Journal of Conference Abstracts

Session L10:1P

L10 : 1P/01 : PO

Collision-Related Granites from the East Slope of the Middle Urals

Vladimir N. Smirnov (root@igg.e-burg.su)

Inst. of Geology and Geochemistry, Russian Academy of Sciences, Ekaterinburg, Russia

The Urals is the Paleozoic orogenic belt which appeared as a result of collision of the East European Craton with continental plates of Siberia and Kazakhstan. The process of collision was accompanied by the lardge-scale intrusion of granites different in composition. These granites are investigated more closely on the territory of the east slope of the Middle Urals. According to the available data the initial period of the collision (Mid and Late Carboniferous) was quite amagmatic here. The earliest magmatites of the collision epoch were represented by the dikes of the granites, granodiorites, pegmatites and aplites which have been intruded at the turn of Carboniferous and Permian (K-Ar-age - 285 Ma). The generation of the veined granitoids precedes the formation of the large bodies of calc-alkaline granies of the Early-Permian age (287-275 Ma, K-Ar and Rb-Sr-data). The distinguishing feature of these granites is a moderate content of the potassium (K₂O/Na₂O=0,6-0,8). The calc-alcaline granitic magmatism was followed by monzodiorite-granitic one. According to the K-Ar-dating the formation of this type of rocks was taking place in the age interval of 271-243 Ma (to the end of Permian). The composition of the massifes is greatly varying. On the whole acid rocks dominate, intermediate rocks are less prevailing, basites are extremely rare. All the rocks have mid-alkaline composition (K₂O+Na₂O (8%, K₂O/Na₂O close to 1). In the second half of Permien in the east part of the Middle Urals there were formed the massifes of the palingenetic granites. According to the complex of the isotope data the age of these granites is about 250 Ma. In the magmatic areals of this type the long range of the granitoid associations is selected (migmatite-granitic, adamellite-granitic, granitic, pegmatitic and leucogranitic) but all of them were generated as result of the same process - anatectic melting of the crust rocks. As a rule the granitoids are oversaturated by alumenium. They differ from the Ealy-Permien granites in higher content of potassium (K₂O/Na₂O close to 1) and most of rare elements.

The revealed sequence of the granitoids formation differs from known schemes of the collision-related magmatism (Harris et al., 1986; Shinkariov, Grigojeva, 1995). The main features are the following. There are no sincollision magmatic rocks in the Urals. On the opposite late-collision associationsare are widely distributed. They include calc-alkaline and mid-alkaline series usual for this stage and, besides, palingenetic granites typical for sincollision stage. Post-collision magmatic series (usually represented by alkaline rocks) are unknown too.

Harris NBW, Pears IA, Tindle AG, Collis. Tectonic. Oxford, 19, (1986).

Shinkariov NF, Grigojeva LV, Vestnik St-Piter. Univ. 7, 4, (1995).

L10 : 1P/02 : PO

Embedding and Tectonization of the Kamensky Granite Pluton Group (South Urals) as Result of Evolution of the Kopeysk Strike-Slip Zone

Alexander V. Tevelev (atevelev@geol.msu.ru),

Arcady V. Tevelev (tevelev@geol.msu.ru) &

Irina A. Kosheleva (irina@geol.msu.ru)

Geological Faculty, Moscow State University, Vorobjevy Gory, Moscow, Russia

The Kamensky group of the Early-Middle Carboniferous granite massifs (Kamensky, Redutovsky, Novoukrainsky and Kosobrodsky) is situated in area of the Kopeysk strike-slip zone, which is a regional suture that separates the East Uralian and Trans-Uralian terranes. The fault zone constitutes of lens-shaped vertical dipping, faulted slabs including formations from near vicinity of the suture, and among them clastics and volcanics from the Early Mesozoic rift basins. All massifs of the Kamensky group are of droplike, south elongated shape and of nearly alike size in 15-17 km length and 4-5 km width. Massifs attach to the master fault of the Kopeysk zone from the west. Each of them composes of three successive magmatic phases: (1) gabbro, gabbro-diorite; (2) grano-diorite, tonalite; (3) plagiogranite. The rocks from commenceable phases make up the periphery of the massifs, and plagiogranites of third phase are in their core. Analysis of general geological situation, deformational patterns and kinematics of mesostructures within and around the Kamensky massifs region allows to reconstruct a history of these intrusions. Their initiation involved formation of oblique (to NS striking master fault) flat conduits under condition of left-lateral transpression. Successive fast opening of oblique escapes was compensated by impulsive influx of magmatic melt. kinematical mechanism of intrusion was similar to way of formation of sheeted dykes because every following portion of melt intruded in axial part of the solidifying pluton. After the Sudetic orogeny, dynamical situation in area between the Magnigorsk arc and the Trans-Uralian terrane changed abruptly from general compression to extension, possibly because of that a subduction rate began to exceed a convergence rate. Consequently a sense of motion along the Kopeysk strike-slip zone was changed from sinistral to dextral, and deformation mode on the edge of the basin transformed into right-lateral tranpression. This determined a style of tectonic remaking of the massifs that is well studied for the Kamensky intru-sion. Most tectonized are endocontacts of the massif. Rocks of different magmatic phases are mixed tectonically, and magmatic contacts are detached. The marginal magmatic suits are schistizied in-tensively and even locally converted into gneiss, and planes of schistosity are collapsed in small asymmetric folds. By that the central part of late plagiogranite displays a complete absence of tec-tonization. Remarkably that on the place of the synchronous to intrusions local compressional structures there were developed on the stage of massif tectonization the deep-seated magmatic chambers for more young granitoids.

L10 : 1P/03 : PO

Geochemistry and Sm-Nd Isotopic Data of the Late Archaean Granites in the Oster Lake Region of the Central Karelia, Russia

Alexei Kovalenko (granite@comset.net)

Zagrebski bul. 39-2-79, St.-Petersburg, Russia

The geology of the region is characterized by the multi-block structure. The blocks consist of the late Archaean plutonic and volcanic rocks varied in composition from ultramafic throw intermediate to acid rocks occurring on the small area. The granites can be divided in two groups refereed to tectonic events: syn-kinematic Oster granite pluton, plagioporphyre dykes and post-kinematic Geine-oja granite pluton. Oster pluton consists of the coarse-fine-grain mesocratic-leucocratic granites containing quartz, plagioclase, K-fieldspar, biotite and accessories as apatite, sphene, zircon, magnetite and orthite. Granites belong to calc-alkaline series according classification of Sylvester (1988). On Pearce's diagram the granites plot on Volcanic Arc Granites field. The chemical composition allows to consider the granites as I-type. On spider diagram normalised on the upper and average Archaean crust (Taylor and McLennon, 1988) Oster granites are characterized by high positive anomalies of Rb, K, Th, LREE with negative anomalies of Sr and Nb and high negative anomalies of Ti and Eu. The fractionation of the plagioclase and/or accessories can be responsible for the negative Eu anomaly.Isotopic composition of three Oster granite samples varies in wide range. The <epsilon> Nd (2.8 Ga) values are +1.8 (1116), -2.9 (241) and -11.3 (241-1). T2stDM (Leiw and Hoffmann, 1988) are 3.0 (Ga), 3.4 (Ga) and 4.0 (Ga) relatively. We can offer two models of granites formation, explaining isotopic variations.According to the first model the andesites similar with those of this region were the source material. The <epsilon> Nd (3.0 Ga) of anesites is +1.6 and +2.3. The granite sample 1116 has a most evolved composition closed to Qu-Ab-Ort eutectic and its Nd isotopic composition corresponds real value, probably. The value T2stDM of sample 241-1 is meaningless and may be explained by fractionation of Sm and Nd during disequilibrium melting of andesites with apatite and/or zircon within restite.The second model includes contamination of the granite samples 241 and 241-1 by older than andesites crust material. But the isotopic composition of sample 241-1 also must be explained by fractionation of Sm and Nd.

