waters during final emplacement. During Cambrian time (600-300 Ma), the Baltic craton was subjected to extensional lithospheric tectonics, forming thin spots, perhaps as a precursor to the Permian Oslo rifting event (Dahlgren, 1994), allowing mantle melting and rapid ascent as evidenced by the zoned phlogopite in the Bekkestul ultramafic lamprophyre.

Hort M, Journal of Petrology, 39-5, 1063-1076, (1998).

Dahlgren S, Lithos, 31, 141-154, (1994).

O08 : 5P/30 : PO

Comprehensive Thermochronological Modelling of Voronezh Crystalline Massif Gneiss Age (Russian Platform)

Vladimir Gerasimov (gera@igem.msk.su)¹,

Konstantin Savko²,

Vladimir Lebedev³ &

Mayina Arakelyants

¹ 109017, Moscow, IGEM, RAS, Staromonetny, 35, Russia, Russia

² Voronezh State University, Voronezh, Russia, Russia

³ 109017, Moscow, IGEM, RAS, Staromonetny, 35, Russia, Russia

The correct evaluation of the closure temperature of isotope systems and connected with them age dating are impossible without taking into consideration a rate of cooling of the system (Dodson, 1973). Because of kinetic properties the closure of petrological and isotopic part of the system happens in different time-temperature windows. In this work, part of the results of the study of Voronezh crystalline massif (VCM) metamorphic evolution by methods of petrological and isotopic thermochronology are presented. In petrological part of the investigation rock-forming minerals from the high temperature metapelites of vorontsov series of VCM (Gerasimov, Savko, 1995) were studied in detail by electronic microprobe. In garnet on the contact with biotite and cordierite two-component retrograde diffusion zoning was found with counter-flow of iron and magnesium. The rate of temperature evolution of metamorphic system was calculated by diffusion profiles in garnet. It was shown that at the regressive stage beginning with the temperature of 670^oC metamorphic complex cooling rate was about 3^oC/Ma, the pressure did not exceed 4 kbar and average speed of sub-geothermal uplift achieved 0,03 mm/year. To define an absolute geological age laser variant of isotopic dating ³⁹Ar/⁴⁰Ar (Megrue, 1973) was used. Main attention was paid to defining a degree of the breach of K-Ar isotopic system in minerals and an opportunity to its further use for defining the age of metamorphism. Isotopic measurement was conducted with the laser mass-spectrometric complex of IGEM RAS in static regime. As preparations we used special transparent sandwich thin sections with the underlying quartz glass with the built-in monitors of neutron flow. Such preparations contain small amount of substance and have low activity after irradiation. They are convenient for phase diagnosis in polarised light which allows controlling of homogeneity K-containing minerals, their crystalline orientation as well as possible diffusion loss of argon from core to the rim of the grains. For sandwich thin section producing even usual petrographic sections can be used. Only the grains of biotite were analysed. Age assessments for the large grains (600 µm) are quite homogenic, average value is 2275 Ma. The age of the small grains (100 µm) is probably rejuvenated due to the diffusion loss of argon at the regressive stage of metamorphism. To synchronise metamorphic event (thermal peak) with the scale of absolute geological time, petrological evolution P-T-t trends and equation for defining isotopic system closure temperature in the process of cooling (Dodson, 1973) were used. The calculation shows that the difference in time between the moments of closure of petrological and isotopic systems was 270 Ma. This means that the real age of regional metamorphism of VCM can be assessed as 2,54 Ga.