Journal of Conference Abstracts

Session L14:1B

L14 : 1B/26 : H2

Defects and Topotactic Reactions of Carpholites from Different Low-Grade Metamorphic Outcrops

David Schmidt (schmidt@crmc2.univ-mrs.fr) &

Alain Baronnet

CRMC2-CNRS, Campus de Luminy, 13288 Marseille, France

Samples from three (Mg,Fe)-carpholite [1,3,4] (high-P, low-T) and one Mn-carpholite outcrops [2] (low-P, low-T) have been investigated by TEM and EMP. Backscattered images revealed that chlorite and mica occurs pseudomorphically after (Mg,Fe)-carpholite surrounded by quartz. However, if the (Mg,Fe)-carpholite is investigated by HRTEM the structure is almost perfect and shows no reaction microstructures except for one individual grain that is in close contact to K-rich mica layers.

In contrast, Mn-carpholite reveals a variable amount of parageneses with sheet silicates as well as abnormal internal structural features:

1) Electron microscopy shows that chlorite and mica mainly precipitate topotactically with their layers parallel to the cleavage planes of Mn-carpholite (Mn-car). The interfaces between the sheet silicates (sh) and Mn-carpholite are tight and largely coherent. The sheet silicates tend to grow with their c^*_sh || to a^*_Mn-car or c^*_sh || to [110]^*_Mn-car and less often c^*_sh || to b^*_Mn-car.

2) Mn-carpholite exhibits locally clustered precipitates (size: 50-100Å) of serpentine-like layers that grew within the negative form of the host crystal. The serpentine-like layers almost precipitate along a^*_Mn-car or b^*_Mn-car. Other mineral structures may be present as precipitates but they could yet not be identified unequivocally.

3) HRTEM of Mn-carpholite revealed that silicate chain-width errors occur analog to those that have been found in pyriboles. These defects can be explained either by the insertion of talc-like slabs that grew successively in [010]-direction or by removing the octahedral walls within the carpholite structure.

We suggest that high-P favors the perfection of the (Mg,Fe)-carpholite crystals similar to other silicates. However, the lack of defects and topotactic growth of sheet silicates with (Mg,Fe)-carpholite may also be due to: (i) the small amount of (Mg,Fe)-carpholite samples investigated by us; (ii) the metastability of (Mg,Fe)-carpholite during retrograde metamorphism that leads to its "immediate" break down if hydration occurs; (iii) the differences in chemistry of the carpholites and the different P-T-t paths (see Ref.) of the carpholite outcrops that may favor the precipitation of serpentine-like layers in Mn-carpholite.

Theye et al., Eur. J. Min, 4, 487-507, (1992).

Theye et al., J. Petrol, 37, 767-783, (1996).

Theye et al., Eur. J. Min, 9, 859-873, (1997).

Weh et al., 3rd workshop on alpine geological studies, Biella, Italy, (1992).

L14 : 1B/27 : H2

Environmental and Local Controls on Oscillatory Mineral Zonation

Bjørn Jamtveit (bjorn.jamtveit@geologi.uio.no)¹,

Terje Holten² &

Paul Meakin³

¹ Department of Geology, University of Oslo, P.O.Box 1047 Blindern, N-0316 Oslo, Norway, Norway

² Department of Geolog, University of Oslo

³ Department of Physics, University of Oslo

Oscillatory mineral zonation is in most cases the result of crystal growth in an open system. Familiar examples are hydrothermal garnets grown in porous or fractured rocks which have been open to large scale fluid flow, and magmatic plagioclase grown during periods of magma mixing or degassing. In most cases such open systems will be out of thermodynamic equilibrium and often they may not even be in a time-invariant or steady state on the time-scales of crystal growth. To interpret an observed oscillatory zonation pattern in terms of the underlying geological processes operating at large or small scales, we need to understand both 1) The crystal growth dynamics and thus its sensitivity to variable boundary conditions (i.e. fluctuations in the environment) and 2) The coupling between the crystal growth processes and processes operating on a scale much larger than the size of the growing crystal. Although numerous crystal growth models have been forwarded that produce oscillatory zoning patterns, we still don't have an adequate understanding of the processes taking place near the crystal surface during the generation of such patterns (i.e. processes responsible for dissipation effects, lattice strain effects, autocatalysis etc.). In this contribution we have analysed the effects of variable boundary conditions on existing crystal growth models. Some of these models are very sensitive even to low amplitude noise, implying that an observed zonation pattern will be significantly affected by other processes than the local growth processes. In some cases fluctuations in the external environment may cause synchronisation so that different crystals develop similar zonation patterns even if their growth is controlled by highly non-linear local dynamics. This implies that intercrystalline similarity in zonation pattern is not always a sufficient argument for external (environmental) controls on the observed patterns. However, with the current lack of reliable crystal growth models, the origin of the patterns may only be deciphered from a detailed chemical characterisation of the zoned crystal, focusing particularly at concentration variations in externally derived components.

L14 : 1B/28 : H2

Garnet Microstructures: Implications for Metamorphic Studies

David J. Prior (davep@liv.ac.uk)¹,

Alan P. Boyle¹,

John Wheeler¹,

Frank Brenker²,

Bill Carlson³,

Christopher Daniel⁴,

Ben Harte⁵,

Dave Hirsch³,

Luca Peruzzo⁶ &

Richard Spiess⁶

¹ Department of Earth Sciences, Liverpool University, L693BX, UK

² Institut für Mineralogie und Geochemie, Universität zu Köln, Germany

³ Department of Geological Sciences, University of Texas at Austin, USA

⁴ Department of Geological Sciences, Princeton University, USA

⁵ Department of Geology and Geophysics, University of Edinburgh, UK

⁶ Department of Mineralogy and Petrology, University of Padua, Italy

Orientation contrast imaging, using forescatter detectors in the Scanning Electron Microscope, facilitates imaging of the internal variations in crystallographic orientations in minerals. Electron Backscatter Diffraction enables the variations to be quantified. Analysis of garnet porphyroblasts from a variety of environments, using these techniques, show that garnets commonly contain internal substructures.

Deformation Microstructures: Deformed mantle nodules and continental eclogites contain garnets with 10 to 50 micron cellular subdomains of different crystallographic orientation. The mismatches between subdomains are small, typically less than 3 degrees. Where misorientation axes can be constrained, they correspond to rational crystallographic zone axes. The subdomains are more clearly developed and smaller where deformation is more intense. The subdomains are interpreted as subgrains. Individual dislocations and dislocation arrays are imaged lending support to this conclusion. Compositional variations correspond to the intensity of garnet substructure.

Nucleation Microstructures: Some amphibolite facies garnets contain substructures that can be related to the coalescence of individual garnets during growth. This is unsurprising. What is surprising is that many of the individual domains have very similar orientations. Misorientations are generally less than 5°. In contrast to the subgrains described above, misorientation axes do not correspond to rational crystallographic zone axes. Coalescence of the nuclei must be controlled by some mechanisms which favoured aggregation of similarly oriented nuclei and destroyed or reoriented strongly misoriented ones and could be driven by some surface energy mechanism. Garnet aggregates show concentric zonation patterns: individual garnets, even though sometimes concentrically zoned, have compositions correspondent to their position in the aggregate.

Growth Microstructures: Amphibolite facies garnets with sector zones defined by inclusions/intergrowths often contain radial crystallographic misorientation boundaries. The misorientation boundaries do not correspond in position to the sector boundaries. The misorientations across the boundaries vary between 1 and 15°. Misorientation axes correspond to rational crystallographic zone axes. These are interpreted as structures related to the propagation of growth defects. The garnet compositions are concentrically zoned with no compositional changes corresponding to misorientation boundaries.

Recognition of widespread garnet substructures has significant implications for the use of garnets in thermobarometry and in radiometric dating. Better understanding of garnet substructures will enable us to extract more information from the textures of metamorphic rocks.

L14 : 1B/29 : H2

Lewisian Granulite-Facies Metamorphism: Regional Event or Local Contact Effect?

J. M. Watkins (J.Watkins@kingston.ac.uk)

School of Geological Sciences, CEESR, Kingston University, Penrhyn Rd, Kingston-upon-Thames, Surrey KT1 2EE, UK

Lewisian grey gneisses are a suite of tonalite, trondhjemite and granodiorite (TTG) rocks similar to those found in other Archaean terranes around the world. The mainland Lewisian is divided into three - the northern, central and southern regions. The northern and southern regions attained only amphibolite grade. It has been assumed that the entire central region was affected by a granulite-facies event, the Badcallian metamorphism (2700 Ma), and that some areas then underwent retrogression to amphibolite facies. P-T estimates for the Badcallian range from 850 to 1050°C and 0.8 to 1.5 GPa depending on the method employed. However, most estimates come from mafic bodies and not the predominant tonalites.

The central region actually displays a range of grades and evidence for varying degrees of partial melting. For example, in some areas the rocks contain a granulite-grade mineral assemblages and cm-scale felsic (presumed melt) segregations whereas, in other restricted areas, the rocks show a totally anhydrous mineralogy and no evidence for the (remaining) presence of melt. In view of the difficulty of retrogressing such rocks in the absence of melt, it is considered unlikely that rocks of this grade once occurred throughout the central region.

The presence of these highest-grade tonalites coincides with the proximity of mafic-ultramafic bodies that could, potentially, be the heat source for locally more intense metamorphism. They have, however, been described as predating the intrusion of the grey gneiss protoliths, although published ages (eg. Cohen et al. 1991) suggest some may post-date emplacement of the tonalites.

New experimental data suggest that T ~ 950 to 1000°C would have been required to 'dehydrate' the protoliths (assumed to be similar to northern-region amphibolite-facies biotite and hornblende tonalites). In view of the restricted occurrence of tonalites that would have exceeded 950°C, and their coincidence with mafic bodies, it is suggested that the Lewisian, already undergoing a regional high-grade event, was invaded by mafic to ultramafic magma, causing local ultra-high-grade metamorphism.

The experimental evidence also suggests that melt fractions would have been small, even at temperatures around 900°C. Since there is evidence for a small amount of melt retention in many areas, it seems probable that there was little melt segregation from all but the highest-grade areas of the central region. Thus, it seems likely that the 'depleted' nature of the terrain was not caused by the loss of a melt phase.

Cohen AS, O'Nions RK & O'Hara MJ, Contrib. Mineral. Petrol, 106, 142-153, (1991).

L14 : 1B/30 : H2

Granulite Facies Metamorphism and Deformation of Metabasites and Meta-Sediments, South Harris, Scotland

Tamsin Lapworth (lt13zx@liverpool.ac.uk),

John Wheeler (johnwh@liverpool.ac.uk) &

Dave Prior (davep@liverpool.ac.uk)

Dept. Earth Sciences, University of Liverpool, Liverpool. L69 3BX, UK.

In the southern part of Harris of The Western Isles, Scotland, meta-igneous bodies intrude a series of meta-sediments. The bodies have elongate outcrop patterns and trend NW-SE. They comprise metagabbros, meta-anorthosite, metadiorite and amphibolite bodies. The meta-anorthosite comprises coarse polygonal plagioclase (An₆₀) plus minor quartz, sphene and apatite with coarse garnet (ald)-cpx bands and schlieren with varying amounts of retrograde hornblende, biotite, prehnite, epidote and opaque. Banding becomes less frequent towards the centre of the body, from 0.5-1 m thick banding at the margins to cm thick bands, and the centre of the body is unbanded. The metagabbros comprise coarse gt (ald)-cpx (salite-augite)-opx-pl (An₆₀)-qtz assemblages. The sediments are highly aluminous and are mostly metapelites with gt-ky-qtz-bi-ksp assemblages. Ca, Al rich and K deficient sediments with pl-hb-qtz assemblages and thin band of forsterite marble also occur.

Granulite facies metamorphism is indicated from thermobarometry and the assemblages. Mineral textures suggest an anticlockwise P-T path evolving from the sillimanite field to the kyanite stability field at peak conditions, with a period of initial isobaric cooling on retrogression.

Deformation prior to metamorphism is indicated by an LS fabric within a small (1.5 km long) granulite body which is overprinted by granulite cpx-gt banding. The nature of such deformation is hard to determine on the limited data available. The meta-anorthosite body has a polygonal texture and lacks fabric, yet fine banding within the meta-anorthosite is isoclinally folded on a cm-m scale. It is possible that such deformation occurred prior to metamorphism, and that any fabric has been overprinted. Crystallographic orientation data may reveal a pre-existing fabric.

Deformation during or after metamorphism was associated with the formation of a steeply dipping NW-SE trending LS fabric in the meta-sediments and the margins of the meta-igneous bodies. Fabric mapping reveals the presence of a km scale steeply NW plunging, tight, antiform in the meta-anorthosite. Banding trends NW-SE, dipping steeply NE and SW either side of the body. The fabric in the nose region of the anorthosite forms successive shear zones, whilst the body margins are stretched; banding is symmetrically boudinaged. This could support folding by shear. Witty (1975) also identifies an antiform, and suggests it is overturned based on grading in banding. Possible grainsize changes during metamorphism and deformation may render grading indeterminate. No evidence has been found to support overturning.

Continued retrogression after deformation is indicated by randomly oriented retrogression products and rounded coronas which crosscut the LS fabric. Arrays of NW-NNW trending cm-m scale shear zone in the meta-sediments and meta-anorthosite crosscut the LS fabric, so post-date large scale shear. They are amphibolite grade and may post-date or reset retrogression.

Witty GJ, Unpublished PhD thesis, (1975).

L14 : 1B/33 : H2

Thermal Modelling, Reaction Textures and Mineral Zoning in Granulites at Rogaland, Southwestern Norway

John C. Schumacher (j.c.schumacher@bristol.ac.uk)¹ &

Mathias Westphal (westphal@eos.ubc.ca)²

¹ Dept. of Earth Sciences, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK

² Dept. of Earth and Ocean Sciences, Univ. of British Columbia, 6339 Stores Road, Vancouver, BC, Canada V6T IZ4

The Proterozoic Egersund Anorthosite Complex is composed of anorthosite and related rock types, which intrude intercalated charnockitic and garnetiferous migmatites. The highest metamorphic temperatures are spatially associated with the intrusion, and previous workers recognized high-temperature mineral isograds for pigeonite-in, osumilite (OSM)-in and orthopyroxene (OPX)-in. The P-T conditions at a distance of 16 km from the contact are about 700°C and 5 kbar and increase to more than 1000°C and 5 kbar at 2.5 km.