L10 : 1P/04 : PO

Petrology of Granitoids of Metamorphic Core Complexes in East Siberia (Russia)

Tatyana Donskaya (tanlen@gpg.crust.irk.ru) &

Eugeny Sklyarov (skl@gpg.crust.irk.ru)

Irkutsk, Lermontova st., 128, Institute of the Earth crust SB RAS, Russia

Granitoids make up to 95% of whole volume of rocks in metamorphic core complexes (MCC) of the Late Mesozoic age in Transbaikalia (Sklyarov et al., 1997). Three geochemical types are distinguished among them on composition.Granitoids of the first type composing the basement of Zagan MCC are represented mostly by Bt-granites and granosyenites. They are subalkaline (Na₂O + K₂O = 7,5-12,2%), metaluminous or slightly peraluminous (ASI = 0,9-1,2) rocks. The contents of major elements in these rocks are similar to A-type granites, but they differ from typical representatives of A-type granites by higher abundances of Sr, Ba and lower abundances of Zr, Nb, Y. The REE patterns of these granitoids are characterized by a moderate or strong enrichment of LREE with respect to HREE ((La/Yb)n= 8-45). The basement of Yablonovy MCC comprise mostly Bt- and Bt-Hrnb-granites and granodiorites of the second type which can be classify as I-type granites on the petrographical, mineralogical and geochemical features (Na₂O+K₂O = 4,3-7,7%; ASI = 0,8-1,1; the low Rb/Sr (up to 0,85) and Rb/Ba (up to 0,49) ratios, (La/Yb)n=3-46). Biotite-muscovite-bearing leucogranites and granite-granosyenites composing synkinematic sills in the frame of complexes are referred to the third type. They are characterized by the moderate contents of alkalis, the relatively low contents of some high field strength elements (Zr, Y), the relatively high contents Sr (up to 670 ppm), Âà (up to 1600 ppm). They are peraluminous (up to 6% normative corundum) rocks. The petrogeochemical features of the rocks of these intrusions testify about important role of crustal component in these source. The investigated types of rocks belong to widespread in this region series of granitoids, which have various age and nature. The carried out researches refute traditional representation about rocks of the first and second types as blocks of Precambrian basement. Permian granitoids of Zagan MCC (Sklyarov et al., 1997) is a part of syenite-granite series of the Late Paleozoic age, which connected with continental rift environments (Leontiev et al., 1981). Granites and granodiorites of Yablonovy MCC having the Early Paleozoic age (Kosubova, 1976) compose part of granite-granitodiorite-diorite series and can be linked with environments of an active continental margin (Yarmolyuk, Kovalenko, 1991). The emplacement of granitoids of the third type in the Late Mesozoic with tectonic exhumation of metamorphic core complexes is caused by processes of intracontinental extension.

Sklyarov EV, Mazukabzov AM, Melnikov AI, Metamorphic core complexes of the cordilleran type. Novosibirsk. Published by UIGGM SB RAS, (1997).

Leontiev AN, Litvinovsky BA, Gavrilova SP, Zakharov AA, Paleozoic Granitoid Magmatism of the Central Asian Fold Belt. Nauka, Moscow, (1981).

Kosubova LA, Geologia i geofizika, 6, (1976).

Yarmolyuk VV, Kovalenko VI, Rift-related magmatism of an active continental margin and its metallogeny. Nauka, Moscow, (1991).

L10 : 1P/05 : PO

Petrogenesis of Epizonal Granites from SW Spain

Luis Gonzalez (lgonzale@goliat.ugr.es)

Dept. Mineralogia y petrologia, Fac. Ciencias, Univ. Granada, Fuentenueva sn, Granada 18002, Spain

Introduction: In SW Spain, at the CentralIberian zone, an important number of late Variscan batholiths, are emplazed in 2-3 kb level (7-10 km), intruding Paleozoic metasediments, mainly schits and greywakes (Fig.1). Most of these batholiths are formed by several granites, ranging from peraluminous two mica granites ± Crd, Trm, And, Sill, Gt; peraluminous two mica granites and metaluminous granites with Bt ± Anf, Sph, Ep. The peraluminous typically form more than 80% of the batholiths, and the metaluminous are normally 5-20% or less. These different granites of epizonal batholiths represent different magmatic pulses, usually contemparaneous, where the peraluminous are the first in reaching the emplacement level; the metaluminous bodies can be enclave-like bodies brough up by the other granites, or small later intrusions. Previous studies on SW iberian epizonal granites (Corretge et al, 1985; Ramirez, 1996; Vigneresse & Bouchez, 1997; Menendez, 1998; and Ramirez & Menendez, 1998) confirm these data.

A study case: the Nisa-Alburquerque batholithIn a detailed study of the Nisa-Alburquerque batholith (1000 km²) we identified (Fig.2):1. A peraluminous granite suite with Bt-Ms ± Trm, Crd, And. About 75% outcrop2. A peraluminous granite with Bt-Ms. About 20% outcrop3. A slightly peraluminous to metaluminous granite bodies with Bt ± Ms, Sph, Anf. About 5% outcrop. By the study of field relations, petrography and mineralogy, we consider these as different magmas. The metaluminous granites occur as mega-enclaves within both types of peraluminous, therefore we can establish two main intrusions of peraluminous magmas, both carrying up with them the metaluminous bodies. Geochemistry shows that differences between both peraluminous granite types are very small to argue for different sources (Fig.3); only the pure two mica granite being slightly richer in Al₂O₃ and K₂O. The metaluminous granites are quite different because of their lower K/Na ratio, low P₂O5 content, and higher CaO, Sr, and Zr contents. Therefore, a different source for the metaluminous types is likely. An Sr isotope study (Fig.4) confirms that both peraluminous granite types (Sr/Sr = 0.715-0.717) have probabily a very similar source, while the metaluminous granites have a different source (Sr/Sr = 0.705).

Discussion and conclusions: Geochemical and isotope data suggests a metapelitic source for both peraluminous granites. Higher Al₂O₃ and K/Na relation in the pure two mica granite could be due to a higher proportion of pelitic component compared to the other peraluminous granite. In the metaluminous, the high CaO and Sr contents, the low K/Na ratio and low initial Sr/Sr are compatible with a basic-intermediate igneous protholith, possibly of tonalitic composition. As the different granite types represent comteporaneous magmas emplaced at the same crustal level, we believe there was a unique melting event that affected a hetereogeneous midle to lower crust. Interestingly the discussed granite types are very common and form several epizonal batholiths in SW Iberia. This suggest that a similar melting process took place, and affected a source region probabily composed on metapelites with other minor components

L10 : 1P/06 : PO

Rb-Sr and Single- Zircon Grain ²⁰⁷Pb/²⁰⁶Pb Chronology of Monesterio Granodiorite and Related Migmatites (Ossa-Morena Zone, SW Iberian Massif)

Karmah Salman (ksalman@goliat.ugr.es),

Pilar Montero (pmontero@goliat.ugr.es) &

Fernando Bea (fbea@goliat.ugr.es)

Dept. Mineralogia y petrologia, Fac. Ciencias, Univ. Granada, Fuentenueva sn, Granada 18002, Spain

The Monesterio granodiorite is emplaced in a migmatitic complex in the SW of the Olivenza- Monesterio antiform and is an important plutonic body to understanding the relationships among the magmatism, metamorphism and deformation in th Ossa- Morena Zone ( SW Iberian Massif). This granodiorite has been dated by various methods and gives ages from 400 to 550 Ma. We dated the granodiorite with the single- zircon stepwise- evaporation ²⁰⁷Pb/²⁰⁶Pb method and the related migmatization event with the Rb- Sr method on leucosomes. We selected four coreless zircons and our results indicate that the Monesterio granodiorite crystallised at 510±7 Ma ( 99% confidence level) and its protolith had a componet with Upper Proterozoic zircons with a minimum age of 1696 Ma. Leucosomes (four whole- rock samples and k- feldspar and plagioclase concentrates) give a Rb- Sr age of 511±40 Ma (MSWD=1.7) with initial ⁸⁷Sr/⁸⁶Sr = 0.70914±0.00048. The lower initial ⁸⁷Sr/⁸⁶Sr of the granodiorite ( 0.7049±21), the different geochemical caracteristics from the migmatite, the field- relationships between them (sharp and on occasions more gradual contacts) and the absence of migmatite enclaves in the granodiorite, precludes it from having derived from the same protolith as the migmatites. The existence of different magmatic bodies in the Ossa- Morena Zone ( Pallares granodiorite, Higuera de Llerena and Riscal ortogneis, plutonic complex Taliga-Barcarrota...) with ages clustering around 500-510 Ma reveals the existence of a significant melting event during the Late Cambrian that involved protoliths with very different geochemical and isotopic signatures.