The high-temperature metamorphism in the country rocks found near the contacts the Rogaland intrusive complex (surface area of about 1000 square km) cannot be explained by assuming the heat source was a simple, single-phase of intrusion. In order to obtain the observed metamorphic temperatures and isograd distribution, thermal modelling indicates that the heat source must have had at least two main phases that were separated by a hiatus of about 3 - 4.5 m.y. In this model, emplacement and crystallization of the anorthosite (30 x 40 km) heats the region to 600-750°C. A second, smaller intrusion (9 x 12 km) that is present in the region provides sufficient heat to obtain the observed high-temperatures. Recent work indicates that the entire magmatic emplacement occurred over a time interval of about 10 m.y. (930-920 Ma) which is consistent with the thermal model of the metamorphism.

The P-T estimates from 5 and 10 km from the intrusive contact and data from thermal model were used to estimate cooling rates and time intervals. Maximum temperatures are 860°C and 800°C and temperatures greater than 750°C (apparent end of new garnet growth) were maintained for 4.5 to 8.5 m.y. About 5 km from the intrusion calculated post-peak cooling rates indicate a decrease from 30° to 10°/m.y. over about 7 m.y., while at about 10 km calculated post-peak cooling rates decrease from 12° to 7°/m.y. over about 7 m.y.

During this time interval, retrograde and fine-grained Gar-Qz rims formed around orthopyroxene and primary garnet (outermost 150-200 µ) present in assemblages of garnet-quartz-plagioclase-orthopyroxene ± spinel. Based on the widths of preserved zoning profiles in garnet and othopyroxene and diffusion data, estimates of cooling rates that can be made and these agree well with the calculated temperatures and cooling rates from the model.

Where garnet and orthopyroxene are in direct contact, the retrograde exchange may have continued as long as 15-25 m.y., which resulted in more highly developed Fe/Mg-zoning at the rims (outermost 40-240 µ) of both orthopyroxene and garnet.

L14 : 1B/34 : H2

Incongruent Melting of Biotite to Spinel in the Quartz-Free Restites of El Joyazo (SE Spain): Textures and Reaction Characterization

Bernardo Cesare (bernardo@dmp.unipd.it)

Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi, 37, Italy

Hercynitic spinel may be a minor phase in the Bt-Grt-Sil restitic xenoliths of El Joyazo, where it is commonly associated with biotite. The possible reaction relationship among biotite and hercynite was studied in well-preserved and straightforward reaction textures developed around biotite at the contact with patches of fibrolitic sillimanite and melt. In these textures, biotite crystals of about 1 mm show resorbed grain boundaries and are wrapped by a layer of glass <200 µ m in thickness, containing hercynite and ilmenite; the same glass also fills embayments in biotite. Hercynite forms euhedral crystals <100 µ m in size, and ilmenite occurs as smaller anhedral crystals or needles, often intergrown with hercynite. The melt-sillimanite aggregate ("mix" for short) appears like an homogeneous felt, richer in glass close to the reaction rim around biotite. Plagioclase and garnet are located >2 mm away from the reaction texture.Biotite is chemically zoned, the inner part (Bt₁) has X_Fe = 0.65 and Ti = 0.58 a.f.u.; a thin rim (Bt₂) has X_Fe = 0.55 and Ti = 0.64 a.f.u.. The hercynite-rich spinel (Her) has low ZnO (<0.50 wt.%), and X_Fe = 0.74±0.04. The variable chemical compositions of the mix aggregate represent a linear combination between stoichiometric sillimanite and a silica-rich melt. Such melt must be different (lower Si/Al) from that of the layer around biotite (melt), that has a composition at the limit trachydacite-rhyolite. Garnet (Grt) has low Ca and Mn, and X_Fe = 0.86. Plagioclase is characterized by large homogeneous cores (Pl₁, An_31±2) and a thin, more calcic rim (Pl₂, An_49±6).Matrix analysis in the 9-component (Al-Ca-Fe-K-Mg-Mn-Na-Si-Ti), 9-phase (Bt₁-Bt₂-Grt-Her-Ilm-melt-mix-Pl₁-Pl₂) system provides the mass balance: 0.69Bt₁ + 1.28Pl₁ + 3.24 mix + 0.43 Grt = 0.23Ilm + 5.00 melt + 0.22Bt₂ + 2.82Her + 0.64Pl₂. This relationship is considered a good model for the biotite melting reaction, as it is in perfect agreement with the textures observed. The mass-balance indicates that Grt and Pl must be involved in the melting of biotite, so that the equilibration volume is larger that the reaction site. The incongruent melting of biotite, constrained at =850° and =5 kbar, is not a terminal reaction, as its variance in the multisystem is ~3.

L14 : 1B/35 : H2

<alpha>-Quartz and Talc Decompression Domains Inside the Ultrahigh-Pressure 3T-Phengite from Dora-Maira Massive (Western Alps): HRTEM and AEM Data and Petrological Considerations

Cristiano Ferraris (cristiano.ferraris@unifr.ch)¹,

Christian Chopin (chopin@geologie.ens.fr)²,

Bernard Grobety (bernard.grobety@unifr.ch)¹ &

Roland Wessicken (wessicken@solid.phys.ethz.ch)³

¹ Institute of Mineralogy and Petrography, University of Fribourg, Pérolles, CH-1700, Switzerland

² Laboratoire de Géologie de l'Ecole, normale supérieure, URA 1316 du CNRS, Paris, France

³ Swiss Federal Institute of Technology, ETH Hönggerberg, Zurich, Switzerland

High-silica phengite-3T, formed under ultrahigh-pressure conditions (P = 2.7 to 3.7 GPa and T = 1000 K, Schertl et al., 1991) occurs in metamorphic rocks from the Dora-Maira massif, Western Alps, Italy. A systematic HRTEM and AEM study of phengite-3T crystals (K_0.93Na_0.01) (Al_1.44Mg_0.56Ti_0.02) (Si_3.54Al_0.46)O₁₀(OH_1.93F_0.07) were carried out to explain the presence of quartz and talc suggested by recent neutron- and X-ray diffraction and electron-microprobe data obtained from apparently pure crystals (Pavese et al.,1997; Ferraris et al., 1998a, 1998b).

SAED diffraction pattern and HRTEM images show the presence of 100 to 700 Å thick <alpha>-quartz platelets elongated in the cleavage direction of the mica. The platelets have often well defined, rounded to slightly faceted terminations.

In close proximity to the quartz domains, and only there, the 29.7 Å period of the 3T-phengite matrix is faulted by a different type of layer silicate with 18.9 Å periodicity as determined from HRTEM images (Ferraris et al., 1998b). The faulted sections were less stable than phengite under the electron beam and AEM profiles (electron beam diameter= 3nm) taken across the layers showed a slight but statistically significant increase in Mg- and simultaneous decrease in K-concentration relative to the matrix phengite. The observed periodicity and the change in chemistry suggest the faulted sequences to be talc-2M.

Examination by optical microscopy of (001) sections of phengite in petrographic "thick" sections (50-60 µ m thick) reveals the presence of thin platelets, with characteristic "amoebic" contour, interlayered at various depths parallel to (001) of mica, and sometimes containing fluid inclusions showing a mobile vapour bubble (Ferraris et al., 1998b). The low birefringence, low refringence and enhanced Si EDS signal suggest that these platelets are quartz domains similat to those observed by TEM techniques. With the optical microscope, talc can be seen as a coarse-grained primary phase within the matrix and as late lamellae growing on the mica edges, but cannot be detected within the phengite grains.

The absence of deformation features in the phengite matrix around the quartz domains, the strict shape control exerted by the host mica on the quartz crystals and the coeval presence of talc suggest that quartz and talc are the products of a reaction that took place within mica, after the rock had left the coesite stability field, i.e. upon decompression. Taking into account the observed volume ratio between talc and quarz, the following mass-conserving reaction can be written:

3 alumino-celadonite = 1 muscovite + 1 talc + 5 quartz + 2KOH

The above net reaction corresponds to an increase in the muscovite component of the remaining phengite during decompression and is compatible with the trend observed in experiments (Massonne et al., 1989).

Petrographical examination of thick sections of white micas from other well-preserved high-pressure metamorphic rocks (Monte Mucrone and Sesia Zone in Western Alps and from Dabie Mountains in central China) revealed also the presence of similar quartz platelets. The features observed in the Dora-Maira phengites seem, therefore, to be typical decompression textures for white micas in UHP-rocks.

Phengite composition is an important pressure indicator for HP and UHP rocks but the possible presence of submicroscopic quartz and talc platelets has to be kept in mind, when analysing these micas and applying this geobarometer.

Ferraris C & Wessicken R, Suppl. no. 1 Terra Nova, 10, 419, (1998a).

Ferraris C & Chopin C, 17th IMA abstracts vol, A101, (1998b).

Massonne H-J & Schreyer W, Eur. J. Mineral, 1, 391, (1989).

Pavese A, Ferraris G, Prencipe M, Ibberson R, Eur. J. Mineral, 9, 1183, (1997).

Schertl H-P, Schreyer W & Chopin C, Contrib. Min. Petr, 108, 1, (1991).

L14 : 1B/36 : H2

High and Ultrahigh Pressure Metamorphic Overprint in Non-Basic Lithologies, Dabie Shan, China

Franco Rolfo (rolfo@dsmp.unito.it) &

Roberto Compagnoni (compagn@dsmp.unito.it)

Dipartimento Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, 10125 - Torino, Italy

Eclogite-facies metamorphism, traditionally defined in basic rocks, is difficult to study in non-basic lithologies due to easy retrogression of peak assemblages during exhumation. The Dabie Shan in China provides good opportunity to study such non-basic lithologies, since high pressure (HPM) and ultrahigh pressure metamorphism (UHPM) have been found in a number of different rocks, including meta-graywacke, meta-aplite and metamorphic veins.

In the Eclogite Complex of Dabie Shan, the most peculiar lithology preserving UHPM mineral assemblage is a meta-graywacke, which is part of a sedimentary association including marble and eclogite. The graywacke was converted to garnet-jadeite quartzite, with peak assemblage consisting of coesite, jadeite, garnet and accessory rutile, apatite and zircon. Rare zoned glaucophane to Mg-riebeckite porphyroblasts developed just after the peak assemblage. A minimum P of 2.8 GPa and T of 690±30°C have been estimated for the UHP metamorphic peak.

An unusual UHPM assemblage with the kyanite-talc association typical of "whiteschist" has been found in a leucocratric layer, crosscutting coesite-bearing eclogite, interpreted as deriving from a former aplitic dyke. The UHPM mineral assemblage consists of quartz and minor epidote, kyanite, talc and omphacite, with accessory rutile, apatite and zircon. The Fe-rich epidote is strongly zoned with up to 5wt% REE in the core and is crowded with inclusions of quartz after former coesite, minor kyanite, omphacite and rutile. REE- and Sr-rich epidote, a common mineral in felsic lithologies of the Dabie - Su Lu orogen (Nagasaki & Enami, 1998), may be a good marker of the UHPM in retrogressed rocks. A minimum P of 2.6 GPa and a T of 710±20°C have been estimated for the metamorphic peak of the meta-aplite.

Metamorphic veins, consisting of kyanite ± zoisite ± omphacite ± rutile ± apatite ± clinozoisite in a quartz matrix, locally occur in HPM quartz-bearing eclogite. The vein assemblage is considered to have formed during prograde metamorphism from breakdown of lawsonite, just before the eclogitic peak estimated at P of ca. 2.4 GPa and T of 700°C. The presence in the veins of minerals attaining pegmatite-like grain size suggests the local abundance of a wather-rich fluid during the HPM.

In all these lithologies the local occurrence of relic phases, together with a number of symplectites and reaction rims deriving from the breakdown of HPM and UHPM minerals, constrain a clockwise P-T path which is characterized by a post-eclogitic quasi-adiabatic decompression. This kind of retrograde path indicates a very fast exhumation from eclogite- to amphibolite-facies metamorphic conditions, the latter being responsible for the widespread retrogression of HPM and UHPM mineral assemblages.

Nagasaki A & Enami M, Amer. Mineral., 83, 240-247, (1998).

L14 : 1B/37 : H2

Order/Disorder and Thermoelastic Properties of Alpine Phengites by Neutron and Synchrotron Powder Diffraction ­ Some Petrologic Thoughts

Alessandro Pavese (pavese@p8000.terra.unimi.it)¹,

Giovanni Ferraris (ferraris@dsmp.unito.it)² &

Vittoria Pischedda (vittoria@dsmp.unito.it)²

¹ Dip. Scienze della Terra, Univ. Milano, Italy

² Dip. Sci. Mineral. Petrol., Univ Torino, Italy

Micas are sheet silicates formed by stacking one octahedral (O) and two tetrahedral (T) sheets, so as to obtain TOT layers. Order/disorder phenomena and occurrence of one polytype rather than others have been suggested to be related to the environmental conditions (Sassi et al., 1994). The study of the p-V-T equations of state (EOS) of minerals is a very key to interpret petrologic data, to model the interior of the Earth and to provide insights on the thermodinamics and stability of all phases that experience non-ambient thermobaric conditions. Neutron (ISIS, ILL) and synchrotron (ESRF) powder diffraction measurements have been carried out to study Al/Si and Al/Mg ordering and the EOS of phengite-3T from the UHP outcrop of Dora-Maira massif (Italian western Alps; Compagnoni et al., 1995) and phengite-2 M₁ from Val Passiria (Spiess & Bell, 1996). High pressure was achieved by Paris-Edinburgh-like cells; in particular the ESRF experiment was performed along seven isotherms (300 -1000 K) over 0-50 kbar p-range. Main results are: (1) The Dora-Maira phengite-3T shows both tetrahedral and octahedral cationic order up to 1000K. (2) The Val Passiria phengite-2 M₁ shows tetrahedral order at high temperature only. (3) The bulk modulus shows that phengite-3T (about 58 Gpa) is stiffer than muscovite [about 50 GPa; Comodi & Zanazzi, 1995; Vaughan & Guggenheim, 1986]; (4) An isochor with slope about 0.02 kbar/K was calculated from the EOS of the phengite-3T. Apparent disagreement with the thermobaric conditions of 30 kbar and 1000 K experienced by the natural sample (Compagnoni et al., 1995) can be explained both by the obvious fact that the p-V retrograde path not necessarily is along an isochor and by recent results (Ferraris & Wessiken, 1998) showing that nanometric domains of <alpha>-quartz and talc are formed upon decompression from an originally Mg/Si-richer phengite which, therefore, would have a higher bulk modulus, as shown by point (3).