L10 : 1P/07 : PO

Transition from Calc-Alkaline to Alkaline Magmatism: An Example from Ida Ou Illoun (Siroua, Anti Atlas, Morocco)

Ahmed Touil (fstg@cybernet.net.ma)¹,

Essaid Bilal (bilal@emse.fr)²,

Jacques Moutte (moutte@emse.fr)² &

Abdelmajid Elboukhari (fstg@cybernet.net.ma)³

¹ FST Gueliz, Departement Geologie, Universite Cadi Ayad, BP618 Marrakech, Marocco

² Ecole des Mines, 158 cours Fauriel, Saint Etienne, France

³ FSS, Departement Geologie, Universite Cadi Ayad, BP518 Marrakech, Marocco

The Ida Ou Illoun complex, located in the SW part of the Siroua inlier (Central Anti-Atlas, Morocco), is composed of 1. leucocratic granites, 2. equant granodiorites, 3. coarse-grained porphyritic granites, 4. basic rocks, and 5. late stage granites.

Mineralogy and whole rock chemistry lead to the distinction of two groups of contrasting affinities:

- On one hand, the basic rocks are calc-alkaline. Their chemical es characters (low contents in Ti, Nb, low Fe/Mg ratio, high contents in Zr, Th, K, Rb, Ba) suggest emplacement in orogenic context and thick crust conditions. The primary magma can be be produced by partial fusion ofan enriched lithospheric mantle.

- On the other hand, the more acidic rock types have subalkaline characters: alkali content and high Fe/Mg ratio are high. Alkaline affinity is also shown by the composition of biotite (high Fe, low Al) and amphibole (ferro-hornblende and ferro-edenite), but the contents in Rb, Y, Nb, Zr, Ce are relatively low, being intermediate between those of 'volcanic arc granites' and 'within plate granites'.

These chemical features, particularly the high Fe/Mg of the minerals, are close to those of the monzonitic, iron-rich, 'transalkaline granites' defined by Lameyre (1987). The Ida Ou Illoun granites are interpreted as reflecting the transition from orogenic, calc-alkaline magmatism to anorogenic, alkaline magmatism. The parent magmas of the Ida Ou Illoun granite were derived from the strong contamination, by a thick crust, of alkaline basalts produced by the partial fusion of asthenospheric mantle remobilised to shallow depth in the lithosphere after the rupture of the subducted oceanic plate.

Lameyre J, Revista Brasileira de Geociencias, 17, 349-359, (1987).

L10 : 1P/08 : PO

U-Pb Single Zircon TIMS Data in Combination with CL and EMP Studies on Granitoids of the High Tatra Mts. (Slovakia)

Ulrike Poller (poller@mpch-mainz.mpg.de)¹,

Todt Wolfgang (todt@mpch-mainz.mpg.de)¹,

Marian Janak (geolmjan@savba.savba.sk)² &

Milan Kohut (MILAN@gssr.sk)³

¹ Max-Planck-Institut f. Chemie, Postfach 3060, D-55020 Mainz, Germany

² Academy of Sciences, Bratislava, Slovakia

³ Geological Survey of Slovak Republic, Bratislava, Slovakia

This study combines precise U-Pb age determinations under cathodoluminescence control (CLC-method) by TIMS with intense cathodoluminescence (CL) and microprope (EMP) studies on zircons from the High Tatra Mountains (Slovakia).

The High Tatra Mts. in the Western Carpathians are part of the Alpine-Carpathian Arc. In contrast to the geologically very complex Western Tatra Mts., the High Tatra Mts. expose mainly granitoids and migmatites.

Several types of granitoids ranging from diorites to granodiorites and syenogranites have been identified by geochemistry. Of main interest is the situation in the Velicka valley (near Gerlach peak), where dioritic xenoliths are found in granodiorites.

Zircons from different samples were studied by scanning electron microscope-cathodoluminescence (SEM-CL) documenting the diversity of the different High Tatra granites. Whereas the more mafic dioritic samples bear zircons with diffuse structures, the peralumineous granitoids of Velicka valley expose zircons with complex magmatic structures in combination with inherited components. Accordingly, I-Type granitoids like the dioritic xenoliths of Velicka valley are exposed as well as former S-type granitoids.

Electron microprobe analyses on the same zircons done for Hf, Th, U and Y give evidence for coherence of these elements with the intensity of cathodoluminescence. Low-luminescent areas of the crystals are often characterized by low Hf, and increased U, Th and Y contents. However, the correlation of luminescence intensities with the distribution of distinct elements is rather complicated, and additional factors such as crystallinity are involved.

The U-Pb zircon dating of several granitoids by CLC resulted in concordant age data around 340 Ma for the xenolithic diorites and similar ages for the granodiorites and syenogranites. Thus, the magmatism in High Tatra Mts. seems to be slightly younger than the main event in Western Tatra Mts., where we found ages around 360 Ma.

L10 : 1P/09 : PO

Petrography, Geochemistry and Geochronology of the Mönchalpgneiss of the "Hohes Rad" in the Silvretta Nappe, Austria

Ulrike Wokoek (uuullliii@aol.com) &

Ulrike Poller

Max-Planck-Institut für Chemie, Abt. Geochemie, Joh.-J.-Becher-Weg 27, D-55020 Mainz, Germany

The Silvretta nappe, situated in the southwestern part of Austria and eastern Switzerland, belongs to the upper Austroalpine nappes. The crystalline rocks of this nappe are metabasites and polymetamorphic ortho- and paragneisses. The orthogneisses are divided into the so-called "Older Orthogneisses" and the "Younger Orthogneisses" of the "Flüelagranitic Association".

The "Older Orthogneisses" are a group of granitic to ultramafic gneisses including the augengneiss "Mönchalp". The Mönchalpgneiss is characterized as a polymetamorphic S-type granitoid intruded 530 Ma ago as a volcanic arc granite. The "Younger Orthogneisses" containing 10 distinct gneisstypes are also former S-type granitoids characterized as syn- to post-orogen intrusions (420-450 Ma) (Liebetrau, 1996).

In the eastern part of the Silvretta nappe at the outcrop "Hohes Rad" a contact is exhibited between the Mönchalpgneiss and the "Younger Orthogneiss Urezzas". This contact between these two gneisses is diffuse in disagreement with the different ages of the gneisses. The texture at this location is coarser than the common Mönchalpgneiss.

Several samples from this contact have been taken for geochemistry and U-Pb single zircon geochronology. The analyses of the Mönchalpgneiss of the "Hohes Rad" mostly overlap with the geochemistry of the type locality near Davos (~20 km NW), but is not in agreement with any of the "Younger Orthogneisses".

First U-Pb single zircon data yield a Cambrian age (524 ± 20 Ma) defined by a discordia line. This age is in good agreement with the intrusion age of the Mönchalpgneiss at the type locality. Thus, the coarse grained variety of the "Hohes Rad" is in fact a Mönchalpgneiss.