Petrologic thoughts: a) The room-conditions Al/Si disordered phengite-2 M₁ (triggered to order by high T) can be interpreted as a metastable state preserving the situation of the formation conditions (6 kbar and 900 K). b) The higher bulk modulus observed for phengite in comparison to muscovite physically justifies the higher stability of the former at high p. c) Occurrence of cationic ordering in 3T (over a wide T range) but not in 2 M₁ (at room conditions) support Sassi et al. (1994) hypothesis that on retrograde p/T conditions phengite may undergo a 3T -> 2 M₁ order/disorder transition.

Comodi P & Zanazzi PF, Phys. Chem. Mineral., 22, 170-177, (1995).

Compagnoni R, Hirajima T & Chopin C, in Ultra High Pressure Metamorphism, Coleman RG & Wang X Eds, Cambridge Univ. Press, Cambridge, 206-243, (1995).

Ferraris C & Wessiken R, Terra Abs. 1 Suppl. to Terra Nova, 10, 15, (1998).

Sassi FP, Guidotti C, Rieder M & De Pieri R, Eur. J. Mineral, 6, 151-160, (1994).

Spiess R & Bell TH, 8, 165-186

Vaughan, MT & Guggenheim S, J. Geophys. Res, 91, 4657-4664, (1986).

L14 : 1B/38 : H2

Bulk Composition Changes Due to Cooling. The Influence on Retrograde Assemblages

Kurt Stüwe (kurt.stuewe@kfunigraz.ac.at)

Inst. F. Geologie und Paläontologie, Universität Graz, Heinrichstr. 26, Austria

The reaction sequence that a rock can experience during metamorphism is a strong function of the "effective bulk composition" of the rock. This is the bulk composition averaged over the length scale that is in thermodynamic equilibrium at a given time during the metamorphic evolution of the rock. This contribution deals with the changes of this "effective bulk compositon" due to temperature change.

During cooling of rocks, mineral grains may develop zoning profiles as successively larger parts of the grain "close" to the diffusive exchange with the rock. This is a very common observation made in rocks from many metamorphic terrains, but its implication for the retrograde reaction sequence in the whole rock rarely appreciated. One of the consequences of the development of retrograde zoning profiles is that successively larger parts of zoned minerals (depending on grain size) are effectively removed from the reacting part of the rock volume. Thus, the effective bulk composition of metamorphic rocks changes during cooling and the rate of its change will be a function of grain size. The sequence of metamorphic reactions seen by a given rock is a strong function of its bulk composition. Thus, two rocks of identical overall bulk composition, but of different grain size, may experience a different sequence of reactions. Qualitatively identical peak paragenesis may react to form qualitatively different retrograde reaction textures.

The model is applied to examples in the pelitic system. There, garnet is usually the slowest diffusing phase developing zoning profiles during cooling and the effective removal of garnet from the reacting rock volume will cause changes of the effective bulk composition. It is shown that, during cooling of pelitic rocks from amphibolite facies conditions, typical aluminous peak parageneses of garnet-muscovite-kyanite-biotite may react to form either staurolite, chlorite or muscovite (or different combinations thereof), depending on grain size. During cooling from the granulite facies, aluminous peak parageneses of garnet-cordierite-sillimanite may form biotite, either on the expense of cordierite or garnet, also depending on grain size. The two examples are illustrated with a series of reaction textures reported for amphibolite and granulite terrains in the literature.

(This contribution is supported by FWF Project P12846-GEO)

Session L14:1P

L14 : 1P/01 : PO

Very Low-T Metamorphism, Fluid Development and Exhumation History of the Helvetic Alps Along the Lötschberg Exploratory Tunnel

Meinert K. Rahn (rahn@ruf.uni-freiburg.de)¹ &

Joseph Mullis (mullis@ubaclu.unibas.ch)²

¹ IMPG, Albertstrasse 23b, 79104 Freiburg, Germany

² Mineralogisches Institut, Bernoullistrasse 30, 4056 Basel, Switzerland

In the Swiss Helvetic Alps, a 9.5 km exploratory tunnel was drilled to gain detailed geologic information along a planned 32 km Lötschberg railway base tunnel. With the consent of ARGE, the private company in charge, sampling along the tunnel was executed and focussed on critical mineral-bearing greywackes (Taveyannaz sandstone), shales and vein quartz samples across two flysch zones and two Helvetic nappes. In addition to the investigations along the tunnel, several neighbouring drilling cores were sampled. In this study, preliminary results on mineral paragenesis, illite crystallinity, vitrinite reflectance, fluid inclusion thermometry, and fission track (FT) dating on apatites are presented and used to decipher the retrograde P-T-t path. Zircon FT ages turned out to be largely unreset by metamorphism.

At the northern end of the tunnel next to Frutigen, rocks underwent zeolite facies metamorphismm (lmt-pmp-ab-chl). Diagenetic illite crystallinities are in accordance with fluid inclusion data that indicate a transition from higher hydrocarbon-bearing to methane-dominated fluids. This transition occured at a temperature around 200°C. Along the tunnel towards south, methane-rich fluids indicate increasing peak temperature conditions, and illite crystallinities enter anchizonal grade in the Kandersteg area, where vitrinite reflectances indicate medium-grade coalification of semi-anthracitic to anthracitic rank.

Apatite FT ages decrease from 8 Ma in the north to 5 Ma in the south, revealing a considerable amount of post-metamorphic tilting of the Helvetic Alps in a N-S direction. The tilting is the result of an updoming of the western Aar-Gotthard massif in late Miocene.

L14 : 1P/02 : PO

The Very Low Grade Metamorphism in the Brabant Massif, Belgium: A Diastathermal Origin

Françoise Larangé (larange@geol.ucl.ac.be) &

Dominique Laduron (laduron@geol.ucl.ac.be)

Université Catholique de Louvain - Unité de Géologie, Place L. Pasteur, 3, 1348 Louvain-la-Neuve, Belgium

Traditionally, the cause of regional metamorphism as been related to processes of crustal thickening based on modelling within convergent zones. In such models, burial by crustal thickening leads to rapid increases in pressure, followed by thermal relaxation with maximum temperatures attained during uplift and decompression. This results in a characteristic clockwise P-T-t path. More recently regional metamorphism occurring in response to extensional processes has been suggested in sediments deposited within extensional basins formed in the upper crust. The term diastathermal metamorphism was proposed in which lithospheric extension results in an enhanced thermal gradient and heat flow. In this model, strata deposited within extensional basins may be rapidly buried with sharp increases of temperature in relation to pressure, followed by slow cooling but increasing pressure during thermal relaxation and finally rapid cooling during uplift. This model shows an anticlockwise P-T-t path (Warr et al., 1991; Robinson, 1987; Robinson & Bevins, 1989).

Identification of a metamorphic facies series can be made by measurement of the b₀ value, which is an indirect measure of the Mg+Fe²⁺ (celadonite) content in the octahedral position of the white micas. It has been shown that the celadonite content increases with an increase in pressure and Sassi & Scolari (1974) have empirically related b₀ values to differing baric types of metamorphism. However there are various precautions that should be considered when using this method, and selection of samples from the Caledonian Brabant Massif in Belgium for b₀ measurement has been made with regard to the guidelines given by Guidotti & Sassi (1976).

The mean b₀ value of the anchizonal to epizonal samples from the Brabant Massif is 8.996 A, which is indicative of a low-P facies series, and is similar to the mean value for the metamorphism of the Bosost area in the Pyrenees (Sassi & Scolari, 1974) or the metamorphism of the Caledonian Welsh Basin (Robinson & Bevins, 1986). The b₀ data show no significant variation with increasing metamorphic grade or stratigraphic age.

b₀ data indicate a low-P facies series and thus high heat flow which imply high geothermal gradients (c. 50° C km^-1). The general features of the metamorphism in the Brabant Massif as described in Van Grootel et al. (1997) are in accord with the diastathermal model of metamorphism suggested by Robinson (1987) and Robinson & Bevins (1989) in which high heat flow is linked to lithospheric extension leading to very low grade metamorphism with an anticlockwise P-T-t path.

Guidotti CV & Sassi FP, N. Jb. Mineral. Abh, 127, 97-142, (1976).

Robinson D, Geology, 15, 966-969, (1987).

Robinson D & Bevins RE, Earth and Planetary Science Letters, 92, 81-88, (1989).

Sassi FP & Scolari A, Contrib. Mineral. Petrol, 45, 143-152, (1974).

Van Grootel G, Verniers J, Geerkens B, Laduron D, Verhaeren M, Hertogen J & De Vos W, Geol. Mag, 134(5), 607-616, (1997).

Warr LN, Primmer TJ & Robinson D, J. metamorphic Geol, 9, 751-764, (1991).

L14 : 1P/03 : PO

Incompatibility of Illite Crystallinity and Metamorphic Grade as Defined by Subgrain Boundaries Produced by Tectonic Stress: A TEM Study of Low Grade Metasediments from Antarctic Mountains

Isabella Memmi (memmi@dst.unisi.it)¹,

Giovanna Giorgetti (giorgett@dst.unisi.it)¹ &

Donald R. Peacor (drpeacor@umich.edu)²

¹ Dipartimento di Scienze della Terra, via laterina 8 - Siena, Italy

² The University of Michigan, U.S.A.

Illite crystallinity (IC) is considered to be primarily a function of illite crystallite size which is in turn a direct function of metamorphic grade (temperature). Where other geological parameters such as tectonic stress affect IC however, conclusions regarding grade may be in error. In order to clarify the effect of tectonically-induced strain on IC, a combined XRD and TEM study has been carried out on low-grade metapelites belonging to three tectonic units in the Transantarctic Mountains which have undergone complex deformation. These include the Bowers Terrane (BT), the Robertson Bay Terrane (RBT), and the Millen Schist (MS). IC indices decrease westward from the Bowers Terrane (average value 0.24°  2<Gamma>) to the Robertson Bay Terrane (average value 0.21°  2<Gamma>) .The lowest IC values occur for the Millen Schist (average value 0.19°  2<Gamma>), which has been interpreted to be a shear zone juxtaposing BT and RBT. In fact, BT and RBT show a penetrative slaty cleavage, whereas MS shows two superimposed axial plane foliations. The mean illite-muscovite crystallite size measured on TEM images increases from BT (ca 500 Å) to MS, with MS and RBT rocks showing comparable results (ca 650 Å). All crystal size histograms are bell-shaped and skewed towards thickness values greater than 2000 Å, which is a sizes compatible with values typical of the epizone, contrary to conclusions based on IC data. TEM images showed that strain features in illite-muscovite crystals, e.g., kinks, microfolds, and sub-grain boundaries, are far more abundant in BT than in RBT and MS. High-angle sub-grain boundaries within former single-crystals (originally thicker than 1000 Å) result in decrease in effective crystal size as measured by XRD. This effect is greater in the BT sample and is consistent with its larger IC value. In the MS and RBT samples lattice strain is lower and the resultant mean crystal size is higher. These results indicate that strain-induced reduction in crystal size resulting from intracrystal slip and sub-grain formation was significant in BT samples. On the other hand, microtextures within the shear zone suggest recovery/recrystallization of subgrain boundaries, resulting in the smallest IC values.The TEM images reveal that metamorphic grade was approximately equal and corresponding to epizone grade in all three formations, but that anomalously high IC values in the BT samples are determined by strain-related crystal size reduction, and those in the MS samples by recovery of such strain. These data demonstrate that tectonic stress-induced strain may result in anomalous IC values relative to metamorphic grade. IC values should be treated with caution, therefore, especially where there is evidence of deformation.

L14 : 1P/04 : PO

New Textures in Antigorite-Serpentinites and Their Relationship with the Antigorite Structure: A TEM Investigation

Bernard Grobety (bernard.grobety@unifr.ch)

Institute of Mineralogy and petrography, University of Fribourg, Pérolles CH-1700, Switzerland

The originality of the structure of the serpentine mineral antigorite (Kunze, 1961) is the regular corrugation of the TO-layers. The tetrahedral layer is always on the convex side of the corrugated octahedral sheet and consequently flips at each inversion point to the adjacent octahedral layer above or below. In order to get a continuous structure, the apical oxygens of the silicon tetrahedra, which flip at each reversal, have to conicide with one of the hydroxyl groups of the adjacent brucite-like layer. For a given wave curvature, a parameter depending mainly on temperature (Mellini et al., 1987), exact coincidence and, therefore, inversion of the wave, is only possible after a certain number n of tetrahedra or multiples thereof. It is possible that the flipping of a tetrahedron, for which coincidence would be achieved, is hindered because TO-layer growths faster or slower, than neighboring layers. The next possible (higher order) inversions are after <kappa> x n tetrahedra, with <kappa> = 2,3,4.... and the aperture angle of the higher order half-waves will be multiples of the basic aperture angle (<kappa> = 1) of 21°. The quantization of the wavelength should become less strict for long wavelengths for which flipping coincidences are possible for a range of n. Antigorite intergrowths found in ophicarbonates from St. Denis (Aosta Valley, NW Italy) and Val Ventina (Valtellina, NE Italy), are the first examples of retardation of tetrahedral flipping. Antigorite aggregates show a systematic topotactical relationship with parallel b-axes and an angle of <kappa> x 21° between the basal planes of adjacent grains. This angular relationship is compatible with a higher order wave connecting the neighboring grains. An epitaxial overgrowth of adjacent sectors of polygonal serpentine can be excluded, because the expected angle for such antigorite pseudomorphs is 12° or 24°. The basal plane of antigorites nucleating on, or replacing tremolite and chlorite in ophicarbonate from Val Ventina form the same angle of 21° with the (010) plane of tremolite and the (001) plane of chlorite. The first wave of the nucleating antigorite crystal suffered, thus, a retardation of the inversion leading to the observed orientation relationship. Antigorites with wavelengths greater then 10 nm present in the same sample show slight changes of orientation of the basal plane that coincide with the location of the inversion points of the wave. This is a characteristic sign for inhomogeneous flipping of tetrahedra, as predicted by Kunze (1961) for long wavelength antigorites.The serpentine texture found in Alpine ophicarbonate strengthen further the antigorite structure model proposed, but not fully refined, by Kunze (1961).

Kunze, G, Fortsch. der Min, 39, 206-324, (1961).

Mellini, M, Trommsdorff, V, and Compagnoni, R, Contrib. Mineral. Petrol, 97, 147-155, (1987).

L14 : 1P/05 : PO

Synmetamorphic Veining in Low-Grade BS Metapelites

Philippe Agard (agard@geologie.ens.fr) &

Bruno Goffe (bruno@geologie.ens.fr)

Lab. de geologie de l'ENS, 24, rue Lhomond, Paris, 75231

The fact that metamorphic veins trace major sites of fluid-rock interaction and may represent fluid pathways has led to considerable work on fluid circulation in the crust, in view of constraining the way volatiles escape metamorphosing rocks. Besides, metamorphic veins appear essential in understanding the amplitude and extent of mass transfer during metamorphism. Yet, the complex interplay between metamorphic (devolatilization) reactions and deformation in triggering veining events was given comparably little attention.