Liebetrau V, Diss. Univ. Freiburg-CH, No. 1087, (1996).

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Geology and Petrology of the Kestanbol Pluton, NW Anatolia

Zekiye Karacik (zkaracik@itu.edu.tr) &

Yucel Yilmaz

Technical Univ. of Istanbul, Mining Faculty, Dept of Geology, Maslak-Istanbul, Turkey

The Kestanbol magmatic center is a good representative of the widespread, late Tertiary magmatism of the northwestern Anatolia. In the region the plutonic and the volcanic rocks are observed to be clearly associated in time and space.

The Kestanbol pluton intruded into a group of late Paleozoic-Early Mesozoic metamorphic rocks during the late Oligocene-Early Miocene period. Partly coevally with the granitic intrusion, hypabissal rocks were emplaced above the pluton. On the surface volcanic rocks of the similar composition were extruded. Within the volcanic association the ignimbrites and pyroclastic fall out deposits were formed extensively. The plutonic and the overlying volcanic rocks appear to have been intimately connected in a caldera-collapsed environment.

The geochemical properties of the plutonic and the associated volcanic assemblages are similar. The major elements indicate that they are metaluminous, high-K and calc-alkaline. The trace element and isotope data indicate that the magmas are hybrid, and were formed from a similar source; representing a mantle derived magmas, contaminated by the crustal materials.

The Kestanbol pluton is monzonitic and granitic in composition. An aplogranite and a catactastic granite are also differentiated along the western part of the pluton.

Granitic pluton is an elliptical body with the long axis extending NE-SW along the fault zones of the same trend. The granite rose into subvolcanic levels in the crust during its final stage of emplacement using these faults. Along this contact zone in the eastern part of granite and the volcanic rocks are spatially related. Along the western border the pluton displays intrusive contact with the regional metamorphic association in which the granite generated pyroxene hornfels-hornblend hornfels facies contact aureole. This part of the pluton represents a deeper level of the granite emplacement.

During the progression of the present N-S extensional system of the western Anatolia young fault were formed. They cut across the pluton and exhumed the cover rocks considerably, and thus the different levels of the pluton emplaced in the different depths were brought together across these faults.

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The Geology and Petrology of the Kozak Pluton; NW Turkey

Safak Altunkaynak (yilmaz@itu.edu.tr) &

Yucel Yilmaz

Technical Univ. of Istanbul, Mining Faculty, Dept of Geology, Maslak-Istanbul, Turkey

In Western Anatolia, a widespread magmatic activity developed during the Oligocene-early Miocene period. This magmatism produced plutonic, hypabyssal and volcanic associations. They display close temporal and spatial relationship. The Kozak magmatic centre is one of the good representative of this magmatism.

The granitic pluton, emplaced into the metamorphic rocks croping out at the center of a dome. The pluton is enveloped by the metamorphic, hypabyssal and volcanic rocks. The granite intruded into the Triassic, low-grade, metamorphic assemblage during the Late Oligocene-Early Miocene period, and formed a well-developed contact metamorphic aureole. Along the immediate contact, hornblende hornfels facies mineral assemblage was formed. In the plutonic association, three different rock groups are differentiated; a) Kozak granodiorite, b) aplogranite and c) microgranite. The Kozak granodiorite consist mainly of granodiorites and granites, which show transitional contacts. There are also subordinate quarz-diorites and quartz monzonites. The aplogranite is a fine-grained, and light coloured variety of the pluton, outcropping along the outer zone. It represents a late differentiate member of the pluton. The microgranites are exposed in three isoloted outcrops. They form arc shaped pattern. The Kozak pluton is an epizonal pluton, which emplaced into the shallow levels in the crust. The hypabyssal rocks form sheet intrusives around the pluton. They appear to have acted locally as feeder dykes for the overlying lavas of the similar composition. In the light of the field and petrological characteristics, the Kozak pluton may be evaluated as the caldera type or subvolcanic pluton.

L10 : 1P/12 : PO

The Late Permian - Early Triassic Orogeny in Indochina: Precise U-Pb Dating Results from Vietnam

Elizabeth Nagy (nagy@ipgp.jussieu.fr) &

Urs Schärer

Laboratoire de Géochronologie, 2, Place Jussieu, tour 24-25, 1e, F-75251 Paris Cedex 05, France

Crystallization ages of intrusive rocks from the NW-striking Sông Ca and Sông Ma ductile fault zones in central and northern Vietnam, respectively, have been determined for the first time using U-Pb geochronology. Zircon grains separated from undeformed granites and diorites within the Sông Ca fault zone imply late Permian emplacement. Samples from three localities, spaced every 100 km along strike of the Sông Ca belt, yield the following results (SE to NW): (1) a lower intercept age of 250 ± 1 Ma (all ages are 2 <sigma>), (2) a concordant age of 251 ± 3 Ma, and (3) a concordant age of 251 ± 1 Ma. All three samples show evidence of a Precambrian hereditary component. For example, zircons from the granite at locality (1) give an upper intercept age of 1933 ± 11 Ma. These late Permian ages are significantly older than Cretaceous and Cenozoic thermal overprint ages identified within the Sông Ca fault zone by ⁴⁰Ar/³⁹Ar geochronology (Lepvrier et al., 1997). Zircons from an undeformed granite within the Sông Ma ductile deformation zone (sampled near Dien Bien Phu) yield a concordant age of 229 ± 1 Ma. This age is slightly younger than ⁴⁰Ar/³⁹Ar plateau ages (~ 245 Ma) from schists and foliated granites of the Sông Ma metamorphic complex (Lepvrier et al., 1997) located ~ 80 km southeast of our sampling site within the Sông Ma belt. The emplacement of the granite from the Dien Bien Phu area clearly postdates the 245 Ma tectonometamorphic event. The late Permian - early Triassic magmatism identified here suggests a magma-generating tectonic phase (subduction- or extension-related) in Indochina which is older than middle-upper Triassic unconformities generally interpreted to mark the initial phases of the Indosinian orogeny (e.g., Lacassin et al., in press). Regional variations in the timing of Indosinian deformation (e.g., ~ 200 Ma in Thailand; middle-Triassic in China) suggest a multi-phase history of deformation. Our results suggest that the Indosinian collisional deformation which marks the closing of the eastern Paleo-Tethys Sea (i.e., convergence between Gondwanaland and Eurasia) began earlier than previously inferred.

Lepvrier C, Maluski H, Nguyen VV, Roques D, Axente V, & Rangin C, Tectonophysics, 283, 105-127, (1997).

Lacassin R, Leloup PH, Phan TT, & Tapponnier P, Terra Nova, in press

Session L10:2A

L10 : 2A/01 : S1

Importance of Magmatic Interaction Processes in the Evolution of Tertiary Granitoids of Serbomacedonian and Rhodope Massifs (Northern Greece)

Diego Perugini (diegop@unipg.it)¹,

Giampiero Poli (polig@unipg.it)¹,

George Christofides (christof@geo.auth.gr)²,

Triantafyllos Soldatos (soldatos@geo.auth.gr)²,

Andonis Koroneos (koroneos@ccf.auth.gr)² &

George Eleftheriadis (gelefthe@geo.auth.gr)²

¹ Department of Earth Sciences, University of Perugia, Piazza Università, 06122 Perugia, Italy

² Dept. of Mineralogy Petrology and Economic Geology, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece

A lot of attention has been given the last few years to the study of microgranular Mafic Enclaves (MME) in granitoid plutons as important indicators of magmatic interaction processes. Their distribution is widely recognized in the Tertiary granitoid plutons emplaced in Serbomacedonian and Rhodope Massifs (Northern Greece). This work points to the study of such geologically complex areas for a better understanding of magmatic interaction processes as important petrogenetic engines not only on a local scale but considering also the regional spatial distribution of MME-bearing granitoid rocks. Granitoid rocks intruding Serbomacedonian and Rhodope Massifs can be roughly divided into two major groups: calc-alkaline and high-potassium - shoshonitic rocks. Whole-rock geochemical and isotopic data reveal that these groups cannot be related by simple processes such as fractional crystallization or assimilation plus fractional crystallization. Much more complex processes acting together have to be considered; mixing/mingling among at least two basic magma end-members with different potassium enrichment and an acid magma end-member has to be primarily invoked as the main differentiation process, possibly coupled with fractional crystallization. The main debate is on establishing the nature of magmas involved in these processes. Regarding some basic shoshonitic magmas, ideas considering them as anatectic crustal melts can be ruled out on the basis of geochemical and experimental petrology data. An alternative hypothesis, supported also by isotopic data of the considered MME, is, for both calc-alakline and shoshonitic mafic magmas, a partial melting of a differently enriched upper mantle. Trace element data support the inference that melting occurred at different pressures. Variable intensities of interaction, possibly coupled with fractional crystallization, of such mafic melts with acid melts, produced by partial melting of a middle-lower crust of amphibolitic composition, can give the observed granitoid compositional spectrum. Plutons of Ouranoupoli, Jerissos, Stratoni and Vroundou have been considered in their more general features, whereas, as a particular and representative case of study, the Sithonia Plutonic Complex has been considered in detail to verify the above hypotheses.

L10 : 2A/02 : S1

Mafic-Silicic Magma Interactions within the Central Bohemian Plutonic Complex, Czech Republic

Franticek V. Holub (frholub@prfdec.natur.cuni.cz)¹ &

Milan Lantora²

¹ Inst. of Petrology and Struct. Geology, Fac. Sciences, Charles University, Praha 2, Czech Republic

² DIAMO, Administration of the Uranium Deposits, Pøíbram, Czech Republic

The Central Bohemian Plutonic Complex (CBPC) of Lower Carboniferous age displays the broadest petrographical and geochemical diversity among Hercynian plutonic bodies of the Bohemian Massif. Several major compositional groups of granitoids can be distinguished (Holub et al., 1997). Four the most voluminous compositional groups that comprise also some subordinated mafic members are as follows: /1/ the calc-alkaline granodiorites, tonalites and diorites to gabbros (CA), /2/ high-K calc-alkaline to shoshonitic granodiorites and subordinated monzonitic rocks (HK), ultrapotassic melasyenites to melagranites of durbachitic type with some highly mafic members (UK) and geochemically related but more acidic high-Mg K-rich granites (MgK). Although there are many papers ascribing the origin of CBPC to processes of transformation and recently to the "isochemical granitization in situ" (e.g., Palivcová et al., 1989; Vladímsky et al., 1992), the rocks display all typical features of true igneous intrusions.

The CA, HK, UK and MgK granitoids and syenitoids contain numerous mafic microgranular enclaves and small bodies, which are geochemically related to the host rocks. Hybridism is quite common and important in origin of these rock groups. The UK durbachite series, e.g., originated by magma mixing of a mantle-derived ultrapotassic mafic magma with acidic crustal melts (Holub, 1997). The mafic members represent magmas originated in heterogeneously depleted and then enriched lithospheric mantle.

Relatively voluminous mafic masses of quartz-dioritic to gabbroic composition occur in a close association with the CA granitoids. At their contacts can be observed diverse interaction phenomena linked with various stages of thermal equilibration and solidification of broadly coeval but physically and compositionally contrasting magmas. Early contacts of mafic masses are lobate or crenulate with chilled margins of the mafic bodies, pillows and blobs, whereas some later stages of interaction are characterized by veining and mechanical disruption of the solidified mafic masses resulting in origin of subangular to angular enclaves.

Near to Pøíbram at the depth of 1 km below the present surface we have observed a stratified complex of interlayered mafic and granitoid rocks with well-preserved original shapes and interfaces. Gently dipping to subhorizontal mafic layers are chilled against and separated by much thinner layers and laminae of cumulitic tonalite. Basal contacts of the mafic layers are lobate with crests filled by a mobilized granitoid which sometimes form also small pipes and veins extending upward and injecting the mafic body. Such phenomena are distinctive for the mafic-silicic layered intrusions (Wiebe 1996 and references therein) and demonstrate the former existence in CBPC of dynamically evolving granitoid magma chambers that were periodically injected by mafic magma batches. Although such chambers probably could be linked with some volcanic apparatuses, there are no coeval volcanic rocks preserved in the deeply eroded area under study.

Holub FV, J. Geol. Sci. Econ. Geol. Mineral (Praha), 31, 5-26, (1997).

Holub FV, Machart J, Manová M, J. Geol. Sci. Econ. Geol. Mineral (Praha), 31, 27-50, (1997).

Vladímsky P, Ledvinková V, Palivcová M, Waldhausrová J, Èas. Mineral. Geol. / J. Mineral Geol(Praha), 37, 31-44, 37, 31-44, (1992).

Wiebe RA, Trans. Roy. Soc. Edinburgh, Earth Sci, 87, 233-242, (1996).

L10 : 2A/03 : S1

Shoshonitic Plutonic Products in the Mediterranean Area: A Comparison between the Hercynic Complex of Ile-Rousse (NW Corsica, France) and the Alpine Pluton of Valle del Cervo (NW Alps, Italy)

Daniela Querci (dany@unipg.it) &

Giampiero Poli (polig@unipg.it)

Department of Earth Sciences, University of Perugia, Piazza Università, 06122 Perugia, Italy

Two different examples of shoshonitic intrusive magmatism have been studied in the Mediterranean area: Valle del Cervo Alpine plutonic complex (VC, North-Western Alps - Italy) and Ile-Rousse Hercynic plutonic complex (IR, North-Western Corsica - France) with the purpose of understanding the development of shoshonitic products in orogenic areas and their possible genetic links with temporally and spatially related calc-alkaline rocks. Major and trace elements geochemistry, Sr and Nd isotopic systematic and SIMS trace element data lead to identify: the coexistence of various type of magmas; the magmatic processes that give rise to the observed rocks; nature and origin of primitive liquids. Within IR complex, shoshonitic products coexist with high-K calc-alkaline rocks. Common features are the presence of microgranular mafic enclaves and basic septa, whereas a peculiar feature of the shoshonitic suite is the occurrence of large, gold-colored titanite crystals that almost gain the attribute of a major mineral phase. Trace element data and linear programming models using major mineral phases constrain a complex mixing process among a magma of granitic composition and two magmas of more basic composition having different K enrichment; Sr and Nd isotopic data support this hypothesis, and further constrain characteristics of the primitive liquids. SIMS data on heuedral and anhedral titanite crystals from shoshonitic and high-K calc-alkaline rocks indicate: (i) differences in trace element contents of titanites from rocks at different affinity; (ii) strong differences along core-rim transects in the shoshonitic rocks; (iii) differences in REE contents among heuedral and anhedral titanites. Within VC complex, absence of calc-alkaline affinity products can be ascribed to the different geodynamic context active during the generation and emplacement of magmas. However, presence of microgranular mafic enclaves, trace element geochemistry as well as linear programming models using major mineral phases, give indications of coexistence of at least two different magma types of monzogranitic and syenitic composition, that underwent to complex interactions giving rise to the spectrum of observed rocks; Sr and Nd isotope systematic unravel the origin of the two magmas in crustal and mantellic domains respectively. SIMS trace element data seem to corroborate evidence found for IR complex. On the whole, the collected data on IR and VC complexes help to understand relationships between petrology and geodynamics of mature plutonic products in orogenic settings, giving constraints on the nature and origin of shoshonitic suites.