In the following, we present new data pertaining to the mechanism of vein formation in the pervasively strained blueschist-facies (BS) metapelites from the french-italian Western Alps (Schistes lustrés unit, SL) marked by a large fluid budget, - since dense hydrous phase form during the prograde evolution (e.g. carpholite ; prograde metamorphic reactions are water-consuming, unlike in Barrovian metamorphism) and water retentivity is favoured by the almost isochoric prograde path (water-rich fluids) - which show marked differences between BS and greenschist (GS) segregations.

The numerous BS carpholite-quartz-bearing shear veins occur in blackschists as an evenly spaced network (one vein every ca. 50 cm), with up to 1 m large clusters of rotational, boudinage veins, and exhibit the same mineralogy as the wall rock. Incremental formation of the BS veins in response to deformation indicates that the crystallisation of carpholite (i.e. transferring elements to the vein) and subsequent crack sealing were relatively rapid when compared to the rate of progressive deformation. Vein products are shown to have formed from chlorite and white micas through a prograde reaction requiring the presence of a limiting water-rich phase (chlorite-whitemica-water ratio around 1:4:4). In contrast, quartz-calcite±chlorite GS veins are mostly extensional veins formed by hydraulic fracturing related to the breakdown of hydrous phases.

It is suggested that BS veining is triggered by continuous ductile deformation leading to porosity enhancement. Provided an intergranular fluid phase is present in the vicinity of the fracture once nucleation started, element transfer will be maintained by diffusive transport due to existing chemical potential gradients. In the case of the SL metapelites, mass transfer probably occurs at the mesoscopic scale at the most (migration of species < ca. 1 m) and the propagation of the fluid is likely to be limited since the fluid phase is rapidly used up by the crystallisation of hydrous minerals. It is proposed that BS veins reflect local (mesoscale) combined petrological and tectonic processes rather than effective fluid circulation. In the light of such an interpretation, BS vein formation could span the entire period of deformation, as long as rocks remain in the stability field of carpholite.

L14 : 1P/06 : PO

Origin of Amphibolites and Amphibole-Bearing Schists of Crystalline Complex of Moslavacka Gora (Croatia)

Drazen Balen (drbalen@public.srce.hr) &

Darko Tibljas

Institute for Mineralogy and Petrology, Department of Geology, University of Zagreb, Horvatovac bb, HR-10 000 Zagreb, Croatia

Amphibolites and various amphibole-bearing schists occur as large and small bodies and lenses within polymetamorphic amphibolite facies rock complex of Moslavacka gora (Croatia). Amphibolites are closely associated and intercalated with different types of gneisses, mica-schists, cordierite schists, hornfels and marbles beeing anclaved in pre-Alpine(?) andalusite/sillimanite bearing granitoids. The crystalline complex is strongly folded and tilted and shows a fairly significant migmatization.

Amphibole bearing schists containing zoned amphibole posses layered structure ("sandwich") with three different mineral associations: garnet association in "centre" of the rock (grossular-clinozoisite-clinopyroxene-plagioclase), surrounded by the clinopyroxene association (clinopyroxene-plagioclase-titanite) and embedded in the amphibole association (hornblende-plagioclase-clinopyroxene- ilmenite-quartz). A polymetamorphism is characterised by first stage metamorphic event with paragenesis metamorphosed with geothermal gradient 28-35°C/km. In the second metamorphic event intrusion of granite body caused HTLP metamorphism with geothermal gradients over 60°C/km. The pneumatolysis associated with granite pegmatites occurred in third phase. Changes in P-T conditions are recorded in the growth of amphibole grains (general prograde metamorphism) and changes in modal compositions of related parageneses.

Due to their geochemical features the amphibole-bearing rocks could be important for the correlation with other rocks of crystalline basement of Pannonian Basin in general, and especially for those from the Tisia unit. Distribution of main constituents together with distribution of minor and trace elements, petrographic data (relict textures) as well as field features observed in rock-columns, suggest that protholite of amphibole-bearing rocks was:1) basalt of oceanic tholeiite affinity for orthoamphibolites;2) marls or tuffs alternating with other calcareous metasediments for paraamphibolites.

This magmatic-sedimentary complex might have been partly generated in an oceanic realm and partly in arc trench and than metamorphosed in a presumed subduction zone. However to thoroughly validate this model more data are required to confirm especially the relationship between the P-T increments and the time scale.

L14 : 1P/07 : PO

Evolution of Microstructure with Natural Deformation of Clinopyroxene from the Monviso Eclogite Shear Zone

Alexandra Mauler (alexm@erdw.ethz.ch)¹ &

Karsten Kunze (kunze@erdw.ethz.ch)²

Geological Institute, ETH Zentrum, Zurich, 8092, Switzerland

Microfabric analysis of eclogites can be used to characterize the deformation parameters and events that have been operative in deep parts of subducted slabs. As garnet is known to act as a rigid body, attention has to focus on the deformation mechanisms of omphacite, the major stress-supporting mineral in eclogites. Four samples displaying different strain intensities were collected in a well-preserved shear zone (Monviso meta-ophiolitic complex, Western Alps) that formed during eclogite facies metamorphism (P>10 kbar, 400<T<550°C). All the samples contain the mineral assemblage omphacite, garnet, rutile and quartz. Incipient retrogressive metamorphism is indicated by minor amounts of amphibole, plagioclase, epidote and white mica. Evidences of intracrystalline deformation such as subgrains, undulous extinction and deformation lamellae are unevenly present in the four samples. In the low deformed sample, recrystallization started at the boundaries of large relict grains (500µ m - 2 mm), and narrow bands of recrystallized grains crosscut these porphyroclasts. In the more strained sample, both types of omphacite are recognized but recrystalllized grains form a matrix representing most of the rock. An average recrystallized grain diameter of 30-80µ m is characteristic of this sample. The higher strain samples consist almost completely of recrystallized grains exhibiting an elongate shape fabric and straight grain boundaries. Smaller grain sizes are observed in these samples (the grain diameter is about 10-30 µ m in the third sample and 20-60µ m in the most deformed one). The latter sample also presents frequent atoll-shape garnets, which include omphacite grains of a higher average diameter than the matrix.Electron backscatter diffraction (EBSD) was used to measure omphacite crystallographic preferred orientations (CPO). In both the porphyroclasts and the recrystallized grains of the low-strained samples, [001] is parallel to the lineation while (010) has a dominant orientation parallel to the foliation plane. Omphacite inclusions in garnets do not show any CPO. Higher strained samples display slightly weaker fabrics with [001] progressively dispersed within the foliation plane while (010) remains parallel to foliation. The omphacite isolated in the hollow garnet cavities present a similar yet weaker CPO as the matrix of the same sample. The observed trend towards an increasing homogeneous microstructure with smaller grain size will be discussed in terms of deformation mechanisms and associated growth and recrystallization processes.

L14 : 1P/08 : PO

A Peculiar Ms-Pg Association in a Chloritoid-Bearing Micaschist

Claudio Mazzoli (claudio@dmp.unipd.it)¹,

Raffaele Sassi (raffaele@dmp.unipd.it)¹,

Alan Baronnet (baronnet@crmc2.univ-mrs.fr)² &

David Schmidt (schmidt@crmc2.univ-mrs.fr)²

¹ Dept. Mineralogy and Petrology, University of Padova, C.so Garibaldi 37 - Padova, Italy

² CRMC2, Campus de Luminy, 13288 Marseille Cedex 9, France

A peculiar association of muscovite (Ms), paragonite (Pg) and chloritoid (Cld) occurs in a porphyroblast-bearing Ms and garnet (Grt) micaschists from the Greiner Unit outcropping north of the Lago di Neves (Eastern Alps, Italy). Two different metamorphic stages can be recognised in this rock: 1) a syn-kinematic crystallisation of Cld and Ms along an early foliation S₁; 2) the development of a crenulation cleavage (S₂) associated with the growth of chlorite (Chl) and porphyroblastic Grt and Ms along S₂. Both Grt and Ms porphyroblasts show inclusion trails of Cld, quartz and ores, which are parallel to S₁. Furthermore, Ms porphyroblasts include Pg polygranular domains, which are also parallel to S₁ and display a rectangular stumpy shape in thin sections. When identified as inclusions in Ms by BSE and PLM images, these Pg domains elongate nearly perpendicular to the Ms (001) basal plane while adopting almost the same cleavage trace. The shape and size of these large Pg domains are similar to those of the Cld crystals occurring in the rock matrix. An HRTEM/SAED/AEM study of the Ms/Pg single crystals indicate that: 1) both are 2 M₁ polytypes in structural continuity; 2) the Na-rich Ms and to a lesser extent the K-rich Pg are structurally and compositionally modulated along the layers as inferred from layer-thickness variations; 3) each type of coherent Ms or Pg domain minimises total elastic strain through aligning inclined to (001); 4) the angle between [00l]*Pg and [00l]*Ms varies from 0 to 5° to reabsorb the layer-thickness misfit. Such resulting "tweed texture" suggests a further, spinodal-like decomposition inside the Ms-Pg solvus, also related to the local destabilisation of Cld.

L14 : 1P/09 : PO

Computer Method to Determine 3D Preferred Orientation of Relictual Foliations in Regional Porphyroblast Populations and Application to the Variscan Arc of NW-Iberia

Domingo Aerden (aerden@gugu.usal.es) &

Javier Sanchez San Roman (javisan@gugu.usal.es)

Departamento de Geologia, Univesridad de Salamanca, 37008 Salamanca, Spain

A preferred orientation of a relictual foliation (PORF) included by a regional porphyroblast population can, in principle, be determined by combining the (2D) preferred orientations of inclusion trail lines as measured in differently oriented thin sections. A problem arises, though, when porphyroblasts of different timing in a sample include different foliations, or the same foliation in specific orientations. In other words, multiple PORF planes can be associated with successive mineral growth phases in a sample, and these cannot be reliably discriminated if porphyroblasts have similar compositions and their internal foliations are discontinuous with external ones. To get around this problem, a statistical method was developed that resolves up to three different PORF planes in a sample through best-fit calculations for inclusion trail data from 7 differently oriented thin sections per sample. A FORTRAN program (FitPitch) compares inclusion trail data in all thin sections with the theoretical intersection lines of those sections with a very large number of possible plane combinations, and finds the particular combination that minimizes the difference between actual inclusion trails lines and theoretical intersections. Calculations for single, two and three plane combinations are performed separately, after which the success of each option is assessed by comparing standard deviations, and by checking 2D rose diagrams for inclusion trails on the presence of single, bimodal, or trimodal orientation maxima. Best-fit calculations for 20 samples covering 300 km distance in a particular tectonic unit of NW-Iberia revealed two interesting features: 1) PORF planes exhibit a distinctive preference for subvertical or subhorizontal positions. 2) PORF planes define two distinct spatial foliation fans with horizontal but differently oriented fan axes. The orientations of these fans were quantified using a second computer program (Aerden & Sanchez San Roman, 1998) that fitted the poles of all PORF planes to two planes. The computer results are fully consistent with results obtained with an existing technique to determine relative matrix-porphyroblast rotation axes (Hayward, 1990). The latter match the two foliation fan axes. Qualitative microstructural analysis demonstrates a different age of porphyroblasts defining both foliation fans and, consequently, a change in the direction of regional tectonic transport is inferred. PORFs and relative rotation axes are remarkably constant in a major oroclinal bend of 90° in the study area despite a younger age of this macrostructure. Their regional consistency and marked preference for subvertical and subhorizontal orientations suggests that PORF planes record ancient, pre-orocline orogen trends, and reflect an orogenic evolution characterized by alternating compression and extension. Similar conclusions are systematically indicated wherever this type of data has been collected to date in Australia, China, Europe and North America (Belll & Hickey, 1997; Aerden, 1998, and references in both works).

Aerden DGAM & Sanchez San Roman J, Computers & Geoscs, in rev

Hayward N, Tectonophysics, 179, 353-369, (1990).

Bell TH & Hickey KA, Tectonophysics, 274, 275-294, (1997).

Aerden DGAM, Tectonics, 17, 62-79, (1998).

L14 : 1P/10 : PO

Origin and Trace Element Composition of Poikiloblastic Na-Ca Amphibole in Continental Eclogites (Malpica-Tui, NW Spain)

Javier Rodríguez (npbroalj@lg.ehu.es)¹,

Riccardo Tribuzio (tribuzio@crystal.unipv.it)²,

José Ignacio Gil Ibarguchi (nppgiibi@lg.ehu.es)¹,

Bruno Messiga (messiga@crystal.unipv.it)² &

Gisella Rebay (rebay@crystal.unipv.it)²

¹ Departamento de Mineralogía y Petrología, Universidad del País Vasco, Apdo. 644, E-48080 Bilbao, Spain

² Dipartimento di Scienze della Terra, Università di Pavia, via Ferrata,1 I-27100 Pavia, Italy

Meter size boudins within variably deformed metagranitoids of the Malpica-Tui allochthon (Iberian Massif, NW Spain) record ductile deformation under eclogite-facies conditions. Eclogites have a heterogeneous fabric; the mylonitic foliation is defined by shape preferred orientation of matrix omphacite, amphibole, epidote, white mica and rutile. On the basis of microstructural investigations and major and trace element microanalysis, the mechanisms of amphibole development and its role in the trace element whole-rock budget have been unravelled.

The microstructural study reveals a complex recrystallization history involving various steps of amphibole growth from different types of omphacite-rich domains. Porphyroclastic omphacite (Omp1) mimetically replaced igneous Ti-rich Cpx. Finer grained, omphacite neoblasts (Omp2) dynamically grew after Omp1 and were also aligned in surrounding matrix. Single grain, patchy replacement of Omp1 by amphibole at cores (Amp1) may evolve into colourless to bluish poikiloblasts (Amp2) with inclusions of Omp1+Grt+Czo at the final stage. A similar growth process is inferred to have occurred between Omp2 and Amp2 poikiloblasts which are commonly oriented parallel to Omp2 lineation and enclose matrix garnet and rutile. The amphibole poikiloblasts are frequently thin-rimmed by green amphibole (Amp3).

Omp1 and Omp2 are nearly indistinguishable for major element chemistry, but exhibit distinct trace element features. Omp1 has relatively high concentrations of HREE, Y, Sc and Ti, which may be ascribed to a geochemical signature of the pristine igneous clinopyroxene. Omp2 shows a bell-shaped REE pattern, which attests the achievement of chemical equilibrium with matrix epidote and garnet, which respectively behave as a repository for LREE and HREE, in high diffusion domains.