L10 : 2A/04 : S1

Mafic Precursors, Peraluminous Granitoids and Late Lamprophyres in the Avila Batholith. A Model for the Generation of Variscan Batholiths in Iberia

Fernando Bea (fbea@goliat.ugr.es),

Pilar Montero &

Jose F. Molina

Dept. Mineralogy and Petrology, Campus Funetenueva, Univ. Granada 18002, Spain

The Avila batholith of central Spain is composed of upper Carboniferous peraluminous granitoids, preceded by volumetrically insignificant bodies of mafic-ultramafic hybrid materials and postdated by several dike swarms of camptonitic lamprophyres. Rb-Sr dating indicates continuous magmatic activity from ~350 Ma to ~280 Ma, starting with the mafic precursors and a few midcrustal anatectic leucogranites, followed by massive autochthonous and allochthonous granodiorites and granites, and ending with the camptonitic lamprophyres. Early hybrid mafic magmas (<epsilon>_340MaSr ~25; <epsilon>_340MaN~-1.5) were produced in small batches during or immediately after the main deformation phase by partial melting of a mixture of ~8:2 mantle and biotite-bearing crustal rocks at the crust-mantle interface. These magmas were emplaced in the middle crust at ~340 Ma, advecting a negligible amount of heat. The generation of crustal granites during the main deformation phase was limited to highly fertile protoliths heated by frictional heating. The generation of crustal granitoids at batholithic scale took place from ~330 Ma to ~290 Ma, during the main extensional period. Granites (<epsilon>_310MaSr~45-150; <epsilon>_310MaNd~-2.1 to -9) were produced by the partial melting of fertile crustal rocks (<epsilon>_310MaSr~48-218; <epsilon>_310MaNd~ -2.2 to -9) characterized by high heat production (~2.5-3 mW m^-3). The zone of partial melting, between ~15 and 22 km in depth, was heated by thermal conduction from below after crustal thinning, but the contribution of radiogenic heat and the fertility of source rocks would have been essential for anatexis. The fast thinning of the crust from ~310 Ma to ~285 Ma released the lithostatic pressure in the upper mantle and caused decompressional melting of the metasome layer at ~60 to ~85 km in depth, producing camptonitic melts dated at ~283 Ma. The existence of a fertile metasome layer implies that the lithospheric mantle beneath central Iberia was not actively involved in subduction during the Variscan orogeny.

L10 : 2A/05 : S1

Geochemistry of Hercynian Granites from Northern and Central Portugal

Ana M. R. Neiva (neiva@gemini.ci.uc.pt)

Earth Sciences Dept., Coimbra University, Coimbra, Portugal

S-type Hercynian granites predominate. Biotite granites, two-mica granites and muscovite granites are either syn- to late-tectonic (357-304 Ma) or post-tectonic (290-275 Ma). They have metasedimentary enclaves and rarely granodioritic enclaves. Mainly magmatic and rare restitic andalusite and sillimanite occur in some two-mica granites and cordierite in some biotite granites. These granites are peraluminous (A/CNK>=1.1), with initial ⁸⁷Sr/⁸⁶Sr>=0.707, <epsilon>Nd -8.2 to -4.8 and ¹⁸O 10.2 to 13.2. Rarely they have high P₂O₅>=0.30 wt% of magmatic origin which is retained in feldspars, apatite and monazite. Mainly Sn, W and Au mineralizations are associated with S-type granites.

Most of these ganites were generated by partial melting of heterogeneous metasedimentary source rocks (richer in greywacke than pelite) and correspond to distinct granite magmas (e.g. Neiva, 1993; Silva, 1995; Gomes, 1996). Some two-mica granites and muscovite granites are derived from a peraluminous either granodiorite or granite magma by fractional crystallization (e.g. Neiva et al., 1987; Gomes, 1996). Rarely two-mica granite is derived from biotite-hornblende tonalite by fractional crystallization accompanied by assimilation of metasedimentary country rocks (Gomes, 1996). Biotite granites from Tourem would have resulted from sequential higher extents of partial melting of orthogneiss with progressive less efficient segregation from restitic materials (Holtz and Barbey, 1991).

I-type granites containing biotite, sphene and allanite were found in Gerez mountain. They are peraluminous 302 to 287 Ma old and comagmatic, but during fractional crystallization there was assimilation of metasedimentary materials as shown by modelling of trace elements and initial ⁸⁷Sr/⁸⁶Sr ratio ranging from 0.7047 to 0.7147 from the oldest to the youngest (Neiva, 1993). W-Sn bearing quartz veins are related to the oldest granite.

There are also some hybrid biotite granites containing microgranular enclaves. They are metaluminous to peraluminous, syn- to late-tectonic and post-tectonic , with initial ⁸⁷Sr/⁸⁶Sr of 0.7044 to 0.7085 and <epsilon>Nd of -6.2 to -5.0. They represent mixing of a granite magma (formed by partial melting of either igneous rocks or metasedimentary materials) with a basic magma of the upper enriched-mantle followed by fractional crystallization (Neiva, 1993; Dias and Leterrier, 1994; Silva, 1995).

Acknowledgements Prof. B. J. Wood and Dr. J. C. Schumacher are thanked for the EUGF-Bristol facility, contract ERBFMGECT980128.

Dias G, Leterrier J, Lithos, 32, 207-223, (1994).

Gomes ME P, Unpublished Ph. D. thesis. Univ. Trás-os-Montes e Alto-Douro, Portugal, (1996).

Holtz F, Barbey P, J. Petrol, 32, 959-978, (1991).

Neiva AM R, Chem. Erde, 53, 227-258, (1993).

Neiva AM R, Neiva JM C, Parry SJ, Geochim. Cosmochim. Acta, 51, 439-454, (1987).

Silva MM VG, Unpublished Ph. D. thesis, Univ. Coimbra, Portugal, (1995).

L10 : 2A/06 : S1

Cooling Pattern by ⁴⁰Ar/³⁹Ar Dating of the Blond Granite, French Massif Central

Pavel Alexandrov (pavel@crpg.cnrs-nancy.fr)¹,

Alain Cheilletz (cheille@crpg.cnrs-nancy.fr)¹,

Jean-Jacques Royer (royer@ensg.u-nancy.fr)² &

Christian Le Carlier

(Le-Carlier@ensg.u-nancy.fr)²

¹ CRPG-CNRS, BP20, 54501 Vandoeuvre Cedex, France

² ENGS - Nancy, France

The Blond leucogranite is one of the last Hercynian intrusions in Limousin, French Massif Central. Its annular structure has been defined by petrographical and geochemical features. Previous dating (whole rock Rb/Sr) indicated 301 ± 4 Ma. Here we present the results of ⁴⁰Ar/³⁹Ar dating, and propose a model for the cooling history.

Laser step-heating and VG3600 spectrometry of muscovite monograins provided plateau ages for all of the analyses (with more than 80% of the ³⁹Ar on the plateau). The ages obtained vary from 307.2 ± 1.8 Ma to 311.4 ± 1.6 Ma and form three groups : about 311 Ma (inner zones), about 309 Ma (annular zones) and about 307 Ma (on the western extremity and east of the intrusion; figure). We propose the following model for this age distribution (which seems to be opposite to the normal cooling pattern): 1) emplacement at 313.3 ± 0.6 Ma (based on the age of the Richemont rhyolite dike, situated to the north-east and probably related to the Blond granite); 2) at 311 Ma, rotation about an east-west horizontal axis permited to the northern part of the intrusion to pass the isotope-closure isotherm; 3) at 309 Ma, the whole intrusion was uplifted and crossed the isotope-closure isotherm; 4) at 307 Ma, small intrusions (visible at the western extremity) provoked local resetting of the isotopic systems.

In any case, the previous date of 301 ± 4 Ma seems to be wrong. The emplacement of the granite most probably occured between 312 and 316 Ma and involved several, at least two, intrusion pulses.

⁴⁰Ar/³⁹Ar ages in the Blond leucogranite. The fine lines indicate the facies limits. The errors are on the order of 1.5%.