The REE pattern of Amp1 (Mg-hornblende to barroisite) and Amp2 (barroisite) probably reflects that of parent clinopyroxene, i.e. amphibole is bell-shaped when developing after Omp2, and slightly HREE-enriched when replacing Omp1. Amp3 barroisitic rims, with higher pargasite and lower glaucophane substitutions, have higher Sc, Y, HREE, Ba and K than the cores (Amp2). These relatively high concentrations may be ascribed to the breakdown of garnet and white mica. Further, Amp3 has markedly lower Sr than Amp2 and no significant LREE variations, thus suggesting that its growth was accompanied by the development of feldspar in the presence of epidote.

Microstructural and chemical data reveal that amphibole formed at the expenses of omphacite under short range element diffusion, most likely in the garnet stability field. Late amphibole rims probably developed under lower pressure conditions, possibly in the epidote-amphibolite facies. On the basis of mass balance considerations, amphibole is inferred to have a minor role in the REE and HFSE budget of these eclogites.

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High Temperature Oxide Melt Solution Calorimetry of MgMgAl-Pumpellyite

Klaus-Dieter Grevel (klaus-dieter.grevel@ruhr-uni-bochum.de)¹ &

Alexandra Navrotsky (anavrotsky@ucdavis.edu)²

¹ Ruhr-Universität Bochum, Institut für Mineralogie, D-44780 Bochum, Germany

² Thermochemistry Facility, Dept. of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, U.S.A.

The Mg analogue of CaMg-pumpellyite, MgMgAl-pumpellyite, ideally Mg₅Al₅Si₆O₂₁(OH)₇, is considered to be a possible H₂O-containing phase in cold subducting slabs (Schreyer et al., 1991). Recently, Fockenberg (1998) investigated the stability relations of this phase in the chemical system MgO-Al₂O₃-SiO₂-H₂O while preliminary thermodynamic data were estimated by Massonne (1995). In order to check these estimates and to be able to provide reliable thermodynamic calculations, a project was started to carry out measurements of the basic thermodynamic data of MgMgAl-pumpellyite.

Using about 200 mg of a stoichiometric gel as starting material a pumpellyite sample was synthesized at 5 GPa and 700°C in a piston cylinder apparatus. The chemical composition of the run product was determined on an electron microprobe; the water content of the sample was investigated by Karl-Fischer-titration. The resulting MgMgAl-pumpellyite with the formula Mg_4.93Al_5.15Si_5.92O₂₁(OH)₇ was accompanied by traces of enstatite (less than 2 wt.%).

By using high temperature oxide melt solution calorimetry (Navrotsky, 1997) the enthalpy of dissolution in lead borate (2PbO*B₂O₃)x at T = 973.17 K of this sample was measured. Additionally, similar measurements were performed for brucite (Mg(OH)₂), quartz (SiO₂), and corundum (Al₂O₃).

These quantities as well as  _fH⁰(enstatite) (Berman, 1988) were then utilized to obtain the enthalpy of formation from the elements for MgMgAl-pumpellyite: -13794.9 ± 16.7 kJ/mol.

This value is very similar to a value obtained by regression analysis of the reaction 6 MgMgAl-pumpellyite = 10 pyrope + 5 kyanite + coesite + 21 H₂O (Fockenberg, 1998). Using Berman's internally consistent data base (Berman, 1988) for the product phases and Massonne's (1995) estimates for the heat-capacity and P-V-T behavior of pumpellyite, the following values could be calculated:

 _fH⁰(MgMgAl-pumpellyite) = -13797.1 kJ/mol,

S⁰(MgMgAl-pumpellyite) = 711.7 J/(mol*K)

On the other hand the standard enthalpy of formation of this phase estimated by Massonne (1995), -13893.5 kJ/mol, is approximately 100 kJ/mol different to our values.

Additionally, P-V-T data of MgMgAl-pumpellyite will be determined using powder X-ray diffraction methods at high temperatures and pressures (MAX-80, HASYLAB, Hamburg).

Berman RG, J Petrology, 29, 445-522, (1988).

Fockenberg T, Am Mineral, 83, 220-227, (1998).

Massonne H-J, Ultrahigh Pressure Metamorphism (Coleman RG & Wang X, Eds. ), Cambridge University Press, U. K, 33-95, (1995).

Navrotsky A, Phys Chem Minerals, 24, 222-241, (1997).

Schreyer W, Maresch WV & Baller T, Progress in Metamorphic and Magmatic Petrology (Perchuk LL, Ed.), Cambridge University Press, U. K, 47-64, (1991).

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Determination of Balanced Reactions for the Lower PT Overprint of UHP Metamorphic Rocks from Lago di Cignana (Western Alps, Italy)

Sebastiaan van der Klauw

(klauw@geo.uni-jena.de)

Institut für Geowissenschaften der, Friedrich Schiller Universität Jena, Burgweg 11, 07749 Jena, Germany

At Lago di Cignana (Western Alps, Italy) metabasic rocks crop out that experienced ultra high pressure (UHP) metamorphism. The UHP mineral assemblage in these rocks is at least partly overprinted by minerals stable at lower PT conditions (omphacite is partly transformed to a symplectite of blue green amphibole and albite, garnet is rimmed by a different blue green amphibole, glaucophane is rimmed by yet another blue green amphibole, rutile reacted to ilmenite and titanite and paragonite is overgrown by new clinozoisite). The rocks with the smallest amounts of replacement products (15 to 20 vol.%) have very similar amounts of all replacement products. Similar amounts of some of these replacement products are also observed in more completely overprinted rocks. This indicates that a first stage of replacement reactions occurred in all rocks and later replacement stages were localised. Only this first stage of lower PT overprint will be discussed here.

In the rocks that experienced only the first stage of overprinting the replacement minerals are restricted to domains with a maximum size of one square mm in thin section. These domains are not connected, but all have similar relative amounts of the replacement products. This suggests that in all domains the same reaction was responsible for the formation of the replacement products. Assuming that during this reaction the domain was a closedsystem, a balanced reaction must have existed between product and educt phases that produced the observed relative amounts of product phases.

Decomposing the matrix consisting of the compositions of product and educt phases in the system SiO₂, TiO₂, Al₂O₃, FeO, MgO, MnO, CaO, K₂O, Na₂O, H₂O, using singular value decomposition, gives two linear independent reactions between the phases. A combination of these reactions can however not produce the observed relative amounts of product phases. Inclusion of other phases in the matrix (e.g. water or quartz) or exclusion of components present in minor amounts shows that balanced reactions with the observed amounts of reaction products can only be obtained with quartz as additional phase.

This indicates that the initial replacement of the UHP mineral assemblage started in domains where quartz was available and ceased when quartz was no longer available. This inference is supported by the observation that quartz is rare in the metabasic rocks and if present always surrounded by a rim of lower PT replacement products.

Although quartz is no longer available in most reaction domains, the necessity of quartz in a balanced reaction between product and educt phases shows that it must have been present. The inferred presence of quartz allows the use of geothermo- and geobarometers that require the presence of quartz to estimate PT-conditions for the onset of this lower PT-replacement.

L14 : 1P/13 : PO

Mechanisms of Retrogressive Dolomitisation of Marble, SW Highlands, Scotland

Robert Guest (R.Guest@earth.leeds.ac.uk)¹,

Andrew McCaig (A.McCaig@earth.leeds.ac.uk)¹ &

Colin Graham (cgraham@glg.ed.ac.uk)²

¹ School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK

² Department of Geology and Geophysics, The University of Edinburgh, Grant Institute, West Mains Road, Edinbugh, EH9 3JW, UK

Retrogressive fluid infiltration in the Loch Tay Limestone, SW Scottish Highlands, resulted in dolomitisation of calcite-rich marble layers and ¹⁸O enrichment in dolomite (Fein et al., 1994). Coincident dolomite reaction fronts and oxygen isotopic fronts propagated into and along calcite-rich marble layers in wall rock away from fault/veins suggesting that oxygen isotopic exchange between fluid and the host marble was facilitated by the replacement of calcite by dolomite. Retrogression under lower greenschist facies conditions was driven by the infiltration of high  ¹⁸O, low  ¹³C brines (Fein et al., 1994). Textures at reaction fronts revealed by cathodoluminescence imaging include mottled interiors of calcite grains and zoning of calcite grains adjacent to grain boundaries. Electron probe analyses of these zones shows small (0.8 wt.% FeCO₃) increases in Fe content compared to the centres of calcite grains. Initial formation of dolomite occurred principally at calcite-calcite grain boundaries and calcite triple junctions; dolomite is also found forming in the interiors of calcite grains apparently by dissolution-reprecipitation. This suggests that grain boundaries were a pathway for dolomitising fluid to migrate away from faults and veins, and that this fluid migration altered the chemical composition of the calcite grains and generated porosity. In addition, there was alteration of the interiors of calcite grains prior to or during dolomitisation. The dolomite that replaces the calcite exhibits oscillatory zoning from an Fe:Mg ratio of 0.09 to ankerite with an Fe:Mg of 0.8 at the rim. The fluid infiltration may therefore have been pulsed. The centres of some dolomitised areas show patches of ankerite with comparable Fe-compositions to ankerite at the interface with calcite. These ankeritic patches appear to have replaced initial dolomites with lower Fe-contents. This suggests late permeability enhancement by dolomite dissolution. In phyllitic layers, which alternate with marble layers, fluid has apparently exploited deformed zones, which are now filled with dolomite. Dolomitisation appears to have occurred further from the fault/vein in these phyllitic layers than the adjacent marble layers. The phyllitic layers therefore provided a pathway for fluid to interact with the marble beyond the dolomite reaction front in the adjacent marble layers. This indicates that some mass transfer has occurred across the layers in addition to layer parallel mass transfer. It may be that phyllitic layers acted as foci for fluid flow in addition to fault zones. This study shows that permeability was controlled by a dissolution reprecipitation mechanism that operated both along grain boundaries and within the interiors of calcite grains and along discrete fractures. Chemical reaction can be interpreted in terms of a balance between fluid supply and dolomitisation reactions.

Fein JB, Graham CM, Holness MB, Fallick AE & Skelton ADL, J. metamorphic Geol, 12, 249-260, (1994).

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⁴⁰Ar/³⁹Ar Age Constraints for Metamorphic Rocks of the Yuzhno-Chuyskii Complex (Southern Gorny Altai, Russia)

A. V. Plotnikov (plotn@mx.uiggm.nsc.ru)¹,

P. Monie (monie@dstu.univ-montp2.fr)²,

A. V. Titov,

N. N. Kruk &

A. G. Vladimirov

¹ United Inst. Geology, UIGGM SB RAS, Novosibirsk, Russia

² Lab. Geochronology, Univ. Montpellier, Montpellier, France

The Yuzhno-Chuyskii range is a collisional complex exposed at the northern margin of the Altai-Mongolian microcontinent and formed during the Middle-Paleozoic accretionary events. It is an E-W trending massif (~20x100 km) that consists of several tectono-metamorphic zones, surrounded by sedimentary rocks of Vendian-Cambrian age. Five metamorphic zones have been recognized that are interpreted to record the succession of two main tectonometamorphic events. During an earlier stage of accretion and collision with the Siberian continent, a kyanite-sillimanite type of metamorphism (P=5-7 kbar, T=580-670°C) developed contemporaneously with a strong ductile deformation marked by N-S trending mineral lineations and sublatitude folding. This early stage is mainly preserved in the western part of the Yuzhno-Chuyskii complex. It has been overprinted by an andalusite-sillimanite HT/LP type of metamorphism (P=1.5-3.5 kbar, T=500-680°C) in the eastern part of the ridge, associated with E-W dextral strike-slipfaulting.Geochronological investigations have been initiated using laser-probe ⁴⁰Ar/³⁹Ar, Rb-Sr and U-Pb methods. Amphiboles from the northeastern metasedimentary Katun suite give discordant results (450-610 Ma) that could be interpreted to reflect an early accretional-collisional event or to indicate excess argon contamination. The age of low-pressure M2 metamorphism is constrained by Rb-Sr whole-rock dates of migmatites (380 ± 19 Ma) from the central and late tourmaline granite (357 ± 21 Ma) from the eastern parts of the ridge. ⁴⁰Ar/³⁹Ar mica ages are in agreement with these dates. In the western part of the ridge, hornblende from M1 crystalline schists displays a saddle-shaped spectrum with an intermediate plateau date of 407 ± 4 Ma related to the collisional event. Hornblende from the Karasu granodiorite gives a well-defined plateau date of 210 ± 2 Ma which is consistent with Rb-Sr ages from Li-ore bearing granitic plutons to the south of the ridge. The emplacement of these late granitoïds resulted in a partial argon resetting in the micas from country rocks. These dates and forthcoming results are used to reconstruct P-T-t paths in the studied area of Altai-Mongolian range.

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Seeing Through Discordant Ages by Matching Ar Isotopes with Electron Microscopy in a "Heterogeneous" Amphibole from the Limpopo Belt, Africa

Igor Villa (igor@mpi.unibe.ch)¹,

Elena Belluso (belluso@dsmp.unito.it)²,

Raffaella Ruffini (ruffini@dsmp.unito.it)³ &

Myriam Schaller¹

¹ Isotopengeologie, Erlachstr. 9a, BERN, Switzerland

² Dipertimento Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, Torino, Italy

³ CNR-C. s. geodinamica delle catene collisionali, Via Accademia delle Scienze 5, Torino, Italy

Amphiboles from high-grade rock types in and near the Palala shear zone (a Proterozoic suture zone between the Central and Southern Marginal zones of the Limpopo Belt) were analysed by ³⁹Ar/⁴⁰Ar. One special case from a Cpx+Opx+Hbl gabbro gave a discordant age spectrum. Several step ages are older than the most probable age of peak metamorphism at 2.02 Ga, as determined by U/Pb on titanite.

Rather than dismissing this sample as "useless due to excess Ar", we applied Cl/K vs. Ca/K correlation obtained from Ar isotopes (cfr. Villa, this session). By this approach we identified mixing trends between 4 different end-members. However, chemical analyses performed by a WDS electron microprobe gave homogeneous compositions for all analysed amphiboles.

In order to look for different mineralogical compositions at a scale smaller than the 5 µm resolution of the WDS data, we analysed the amphibole grains by transmission electron microscope (TEM) equipped with a EDS microanalysis. With this investigation technique different amphiboles intergrown at a scale 0.5 µm were detected: Tschermakite, Mg-horneblende/edenite, and actinolite. Furthermore fine (0.2-0.3 µm) layers of chlorite overgrowing the originary amphibole were revealed.

Extrapolating the mixing trends to pure end-member amphiboles, the influence of foreign phases can be modelled away. Minor chlorite alteration and late actinolite overgrowths account for higher and lower outliers, respectively. The ages pertaining to tschermakite and Mg-hornblende are indistinguishable at 2.02 Ga. The accurate age of the metamorphic peak is thus unravelled.

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Significance of U-Pb and Sm-Nd Ages in Eclogites in the Light of Petrological

Valerie Bosse (bosse@unice.fr)¹,

Michel Ballevre

(Michel.Ballevre@univ-rennes1.fr)²,

Gilbert Feraud¹,

Koen de Jong³,

Jean-Jacques Peucat² &

Gilles Ruffet¹

¹ UMR 6526 Geosciences Azur - Universite' de Nice Sophia Antipolis - France

² UPR 4661 Ge osciences Rennes - Universite' de Rennes I - France

³ Geological Survey of Japan - Tsukuba - Japan

The Champtoceaux nappe is a crustal-scale thrust located in the South-Armorican Massif between the Nort-sur-Erdre Fault to the north and the southern branch of the South Armorican Shear Zone to the south. The nappe consists of several imbricated units, the lower one (Cellier Unit) being characterised by well-preserved eclogite-facies rocks. We report here new petrological and Sm-Nd data in order to constrain the P-T conditions and the timing of the high-pressure event.

The studied sample is a fine-grained eclogite similar to the one described by Godard et al (1981). The primary assemblage consists of garnet, clinopyroxene, phengite, glaucophane, rutile and minor pyrrothite. Garnet grains are either idioblastic or atoll-like with a phengite core and preserve growth zoning (increasing Prp and decreasing Grs from core to rim). The jadeite content of the clinopyroxene varies between 45 and 55 mole per cent. Glaucophane occurs as poeciloblastic, slightly zoned (increasing Fe content from core to rim) crystals in equilibrium with garnet and omphacite. P-T estimates are 15-20 kbar at about 600°C, the growth of glaucophane idioblasts recording the earliest stages of the cooling history.

Zircons extracted from this sample have been studied by Paquette (1987), who obtained an U-Pb age of 358 ± 2 Ma (upper intercept). This age has been interpreted as (i) a minimun age for the eclogite-facies event (Paquette, 1987), (ii) a possible age for the HP event (Ballevre et al, 1994), or (iii) a resetting of the U-Pb clock at 450-500°C (Faure et al, 1997). A new Sm-Nd analysis of this sample gives an isochron age (whole rock-garnet) of 359 ± 17 Ma, which is consistent with the U-Pb age. The preservation of growth zoning in garnet and glaucophane, and the lack of amphibolite-or greenschist-facies overprint indicates that resetting of both the U-Pb and Sm-Nd systems is highly unlikely. Moreover, ³⁹Ar-⁴⁰Ar data in a large variety of samples from different lithologies (eclogites, garnet-kyanite micaschists, leucocratic gneisses) from the same unit show that closure of the argon clock took place at about 350-360 Ma.

The above data show that the high-pressure event in the basal unit of the Champtoceaux nappe took place in the uppermost Devonian (about 360 Ma), i.e. is much younger than expected, and that cooling occurred shortly after the pressure peak.

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Well-Preserved Prograde P-T Path from the Variscan Basement of Tisza Unit, Pannonian Basin, Hungary

Péter Horváth (phorvath@sparc.core.hu) &

Péter Árkai (arkai@sparc.core.hu)

Laboratory for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budaorsi ut 45., Budapest, Hungary

The polymetamorphic basement of the Tisza Unit, SE Transdanubia, Hungary forms a detached fragment of the Variscan European foreland of the Neotethyan realm (Kovács, 1982; Szederkényi, 1996). The prograde evolution path of a gneiss-amphibolite complex characteristic of the main part of the Tisza Unit was reconstructed, investigating the polymetamorphic rocks of the borehole Baksa-2. The results obtained by detailed microstructural and mineral paragenetic observations, mineral chemical analyses, and thermobarometric calculations and the shape of the P-T loop suggest a complex Variscan polyphase polymetamorphic model (Árkai et al., 1985) rather than a polycyclic (pre-Variscan-Varican) one (Lelkes-Felvári and Sassi, 1981). On the basis of preserved relic, large garnet porphyroblasts and their inclusions, as well as adjacent (matrix-forming) mineral assemblages, the physical conditions were estimated. The early part of the prograde path with kyanite as index mineral, is characterized by T-P conditions of 480°C and 460 MPa, respectively. The metamorphism reached its peak conditions of 660 ± 25°C, and 750 ± 50 MPa, when both kyanite and staurolite were stable. The estimated metamorphic thermal gradient of the prograde part might be between 24 and 28°C/km. This metamorphic clymax was followed by a nearly isothermal decompression at 650 ± 40°C, which resulted in a significant decrease of pressure down to 440 ± 20 MPa. This event is marked by the presence of sillimanite and the second generation of garnet, and was closely related to the synkinematic post-collisional Variscan granitoid magmatism observed in considerable parts of the Tisza Unit (Buda, 1981). The thermal gradient of this phase proved to be of ca. 37°C/km. In amphibolites intercalated with gneisses, only this last event was preserved, providing less precise T-P estimates (ca. 650-690°C/400-500 MPa). The present paper demonstrates the first, almost complete, continous, clockwise P-T-relative time path in the Variscan metamorphic basement of the Hungarian part of the Tisza Unit.

Árkai P, Nagy G & Dobosi G, Acta Geol. Hung, 28, 165-190, (1985).

Buda Gy, Acta Geol. Acad. Sci. Hung, 24, 309-318, (1981).

Kovács S, Geol. Rundschau, 71, 617-640, (1982).

Lelkes-Felvári Gy, Sassi FP, IGCP Project No. 5 Newsletter, 3, 89-99, (1981).

Szederkényi T, Acta Miner. Petr. Szeged, 37, 143-160, (1996).

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P-T-Paths of Subduction-Related High-Pressure Rocks, Rio San Juan Complex, Northern Dominican Republic

Martin Krebs

(walter.v.maresch@ruhr-uni-bochum.de)¹,

Walter V. Maresch¹ &

Grenville Draper (draper@fiu.edu)²

¹ Institute for Mineralogy, Ruhr-Universität Bochum, 44780 Bochum, Germany

² Dep't of Geology, Florida International University, Miami FL 33199, USA

Draper and Nagle (1991) mapped and described the Rio San Juan Complex, the largest exposed area of Cretaceous basement rocks in the North Coast Zone of Hispaniola. These authors suggest that the predominantly mafic schists of the northern part of the complex were metamorphosed in a Late Cretaceous subduction zone and later protruded at depth by serpentinite melanges entraining tectonic blocks of various rock types. We are carrying out petrographic and microanalytical studies of samples from the mafic schists (Hicotea schist, Puerca Gorda schist) and from blocks of metamorphic rock in the melanges (Jagua Clara melange, Arroyo Sabana melange) to gain more insight into subduction zone processes. Classical thermobarometers and petrogenetic grids, in conjunction with information on relative timing from compositional zoning in porphyroblasts and inclusion relationships, allow P-T-conditions and preliminary P-T-paths to be delineated.

The mafic schists have been subjected to a pervasive greenschist overprint, but a lawsonite-blueschist mineralogy is still preserved. Maximum P-T-conditions are estimated to have been between 5 and 10 kbar, at temperatures below 400°C. Blocks from the Arroyo Sabana melange show distinctly heterogeneous P-T histories. A granitic igneous rock exhibits lawsonite growth at the expense of feldspar and muscovite. An eclogite yields peak P-T-conditions of 780°C and a minimum of 17 kbar and an exhumation path to 500°C, with P still higher than 6.5 kbar. An epidote-amphibolite shows no evidence of high-pressure metamorphism. Metamorphic blocks from the Jagua Clara melange yield an intriguing and relatively uniform P-T-development. The following P-T-path is entirely recorded in a single specimen of eclogite; segments of this path are observed in three further samples. An original amphibolite-facies mineralogy (600-650°C/6 kbar) is overprinted by a low-grade blueschist assemblage (400-450°C/6 kbar). These blocks then followed a typical subduction-zone prograde path to maximum conditions of 16-20 kbar and as high as 680°C. Exhumation essentially retraces the prograde path at only slightly higher temperatures and can be followed to below 400°C.

Draper G, Nagle F, Geol. Soc. Am. Spec. Pap, 262, 77-95, (1991).

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P-T-Analysis of Low Grade Metamorphic Accretionary Complexes in Eastern Kamchatka, Russia

Sven Lewerenz¹,

Heinz-Gerd Holl¹,

Nikolai Tsukanov²,

Christoph Gaedicke³,

Dmitri Alexeiev² &

Ralf Freitag³

¹ Institut für Angewandte Geowissenschaften II, Technical University Berlin, Germany

² Institute of Oceanology, Moscou, Russia

³ GeoForschungsZentrum Potsdam, Germany

Upper cretaceous up to paleocene volcano-sedimentary and turbiditic sandstones and siltstones of the Kumroch Range and the Kamchatka Mys Peninsula in Eastern Kamchatka have been investigated in order to determine the p-T-conditions during the accretion of the Ozernovsky-Valaginsky Terrane and the Vetlovsky Terrane in the Kumroch Range and of the Kamchatka Mys Terrane. The first results show impressive trends allowing to retrace the accretionary events. We use different analytic approaches in order to determine the p-T-conditions of the low grade metamorphic and higher diagenetic units. Data of chlorite cristallinity mesurement show a variation of 0.357-0.439° 2<theta> (0.7nm-peak) and can be placed into the field of higher diagenesis (Árkai et al., 1995). In this intervall of data the formation conditions of the chlorite authigenesis are ¾200°C (Frey, 1986). Cristallinities increase as a function of stratigraphic age. Illite is found in several samples but difficult to use for mesuring cristallinities due to the superposition with the laumontite-leonhardite (110) peak at 0.95nm. Zeolites could successfully be separated by using a flotation method and will be investigated by spectroscopic and geochemical methods. Corresponding to the results of clay mineralogy we got data of coalification measurements of organic matter in the pelites. The values varie between 1.2 and 0.7% Rm_oil. We can also observe an increasing of the coalification values in function of the stratigraphic age of the rocks. The vitrinite reflection data of a large sample set will be used to calculate the maximum temperatures depending on the effective coalification time using the EASY%-Ro model (Sweeney & Burnham, 1990).The fraction <63 µ m of the volcano-clastic rocks shows obviously the presence of partially dehydrated laumontite-leonhardite. The paragenesis of these rocks is low quartz, albite, chlorite, laumontite, ± illite, ± montmorillonite, ± hornblende (edenite, pargasite), ± augite and therefore they can be plotted into the zeolite zones IIIb and IVa defined by Iijima (1980). Prehnite and pympellyite as facies critical minerals for a low grade metamorphism (subgreenschist facies) are not found. As these minerals will be formed by alteration of the zeolites when pressure and temperature of the described paragenesis increase, we are able to suggest a upper temperature limit of maximum 200-250°C (Frey, 1986). The deformation structures of quartz minerals in the psammites indicate similar conditions. All described parameters of the early pilot project indicate conditions from 100 to ca. 230°C and 0.5 up to 2 kbar. The sample set of now 70 specimen and the use of several analytical methods complementing each other should give us more differentiated data to the points mentioned above, to contribute some new aspects for this accretionary history in the collision zone of Kamchatka.

Árkai P, Sassi FP & Sassi R, Eur. J. Mineral, 7, 1115-1128, (1995).

Frey M, Schweiz Mineral Petrogr. Mitt, 66, 13-27, (1986).

Iijima A, Proceedings of the fifth international conference on zeolites, Rees LV, 103-118, (1980).

Sweeney JJ & Burnham AK, Bull. Amer. Assoc. Petrol Geol, 74(10), 1559-1570, (1990).

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Some Geologic and Petrologic Implications of Compressibility and High-Pressure Crystal Structure of Muscovite and Paragonite in Metapelites

F. P. Sassi¹,

C. V. Guidotti²,

P. F. Zanazzi³ &

P. Comodi

¹ Dept. Mineralogy and Petrology, C.so Garibaldi 37, 35137 Padova, Italy

² Dept. Geological Sciences, Orono, Maine U.S.A

³ Dept. Earth Sciences, P.za Universita', Perugia, Italy

The interpretation of the behavior of a mineral subjected to a range of petrologic and geological conditions in a fashion that integrates all aspects of its crystal chemistry, including its composition, crystal structure and elastic properties, can have relevant implications in deciphering metamorphic processes. Herein we consider the interrelationships among the muscovite crystal-chemical features with respect to P, T of formation and recrystallisation under the influence of a differential stress. The starting point for our considerations has been the recently published compressibility data of three compositionally well constrained white micas and related crystal structure refinements obtained over a range of different pressures. Considering the chemistry of these three crystals, variations in the compressibilities measured and structural responses to increasing P can be ascribed solely to the Na/(Na+K) ratio. This enables one to better understand the effects of isochemical P increase on a white mica structure. Key results are: (1) compressibilities and structural adjustments are clearly a function of Na/(Na+K) ratio; (2) structural adjustments (e.g. <alpha> angle, dimensions of IV and VI sheets, etc.) can destabilize the structure; (3) most of the compression involves shortening of the c cell dimension. The main implications of such results are: a): it is now well recognized that significant increase of P causes the K- and phengite-content of muscovite to increase. This is true even if the mineral assemblage is Al-saturated. The increasing the K- and phengite-content of muscovite also changes the mica structurally in terms of cell dimensions, (<alpha> angle, etc. The key point is that these changes are exactly those needed to mitigate the destabilizing effects caused by isochemical increase of P on muscovite. Hence, we suggest that the well-known increase of K- and phengite-content in natural muscovite in response to P increase can be understood in terms of the structural adjustments needed to maintain stability of the mineral. b): It is generally believed that, during the formation of slates, the platy minerals like muscovite and chlorite become aligned by some combination of rotation and recrystallisation such that their (001) planes are perpendicular to the direction of maximum compression. In several cases, TEM-SEM studies have shown that the oriented muscovite is more K- and phengitic-rich than the unoriented flakes. This is clearly related to the recrystallisation which accompanies the formation of the preferred orientation. Such composition changes cause the muscovite c dimension to shrink, but the a and b cell dimensions increase. Hence the dimensional changes of such muscovite mirror the strain experienced by the rock as a slate forms. Moreover, the above mentioned compressibility data show that as muscovite K-content increases, its intrinsic compressibility increases. Differential stresses are thought to be small during formation of slates. Nonetheless, we suggest that when low-grade metamorphism occurs in association with a differential stress, the above noted crystallochemical aspects of muscovite readily facilitate and favor its recrystallisation into the preferred orientation that typifies slates. In summary, the above two cases provide two examples for which it appears that the petrologic and geologic processes can be intimately related to the crystallochemical response of an affected mineral.