L10 : 2A/09 : S1

On the Origin of Various Post-Collisional Rock Series in the Svecofennian of the Fennoscandian Shield

Olav Eklund (olav.eklund@utu.fi)¹,

Dimitry Konopelko (Konopelk@DK2032.spb.edu)²,

Ulf B. Andersson (ulf.andersson@geo.uu.se)³ &

Sören Fröjdö (soren.frojdo@abo.fi)⁴

¹ University of Turku, Dept.of Geology, 20014 Turku, Finland

² Institute of Precambrian Geology and Geocronology, 2 Makarova Embankment, St.Petersburg, Russia

³ Department of Earth Sciences, Uppsala University, Villavägen 16. S-75236 Uppsala, Sweden

⁴ Åbo Akademi Univ., Dept. of Geology and Mineralogy, 20500 Turku, Finland

The Svecofennian domain formed and accreted to the Archaean craton in NE c. 1.95-1.87 Ga. The subsequent metamorphic peak (high T, low P) was reached at different times in different places in the domain during the period 1.84-1.78 Ga, and was accompanied or followed by post-collisional magmatism that comprises at least three different series. 1. A shoshonitic series, ranging in composition from ultramafic, calkalkaline, apatite-rich postassium lamprophyres to peraluminous HiBaSr-granites 2. A tholeiitic-calcalkaline (T-CA) series ranging from gabbros, norites to met- and peraluminous granites 3. Peraluminous S-type granites.

1. The shoshonites are interpreted to stem from a metasomatically enriched mantle. The relative importance of the fluid species, however, varied regionally over the domain. Primary shoshonitic magmas can be interpreted as parental, deriving the whole series (incl: lamprophyres) up to the HiBaSr-granites, by crystal fractionation. Intitial <epsilon>Nd values are around +1 for all rocks of this series.

2. The mafic rocks of the T-CA series can be interpreted to originate by melting of Phl-Sp-Pl-lherzolites, followed by crystal fractionation. The Phl-Sp-Pl-lherzolites occur at a few localities and are interpreted to represent pieces of metasomatized uppermost mantle. The associated granites are interpreted as derived by intracrustal melting of the pre-existing calcalkaline Svecofennian crust. Intermediate rocks form various hybrids between these endmembers. 3. In the Svecofennian domain, peraluminous S-type granites occur as late-orogenic intrusions generally emplaced during or somewhat after the metamorphic peak. However S-type granites have occasionally overlapping ages and are intermingled with shoshonitic rocks. It seems that, during the post-collisional event in the Svecofennian domain (1.84-1.77 Ga) , melting was initiated in a mantle that was variably metasomatized in different areas. The shoshonite magmas originate from more strongly metasomatized mantle sections, compared with the T-CA magmatism. The T-CA magmas underplated large regions of the early Svecofennian crust, caused extensive melting and mixing with crustal magmas. In contrast, the shoshonitic rock complexes were emplaced at high crustal levels, sometimes together with crustally derived S-type granites.

L10 : 2A/10 : S1

The Tatra Mts. Granitoid Pluton ­ A Collision Related Variscan Intrusion in the Western Carpathians, Slovakia

Milan Kohút (milan@GSSR)¹,

Ulrike Poller (poller@mpch-mainz.mpg.de)²,

Wolfgang Todt (todt@mpch-mainz.mpg.de)²,

Peter Nabelek (nabelekp@missouri.edu)³ &

Marian Janák (geolmjan@savba.savba.sk)⁴

¹ Dionyzstúr Institute of Geology, Slovak Geological Survey, 817 04 Bratislava, Slovakia

² Max-Planck Institut fur Chemie, D-55020 Mainz, Gernany

³ University of Missouri, Columbia, MO 65211, USA

⁴ Slovak Academy of Science, 842 26 Bratislava, Slovakia

The Variscan belt in Europe is characterized by large volumes of felsic magmatic rocks. Granitoid rocks are dominating the Variscan basement in the Western Carpathians, and the Tatra Mts. pluton is good example of the Carboniferous granite intrusion. The granitoid pluton is composed of several rock types ranging from tonalite to leucocratic granodiorite and granite. Petrographically these granites represent common crustal anatectic rocks with magmatic muscovite. The mafic enclaves (MME) and metamorphic xenolithes, can be observed in rather homogenous rocks. Prevalence of plagioclases (An_{20 - 45}) over K-feldspar is typical attribute. Biotite is the dominant Fe-Mg mafic mineral, whereas hornblende occurs only rarely in the dioritic enclaves. Silica contents of the Tatra Mts. granitic rocks vary from 64 to 74 wt.%. These granitoids represent low- to high-potassium calk-alkaline (throndhjemite & monzonite) series of magmatic rocks. Chondrite-normalized patterns of the REE exhibit absence or slight negative Eu anomalies and uniform fractionated distribution trends for various granite types (La_N/Yb_N = 42 - 15). Metaluminous to peraluminous (subaluminous) character is reflected by ASI = 0.8 - 1.2 and/or Peacock's index ALI = 62 - 63. The Rb/Sr ratio range from 0.05 to 1.1 indicates mixing of several, mainly crustal components, albeit initial strontium ratios I_Sr = 0.705 - 0.706 call for lower crust dominance. The <epsilon>Nd₍₀₎ values, varying from -7 to -4 are similar to other European crustal granites. Particularly low ¹⁸O values 8.5 - 9.6‰ SMOW are consistent with infracrustal metaluminous whole-rock chemistry. New single grain zircon U/Pb data suggest the Meso-Variscan emplacement of the Western Tatra Mts. granites between 369 ± 19 Ma and 347 ± 14 Ma. There is none proof (geologic or isotopic) for a direct input of juvenile products of a mantle magma, which could possibly trigger anatectic and/or differentiation processes during the Variscan orogen in the Western Carpathians. All geochemical features suggest that the Tatra Mts. granitoids are analogous VAG (CAG) granites related with subduction processes. However, metamorphic, sedimentary and structural data rule out this scenario, and suggest continental collision processes. This collisional processes with overthrusting of the deep crustal nappes were juxtaposed with crustal reactivation and granitoid production. Field evidence together with P-T-t paths (generally clockwise) suggest tectonic inversion of metamorphism in the Western Tatra Mountains by thrusting of hot, highly metamorphosed slab (migmatites, gneisses, amphibolites) over a cooler parautochthon dominated by mica schists. Suitable source rocks for the Tatra Mts. granitoids could have been more complex, like an old metaigneous rocks and/or greenstones, with contribution from high-grade metapelites.

KEYNOTE

L10 : 2A/11 : S1

Subduction-Related Anatexis: An Example from the Urals, Russia

German B. Fershtater (gerfer@online.ural.ru)

Institute of Geology and Geochemistry, Pochtovy per., 7. Ekaterinburg, 620151, Russia

The eroded ocean to continent transition zones are the only place to study subduction related anatectic granites and the processes of their magma generation. One of such zones can be observed in the Ural Paleozoic orogen. It includes the fragments of accretion prism (subduction melange, suture), island arc and continental margin. Anatectic granite rock series occur in all these parts. The common features of these series are as follows: 1) they include the wide spectrum of the rocks different in nature (protolith, refractory residues, melt and its differentiates) and composition (from basic to acid); 2) the rocks contain primary epidote; 3) basic rocks are an important component of substratum; 4) the main source of the volatiles as a flux for anatexis is the molecular water from the deeping slab; the dehydration melting is not common; anatectic melt is strongly enriched in water: PH₂O/Ptot=0.5-0.8; refractory remains have approximately the same mineralogy as protolith; 5) all magmatic rocks have clear spatial chemical variations toward subduction zone; 6) the tonalitic or granodioritic composition of primary anatectic melt is common; this composition corresponds to about 40% of partial melting of basic protolith. The anatexis usually is connected with underplating of lower crust by basic intrusions. So the subduction-related granitoids are situated in the areas with thick and basic in composition crust. They have long and complex history of emplacement. This study is supported by RFSF (grant 98-05-64826).