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Orthoamphibole Breakdown Reactions in the Oonagalabi Domain, Arunta Block, Australia: Phase Relations and P-T Conditions

Leo M. Kriegsman (leo.kriegsman@utu.fi)¹ &

Johann G. Raith (raith@unileoben.ac.at)²

¹ Department of Geology, University of Turku, FIN-20014, Turku, Finland

² Institute of Geological Sciences, Mining University Leoben, A-8700 Leoben, Austria

The Oonagalabi domain in the eastern Arunta Block, Central Australia, comprises a succession of orthogneisses and subordinate supracrustal units that were metamorphosed under transitional amphibolite to granulite facies conditions. Metapelites show evidence for partial melting and segregation of melt, leaving a restitic residue. Metabasites display orthopyroxene ± garnet-bearing segregation pods, in which orthopyroxene and garnet are commonly rimmed by calcic amphibole. Hence, high-grade partial melting was followed by cooling and minor back reaction.

Aluminous lenses in a concordant orthoamphibole-dominated unit show early orthoamphibole + spinel ± corundum, overgrown by sapphirine and garnet. The earliest recorded assemblage in K-poor, silica-saturated rocks is orthoamphibole + sillimanite + quartz, overprinted by garnet and cordierite. In sillimanite-absent domains, breakdown of the anthophyllite component of orthoamphibole has left behind Na-enriched (~2 wt% Na₂O) gedrite enclosed by large orthopyroxene grains. This was followed by back reaction of orthopyroxene with the fluid to produce pure anthophyllite. The combined evidence suggests a prograde evolution culminating at ca. 750-800 ^oC, 8-9 kbar, followed by minor cooling.

The gradation from mono- or bimineralic orthoamphibole (gedrite + anthophyllite) rocks to orthoamphibole + spinel ± garnet and orthoamphibole + spinel + corundum + sapphirine + garnet assemblages in aluminous lenses, coupled with the constant K_D of garnet-orthoamphibole Fe-Mg exchange in all rock types, suggests that the entire sequence witnessed a single, transitional amphibolite to granulite facies event. Hence, the formation of the orthoamphibole-dominated unit and its associated Cu-Zn(-Pb) mineralisation probably reflects a metasomatic event prior to high-grade metamorphism.

The high fluorine content of orthoamphibole and phlogopite, and the presence of fluorite in marble layers and of chondrodite in some orthoamphibole-dominated lenses, suggest a high F content of the metasomatic fluids. We argue that F and the Cu-Zn(-Pb) sulfides were derived from the Oonagalabi bimodal igneous suite, which was emplaced in an early extensional setting (Sivell, 1986). Fluorine probably played an important role in stabilising orthoamphibole and phlogopite during subsequent transitional granulite metamorphism.

Sivell WJ, Contributions to Mineralogy and Petrology, 93, 381-394, (1986).

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The Minor Elements in Coexisting Pyroxenes as Indication of Metamorphic Grade

Sergey Skublov (Vrev@ad.iggp.ras.spb.ru)¹,

Galina Drugova (Vrev@ad.iggp.ras.spb.ru)¹ &

Dmitry Skublov²

¹ Institute of Precambrian Geology and Geochronology, St. Petersburg, Russia

² National Petroleum Institute, St. Petersburg, Russia

We investigate the chemical composition of coexisting pyroxenes from basic granulites with related enderbites and regressively altereted granulites. The content of Y, Yb, Sc in clinopyroxenes increase with a rise of temperature. The content of Mn, Y, Yb, Sc in orthopyroxenes increase as the result of the drop in pressure in the act of enderbite forming. The lowering of V, Cu, Cr in orthopyroxenes is a consequence of the temperature rise. The distribution of Cu, Yb, Sc between coexisting pyroxenes is close to ideal for enderbites. The mineral equilibrium was established during enderbite forming with drop in pressure and approximately fixed temperature.

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The Metamorphic Evolution of the Lapland Granulite Belt

Galina Drugova (Vrev@ad.iggp.ras.spb.ru) &

Sergey Skublov (Vrev@ad.iggp.ras.spb.ru)

Institute of Precambrian Geology and Geochronology, St. Petersburg, Russia

The new data on petrology of the granulites from Lapland Belt testifies that granulitic metamorphism was repeated doubly. The low pressure metamorphism was most conspicuous in the north-east (upper) part of the granulite complex, the high pressure metamorphism appeared in the south-west (lower) part. The area of high pressure granulites declines from the east to the west. Coarse-grained migmatizated rocks with cordierite are the result of the early low pressure granulitic metamorphism. The new data on composition of zoned garnets allows to determine metamorphic conditions as 750-950°C, 5-6 kbar for low pressure stage and 750-800°C, 8-9.5 kbar for high pressure stage of granulitic metamorphism. Conceivably the low pressure granulites represent the mineral forming conditions of Archean rocks in place of Lapland granulites. This conditions are similar with the metamorphism of Late Archean metasedimentary Cola series.

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Chlorite-Phengite Equilibrium: Application to the Retrograde P-T Path of Tinos Island, Cyclades Archipelago

Teddy Parra (parra@cristal.ens.fr)¹,

Olivier Vidal &

Laurent Jolivet (jolivet@lgs.jussieu.fr)²

¹ E.N.S., Lab. Géologie, 24, rue Lhomond, Paris, France

² Dpt Géotectonique, Université Pierre et Marie Curie, 4, place Jussieu, T26-0 E1 case 109, 75252 Paris Cedax 05, France

Metapelites metamorphosed at low-temperature (<600°C) and medium to high-pressure (up to 20 kbar) conditions are widespread in the peri Mediterranean Tethysian orogens resulting from subduction. When these rocks are devoid of the common blueschist facies index minerals such as carpholite ± chloritoid, glaucophane or jadeite, they are classically considered to have been metamorphosed in greenschist facies conditions. In consequence, only minimum or maximum pressure conditions corresponding to blueschist-greenschist facies transition are estimated from metapelite mineralogy, but a more continuous assessment of the shape of metamorphic P-T path is generally difficult or impossible. A possible way to improve this situation is to use the composition of chlorites and micas, which is very sensitive to pressure and temperature. Both these phases systematically occur in metapelites, whatever the metamorphic conditions.

This is the case for metapelites that occur in Tinos island (Cyclades Archipelago, Greece). In these rocks, the degree of the late greenschist overprint is correlated with the gradient of deformation, along a N-S transect crossing a major extensive shear zone. It is noteworthy that blueschist facies metapelites are preserved far from the shear zone, whereas the retrogression is complete in the vicinity of the contact. The Si-content variations in chlorites and micas are correlated together and related to the distance to the detachment. This compositional variation is interpreted in term of progressive increase of retromorphosis towards the shear zone. These chemical variations can also be correlated to a local increase of deformation and probably of the availability of fluids.

The compositional variation of chlorites and micas has been used to constrain the exhumation path. We first localize good indicators of equilibrium in the schistosity and in the porphyroblasts. Analysis of texture and microstructures in the queues of crystallization of albite provides information to discriminate the different generations of chlorites and micas and on their order of crystallization. On the contrary, the systematic study of sodic amphiboles-bearing schists was less successful. For example, it is unknown which part of the amphibole is in equilibrium with the chlorite-micas couples. About 200 chlorites-micas couples were used to compute the evolution of P-T conditions during retromorphosis. The obtained P-T paths allow us to discuss the modalities of exhumation of the high-pressure rocks of Tinos Island.

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Molar Volumes of the Phengite Solid Solution in KMASH (K₂O-MgO-Al₂O₃-SiO₂) and Consequences on Geo-Barometry

Max W. Schmidt

(max@opgc.univ-bpclermont.fr)¹ &

Gilberto Artioli (artioli@iummix.terra.unimi.it)²

¹ CNRS - UMR 6524, 5, rue Kessler, 63038 Clermont-Ferrand, France

² Dip. Scienze della Terra, Via Boticelli 23, 20133 Milano, Italy

Phengites with compositions on the pseudo-binary muscovite-celadonite

KAl₂[AlSi₃]O₁₀(OH)₂-KAlMg[Si₄]O₁₀(OH)₂)

join were synthesized experimentally at conditions of 700-1025°C and 1.5-9.2 Gpa. Syntheses at lower temperatures and higher pressures did not result in a mica phase. Syntheses were considered as successful if (i) phengites were large enough to be measured by electron microprobe analysis, (ii) compositions were close to the pseudobinary above, deviations were accepted in the range 0.95<K<1.0 and 1.98-2.02 octahedra per formula unit, (iii) yields were >90% mica. Successful syntheses yielded phengites between cel(26) and cel(99). Grain sizes were typically 10-30 micron allowing for microprobe analysis with a defocused electron beam. After microprobe analysis, run products were ground, mixed with an internal Si-standard, and measured by X-ray powder diffractometry. Cell parameters were determined by full-profile Rietveld analysis.

Molar volumes as a function of composition were then fit on a data set including our samples having pseudo-binary compositions, a few Na- and Fe-poor natural phengites from the literature, and the end member muscovite volume as reported by Guidotti et al. (1992; V(Musc)= 14.078(6) J/bar). The excess volume deviates positively from the ideal volume of mixing; the molar volume has small variatons in the range cel(26)-cel(40) and then drastically drops, reaching a minimum at the celadonite endmember. The volume decrease is related to the decrease of all cell parameters, with c having the largest and a the smallest variation. A fit with an asymmetric Margules model yielded V(Cel)=13.954, W(MuscCel)=0.244, and W(CelMusc)=0.203 J/bar; r²=0.948 . A fit with a symmetric model yielded V(Cel)=13.956(39), W= 0.222(28) J/bar, r²=0.948 . As both fits were of identical quality we of course retain the simpler symmetric formulation for the excess volume of the muscovite-celadonite solid solution. Our volume data of the phengite solid solution, based on specimen whose composition and cell parameters were independently determined, is largely different from that reported by Massone and Szpurka (1997).

We used the resulting volume data together with experimental data on the assemblage garnet-cpx-phengite to adjust the thermodynamic properties of the muscovite-celadonite solid solution. Preliminary applications on well studied metamorphic terrains, such as the Franciscan belt, yield pressures in accordance to phengite-independent estimates.

Guidotti et al, Eur. J. Min, 4, 283-297, (1992).

Massone & Szpurka, Lithos, 41, 229-250, (1997).

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A Complete Clock-Wise P-T Loop in Continental Gabbros from the Southern Sesia-Lanzo Zone

Gisella Rebay (rebay@crystal.urnpv. it)¹,

Bruno Messiga (messiga~crystal.unipv.it)¹ &

Maria ble Spalla (ble. Spalla@unimi.it)²

¹ Dip. Scienze della Terra, via Ferrata, 1, 1-27100 Pavia

² Dip. Scienze della Terra, via Mangiagalli, 34, 1-20133 Milano

The Cono and Monastero metagabbros are exposed at the southern boundary of the Sesia-Lanzo Zone, the most internal slice of eclogitised continental crust in the Alpine nappe belt. They are almost completely transformed into glaucophane, epidote, garnet, chloritoid rocks, during Alpine times, and are heterogeneously and pervasively deformed and metamorphosed. Some anastomosing shear bands separate less deformed domains that preserve pre-Alpine textural and/or mineralogical relics.

The igneous precursors consist of gabbronorites and Fe-gabbronorites with plagioclase (An_50-70), clinopyroxene, orthopyroxene, Ti-rich amphibole, + apatite, + rutile, + oxides (magnetite and ilmenite). Whole-rock and clinopyroxene trace elements composition suggest that the parental liquids of the gabbros were crustally contaminated MOR basalts.

During the polyphase pre-Alpine evolution the gabbros re-equilibrated under granulite facies conditions (P=5-7 kb e T=800-900°C), with a successive retrograde P-T path through amphibolite facies (P=4-5 kb, T=500-600°C) up to greenscbist-facies conditions (T<450°C & P<4 kb) that was assisted by different fluid phases. Pre-Alpine syn-metamorphic structures consist of narrow amphibolite- to greenschist-facies shear zones. The shallower depths reached by these gabbros are compatible with the intrusion of porphyritic andesitic-rhyolitic dykes with chilled margins.

The P-prograde Alpine evolution is documented by the development of progressively pressure-increasing parageneses: - epidote and paragonite are followed by garnet, glaucophane and chloritoid on plagioclase microdomains; - glaucophane, garnet, + chloritoid replace mafic minerals. The pre-Alpine transformation rate and the bulk composition discriminate the type of high pressure assemblage.

The Alpine peak conditions were evaluated at pressures of about 20 Kb and temperatures of 400-500°C on the basis of the eclogitic garnet-glaucophane-chloritoid parageneses, that are in places associated with omphacite-garnet eclogitic assemblage. In the porphyritic andesitic-rhyolitic dykes jadeite + quartz assemblage is widespread.

The tectono-metamorphic evolution of the Corio and Monastero gabbros was recorded during two main geodynamic episodes. The high T/P ratio characterising the pre-Alpine retrograde evolution suggests that it took place in an abnormally high thermal regime, that can be induced by astenosphere upwelling during lithospheric thinning. Afterwards, typical subduction-related environment 15 testified by Alpine high-pressure, low temperature metamorphism. This event is correlated with the subduction of the Ligure-Piemontese Basin, whose remnants are preserved in the nearby eclogitised ophiolitic complexes. Metagabbros exhumation partly occurred under greenschist facies conditions mainly developed along narrow high-strain rate, chlorite-albite-rich bands cutting through the gabbro bodies and the country rocks.

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The Origin of Reverse Zoning in Metamorphic Garnets: Fluid Flow During Crystallization

Pavel Azimov (az@pa1400.spb.edu)¹ &

Alexander Shtukenberg (sasha@as3697.spb.edu)²

¹ Inst. of Precambrian Geology and Geochronology, Russian Academy of Sciences, St.Petersburg, Russia

² Geological Faculty, St.Petersburg State University, St.Petersburg, Russia

Here we present results of our investigation devoted to intracrystalline zoning in metamorphic garnets grown at fluid flux. We have studied, firstly, the effect of fluid composition on behaviour of fractionation coefficient K_Fe-Mg between aqueous solution and garnet solid solution, which describes the distribution of components between reservoir and growing crystal. The second direction of our study was numerical modeling of zoning profiles at garnet crystallization. The models used have been constructed on the base of kinetic equations for crystal growth from aqueous fluid and mass balance in the system.