L10 : 2A/13 : S1

Single-Zircon Stepwise Evaporation ²⁰⁷Pb/²⁰⁶Pb and Rb-Sr Dating of Major Uralian Batholiths. Constraints on the Timing of Deformation and Granite Generation

Pilar Montero (pmontero@goliat.ugr.es)¹,

Axel Gerdes,

Fernando Bea,

German Fershtater (gerfer@online.ural.ru)²,

Nadiezna Borodina,

Yelena Zinkova &

Galina Shardakova

¹ Dept. Mineralogy & Petrology, Campus Fuentenueva, Unv. Granada, 18002 Granada, Spain

² Institute of Geology and Geochemistry, Pochtovy per., 7. Ekaterinburg, 62015, Russia

Uralian batholiths, regardless of location and apparent age, are usually composed of strongly deformed granodiorites and tonalites together with undeformed granites and adamellites. Since they normally appear related to main shear-zones, a precise knowledge of their ages is fundamental to understanding the tectotonomagmatic evolution of the Uralian Orogen. We present here the up-to-date results of a systematic dating project in which four of the most important Uralian batholiths, the Syrostan, Verkhisetsk, Murzinka and Dzhabyk were dated by single zircon ²⁰⁷Pb/²⁰⁶Pb stepwise evaporation analyses and Rb-Sr. Syrostan and Verkhisetsk are typical subduction-related batholiths, comprising amphibole- and epidote-bearing metaluminous granitoids with a compositional spectrum ranging from gabbros to leucogranites. Murzinka and Dzhabyk are continental-type batholiths, composed of moderately peraluminous granodiorites, adamellites, and granites that have biotite ± muscovite as varietal minerals. Syrostan is a small body composed of gabbros, diorites, granodiorites and leucogranites that crops out between the Main Uralian Fault and the metasediments of the Russian Platform. The main rock type consists of extremely deformed granodiorites that define a nearly constant N-140-E foliation. Deformation is ductile and seems to mark the surface of maximum compression related to dextral strike-slip movements of the Main Uralian Fault. Gabbros and diorites may appear either undeformed or sympathetically deformed with granodiorites. Granites, however, are always undeformed. ²⁰⁷Pb/²⁰⁶Pb zircon systematics reveal that gabbros and granodiorites have exactly the same age, 334 ± 5 Ma and 334 ± 4 Ma respectively, whereas granites are slightly younger with 327 ± 4 Ma. Syrostan granites also have zircon grains with older cores with a minimum age of 1,816 ± 27 Ma, which must be inherited from older crustal materials such as those from the Russian Platform. The Verkhisetsk batholith (250 km N of Syrostan) consists of several intrusive units. The westernmost unit, Tabatuy, is affected by the Serovk-Mauk Fault; the easternmost rocks of the Isetsk unit are affected by the Ekaterinburg Fault; in both cases a gneissic fabric with little cataclasis was produced, thereby suggesting that the emplacement of these units was synkinematic with those structures. Since Tabatuy was precisely dated by Rb-Sr and ²⁰⁷Pb/²⁰⁶Pb single zircon grain stepwise analyses at 316 ± 6 Ma, and Isetsk at 320 ± 20 Ma, we conclude that the Serov-Mauk and Ekaterinburg Faults were active at 315-320 Ma, when the activity of the Main Uralian Fault had already ceased in the region near Syrostan. These data indicates an age of ~335 Ma and ~320 Ma for the transition from subduction to continent-continent collision at the latitudes of Syrostan and Verkhisetsk respectively. The Murzinka batholith (~70 km NE of Verkhisetsk) is composed of three units that have the same age, determined at 253 ± 6 Ma with ²⁰⁷Pb/²⁰⁶Pb single zircon grain stepwise analyses and 257 ± 11 with Rb-Sr. The easternmost granites of the Murzinka batholith are synkinematic with the Alapaevsk Fault, thus enabling us to date its activity at ~250-255 Ma. The Dzhyabyk batholith (~400 km S of Murzinka) comprises several different intrusive units which, according to our data, seem to be coeval. Its age has been determined by zircon ²⁰⁷Pb/²⁰⁶Pb dating at 292±4 Ma and by Rb-Sr at 282±17 Ma. Since Dzhyabyk and Murzinka represent the climax of continental-type granite magmatism in the South and Middle Urals, respectively, our data suggest that that the age of the continent-continent collision and granite magmatism migrated gradually northwards and, at a given latitude, the age of magmatism and main shear zones is also progressively younger eastwards. These observations, together with others derived from the thermal regime during the generation of the Verkhisetsk batholith as well as the geometry of the main shear-zones, suggest (1) an oblique convergence with a SE movement of the converging continent, and (2) that the closure of the Uralian ocean started in the south and migrated progressively to the north.

L10 : 2A/14 : S1

Late-Orogenic Continental-Type Granites of the Urals: Composition, Age and Petrogenetic Implications

Axel Gerdes (agerdes@goliat.ugr.es)¹,

Pilar Montero¹,

Fernando Bea¹,

Tatjana Osipova²,

Nadjezna Borodina² &

German Fershtater²

¹ Deptartamento de Mineralogia y Petrologia, Campus Fuentenueva, 18002 Granada, Spain

² Institute of Geology and Geochemistry, Pochtovy per. 7, Ekaterinburg, 620151 Russia

The late-orogenic magmatism of the Urals is concentrated in the eastern part of the chain and is clearly distinct from early subduction-related calc-alkaline magmatism in the Central zone. Dzhabyk and Murzinka are the largest late-orogenic batholiths in the South and Middle Urals, respectively. They are composite batholiths, formed of several intrusive units mainly composed of felsic, peraluminous biotite and two-mica granites.

Murzinka granites show an extraordinarily large variation in radiogenic isotopes, with from <epsilon>Nd₂₅₅ -12 to +5.2, ⁸⁷Sr/⁸⁶Sr₂₅₅ from 0.709 to 0.704 and Nd model ages from 1900 Ma to 530 Ma. We explain these isotope data by the involvement of different peraluminous melts derived from depleted, geological young orthogneisses and evolved, old meta-sedimentary sources.

The Dzhabyk batholith comprises small amounts of quartzdiorites and quartzmonzodiorites apart from granites. Despite the variation in major and trace elements, Dzhabyk rocks have a surprisingly primitive and uniform Sr and Nd isotope composition, with <epsilon>Nd₂₉₀ from +1.6 to 0.8 and ⁸⁷Sr/⁸⁶Sr₂₉₀ of 0.7037 to 0.7047 in the granite and <epsilon>Nd₂₉₀ from +0.7 to 0.0 and ⁸⁷Sr/⁸⁶Sr₂₉₀ from 0.7046 to 0.7056 in the quartzdiorite units. We propose a homogeneous crustal protolith with a low average crustal residence of about 700 Ma at the time of melting, suggesting fast reworking of a large amount of juvenile material in the mid-Paleozoic and subsequent burial to deeper crustal levels.

Concordant Rb-Sr whole rock and Pb-Pb zircon ages indicate equilibration of the Sr isotopes during the generation of large amounts of granite for both batholiths, with the initial ⁸⁷Sr/⁸⁶Sr ratios most likely representing the average protolith composition. However, Nd isotopes were not generally homogenized, probably due to either slow Nd self-diffusion within the magmas, the involvement of accessory phases or both. The variations in the two batholiths therefore reflect the heterogeneity of the source region.

Continental-type batholiths clearly mark a significant thermal event in the late evolution of the Uralian belt. However, there is no tectonic scenario and no evidence that massive underplating of mantle-derived magmas were involved in the granite generation. Crustal thickening with burial of fertile sources enriched in radiogenic heat production elements and subsequent thermal equilibration are a more likely scenario to explain the crustal melting. This process even better accounts for the northward-decreasing age of formation of the batholith from about 290 to 250 Ma as a result of the oblique collision of the Uralian belt.