When studying the effect of fluid composition on garnet fractionation coefficient we have examined such parameters of aqueous fluid as salinity, acidity and concentration of MgCl₂ and FeCl₂. In the case of fresh water (fluid with low ionic strength) K_Fe-Mg<1, and garnet is enriched by Fe while the fluid is enriched by Mg (Rayleigh fractionation). This leads to Mg enrichment in the rim of garnet grains (normal zoning). Salinity has weak influence on behaviour of the component distribution between garnet and fluid. At high concentration of MgCl₂ and FeCl₂ K_Fe-Mg is near unity and weakly depends on temperature. Therefore zoning in such case should be only slightly manifested. The influence of pH is weak in acid solutions while arising of alkalinity leads to inversion of fractionation coefficient (i.e. K_Fe-Mg becomes more than 1). It means that fractionation in alkaline solution may form the reverse zoning. Such zoning would be contrast enough.

The modeling of zoning profiles in metamorphic garnet has manifested that garnet growth at fluid flow can form non-monotonic profiles (so called complex zoning) or profiles typical for high-temperature garnets with reverse zoning (homogeneous core and narrow rims with sharp increasing of Fe content). Such profiles can be complicated by small inflexions. Analysis of numerous data on geological position of reverse zoned garnets shows that such zoning is common for diaphthorites or narrow zones affected by fluid alteration during retrograde metamorphism. This does not contradict to assumption that the reverse zoning in garnet grains can originate at crystallization under fluid flow through metamorphic rock.

Thus, we can summarize that a possible growth mechanism producing reverse and complex zoning is garnet crystallization in the open system, under fluid flow. Zoning trend is determined by fluid composition (salinity, pH and solute concentration). Combination of the Rayleigh fractionation and fluid flux in the system can create various types of zoning profiles, including non-monotonic ones.

The work has been supported by the RFBR grants 97-05-65518, 96-05-66060 and 96-15-98427.

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Experiments Under Thermal Gradients: New Insights on the Diffusivity and Speciation of Aluminium in the K/Na₂O-Al₂O₃-SiO₂-H₂O System at 3 kbar, 300-550°C

olivier vidal (vidal@cristal.ens.fr) &

leila durin

E.N.S., Lab. Géologie, 24, rue Lhomond, Paris, France

Hydrothermal tube-in-tube experiments involving strong thermal gradients were initially designed to determine if mass transfer between solids and solution and mass transport induced by thermal gradients within closed systems could account for the spatial distribution of naturally occurring hydrothermal sequences of crystallisation.New experiments involving a variation of temperature with time or a strong thermal gradient were conducted in order to study the transport and transfer of aluminium in a closed medium along with dilute water. Results show that in contrast to the common assumption, aluminium is highly mobile at 3 kbar, 300-550°C in a water saturated medium with alkali (K/NaASH). The solubility and the transport of aluminium are controlled by the availability of alkali. Starting from a mixture of kyanite + quartz + muscovite at the hot extremity of a thermal gradient, aluminium is transported toward the cold extremity in form of a complex with an Al/K stoichiometry close to unity. However, since more aluminium than alkali are released by the dissolution of muscovite, an Al-rich phase (kyanite) forms close to the starting minerals undergoing dissolution, although aluminium is mobile in our system. Then, the variation of the solubility of Al-K complex with temperature leads to the formation of muscovite (+quartz) at the cold extremity of the thermal gradient. A quantitative interpretation of the experimental results was done using literature data on aluminium speciation in dilute water. Extrapolation of the laboratory results to natural rocks suggests that 1) the diffusion of aluminium in water is an efficient process of transport under medium-grade, low- to medium-pressure conditions. This conclusion is consistent with the occurrence of Al-bearing phases in synmetamorphic veins. 2) mass-transfer estimates based on mass-balance analyses postulating a fixed aluminium reference frame should be considered with caution. 3) the high fluid/rock ratio calculated from the amount of aluminosilicates occurring in veins of medium-grade metapelites are questionable because such calculations neglect the importance of the transport of aluminium by diffusion.

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Role of External and Internal Buffering in Predazzite Formation in the Debnik Anticline, South Poland

Anna Lewandowska (ania@ing.uj.edu.pl) &

Mariusz Rospondek (ros@ing.uj.edu.pl)

30-063, Krakow, ul. Oleandry2a, Poland

Predazzite in the Debnik anticline was formed during the contact metamorphism and hydrothermal alteration of uniform series of Givetian dolomites. Metamorphism caused by the late Palaeozoic ryodacite shallow intrusion resulted in the formation of ca. 300 m thick contact aureole.

In the inner part of the contact aureole altered rocks occur as white calcite-brucite marble (predazzite). The process of predazzite formation can be explained by dolomite thermal decomposition and hydration in the following reaction:

CaMg(CO₃)₂+H₂O = Mg(OH)₂+CaCO₃+CO₂ (1)

In the outer part of the contact aureole white predazzite dominates along fractures or layers whereas the remaining rock is black, and composed of dolomite, calcite and brucite (Haranczyk & Lewandowska, 1994). Temperature, due to the constant distance from the intrusion, must be ruled out as a factor controlling the observed variation of mineral assemblages in a range of centimetres. In the field, metamorphism is marked by a distinct change in the rock colour. Unmetamorphosed dolomites were brown due to the presence of organic matter (0.5 wt.%). Metamorphic fluids removed organic matter causing predazzite whitening. In the black altered parts, however, the organic matter underwent coalification and constitute up to 0.3 wt.% TOC (Lewandowska & Rospondek, 1998). The differences in mineralogy of the white and black rock can be explained by the influence of the reaction (1) on the composition of metamorphic fluids. In the white zones, fluid composition was externally buffered and CO₂ derived in reaction (1) was removed. The reaction (1) was completed, and predazzite exclusively composed of calcite and brucite formed. Penetrating fluids removed organic matter causing whitening. However, the efficiency of the external buffering decreased away from the fissures. In the black zones reaction (1) internally buffered composition of metamorphic fluids. This process increased XCO₂, dolomite decomposition was restrained, and this mineral is still an important component of the rock. The amount of rock penetrating water declined rapidly and the organic matter was preserved in a coalified form.

Emplacement of ryodacite intrusion at shallow depths caused the thermal expansion of country rock fluid and hydraulic fracturing of the dolomites in the contact aureole. Along fractures composition of metamorphic fluids was externally buffered and predazzite was formed. Away from fissures fluid composition was internally buffered and dolomite decomposition was restrained. Processes of external and internal buffering well explain coexistence of predazzite and dolomite in the zones which underwent metamorphism at the same temperature.

Haranczyk C, & Lewandowska A, Przew LXVZj Pol Tow Geol, 112-114, (1994).

Lewandowska A, & Rospondek M, Pr Spec Pol Tow Min, 11, 133-135, (1998).

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Kyanite in a Low Pressure High Temperature Orogen (Serra da Freita, Variscan Chain, North-Central Portugal)

Maria H M Acciaioli (mMendes@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 Serra of Freita region (Central Iberian Zone) is dominated by pre-Ordovician metasedimentary rocks of the Slate-Greywacke Complex. These formations were intruded by syn, late and post-D3 granitoid plutons during the Variscan orogeny. Data from detailed structural, metamorphic cartography, petrography and mineral chemistry shows that the Variscan regional metamorphism: (a) occurred mainly, during the final period of the D2 deformation and syn-late D3 times; (b) is of low pressure high temperature type (LPHT) with mineral parageneses containing andalusite in medium and high grade metamorphic zones; (c) is prograde and (d) practically isobaric.

Locally, in staurolite-andalusite and sillimanite zones, the regional metamorphic sequence, marked by prograde dehydration reactions, is interrupted. These interruptions correspond to areas where intense fluid circulation occurred (which is testified by: high density of quartz veins; modal importance of hydrated minerals; potassic metasomatism). Kyanite is a common phase in the strongly hydrated metapelites.

Kyanite is not a relic phase and replaces syn to tardi-D3 andalusite, suggesting a late anomalous compressive event of local extension. This episode was probably related to an increase of ductility - and, therefore, an increase of the mean stress - caused by the formation of hydrated minerals during fluid ascent along a shear zone. The fluids responsible by the development of the strongly hydrated parageneses probably resulted from devolatilization reactions in the sillimanite zone.

The stable (as revealed by textural characteristics and mineral chemistry features) coexistence of the three Al₂SiO₅ polymorphs occurs along a narrow domain, with cartographic expression, in the transition between the most hydrated metasediments (containing muscovite, quartz, staurolite, kyanite and andalusite) and the typical sillimanite zone paragenesis. The occurrence of this unusual paragenesis was probably favoured by the increase of variance caused by the additional presence of FeAlSiO₅ in the composition of the Al₂SiO₅ polymorphs.

Regional isograds are apparently folded. However, the observed structural pattern may be explained considering the isograde surfaces as chemical readjustment planes whose final position was established during the late thermal relaxation events, rather than by simply assuming a two phase process of isograde structuration followed by tectonic folding.

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Development of Polymetamorphism in the Pon'goma Area, Southern Part of the Belomorian Belt (Baltic Shield)

Nikolay Alexejev,

Tatijana Zinger,

Boris Belatsky &

Sergei Klepinin

St.Petersburg, Makarova emb., 2, Russia

Four major lithological components are recognized within the high-grade rocks making up the southern part of the Belomorian Belt near Pon'goma village. These are: 1)Late-Archaean Belomorian gneisses; 2) the Late Archaean Chupa metasedimentary suite; 3) the Late Archaean ca 2728 Ma Opx-diorites and 4) the Palaeoproterozoic ca 2415 Ma Hbl-Cpx-granodiorites. The latter two are strongly deformed, metamorphosed and partially migmatized and are cut by several generations of mafic dykes. Primary igneous textures of the granite plutons are however preserved in some places.The Early Palaeoproterozoic tectono-thermal event resulted in development of ca 2.45 Ga (Levchenkov et.al.,1996) steep, N-W-N trending shear zones developing high-pressure and temperature (mineral assemblages within Opx-granodiorites and ca 2.41 metamorphic fabric (foliation and, probably, migmatitic banding) within Hbl-Cpx-granodiorites. These took place under conditions changing from granulite (P=10-11 kbar, T=725-770°C) during the formation of ca 2.45 Ga shear-zones to amphibolite facies (P=8-9 kbar,T=630-700°C) during the formation of oldest metamorphic textures in the granodiorite pluton. The ca 1.9-1.8 Ga metamorphic event were accompanied by substantial strike-slip faulting. As a result, all the lithologies were reworked by EN-SW striking, NW dipping shear zones. These also modified the primary intrusive contacts between Opx-diorites and Hl-Cpx-granodiorites and, probably, reworked the contacts between the felsic plutones and mafic dykes. P-T studies suggest that all the outlined lithologies were affected by ca 1.9-1.Ga metamorphic event. This took place under amphibolite facies metamorphism: P=6-8 kbar and T=600-630°C.

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Difficulties Associated with Interpreting ⁴⁰Ar-³⁹Ar Ages from Muscovite and Biotite in Metamorphic Rocks

Hazel Roberts (H.J.Roberts@open.ac.uk)¹,

Simon Kelley (S.P.Kelley@open.ac.uk)¹,

Peter Dahl (PDAHL@GEOLOGY.KENT.EDU)²

¹ The Open University, Walton Hall, Milton Keynes, England

² Kent State University, Kent, Ohio, USA

With the development of high spatial resolution dating techniques, such as the ⁴⁰Ar/³⁹Ar UV laser microprobe, it is now possible to produce multiple ages from single mineral grains. This has allowed the investigation of the effects of different cooling histories within metamorphic rocks by the systematic measurement of apparent age variations within single mica grains. In theory, micas that have undergone slow cooling will exhibit younger apparent ages at the edge of the grain, with a rapid increase in ages within the rim area and an almost constant age from near rim to core (Dodson, 1986). The profile produced can then be used to interpret the cooling history of that rock. Micas from PreCambrian gneisses from South-western Montana were used in this study to investigate this theory. Our results support the findings of previous work in that although muscovite will often exhibit such a profile, equivalent profiles are rarely found in biotite. This raises questions over the different diffusion characteristics of biotite and muscovite, and what might be the controlling factors.

Age profiles caused by slow cooling are dependent on both the microstructure of the mineral grain in question and volume diffusion being the dominant transport process. In reality, of course, mineral grains are never perfect and contain a number of different defects, inclusions, and cleavage traces, that can all facilitate rapid diffusion of argon out of, or even into, the grain. This, clearly, can have a significant effect on the apparent age obtained from ⁴⁰Ar/³⁹Ar dating on that grain. Although volume diffusion theory predicts that cooling ages and closure temperatures will be affected by grain size, rapid diffusion pathways may override that effect, resulting in cooling ages and closure temperatures that are independent of the observed grain size. Instead, the effective diffusion dimension will be the influential factor.

We have found that incipient alteration of mica is another key factor influencing the apparent age and closure profile. Where unaltered, biotite from South-western Montana yield an age of c.1750 Ma, very minor alteration can produce ages 100 Ma younger and pervasively altered biotite can produce an apparent age as young as c.950 Ma. It is apparent that biotite is more prone to alteration than muscovite and even the early stages of alteration in an apparently unaltered rock can influence the ⁴⁰Ar/³⁹Ar age produced from biotite.

Dobson MH, Material Science Forum, 7 145-154, (1986).

Session L14:2A

L14 : 2A/01 : H2

Characterising and Dating the Effects of Metamorphism on Zircon

David H Cornell (cornell@geo.gu-se)¹,

Bob R J Thomas (bob@geobell.org.za)²,

Paul Hoskin (paul.hoskin@anu.edu.au)³,

Richard Armstrong (prise.armstrong@anu.edu.au)³ &

Martin Whitehouse (martinw@nrm.se)⁴

¹ Box 460, Gothenburg, SE-40530, Sweden

² Council for Geoscience, Box 572, Bellville, 7535, South Africa

³ Research School of Earth Sciences, Australian National University, Canberra 0200ACT, Australia

⁴ National Museum of Natural History, Box 50007 Stockholm SE-10405, Sweden

The SHRIMP and CAMECA IMS1270 ion probes and more recently laser ablation ICPMS instruments can date <30 micron domains within zircon grains. Methods for the recognition and delineation of metamorphic domains large enough to date are still being developed and debated. In our work on polymetamorphic gneisses from the Precambrian shields of Scandinavia and Southern Africa, four main types of metamorphic zircon have been recognised, which developed under amphibolite and granulite facies metamorphic conditions, often during migmatite formation: (1) New metamorphic zircon grains, sometimes with older cores, usually stubby with length/breadth <2 and with several pyramid forms, giving a rounded habit. (2) Rims on older zircon, often overgrowing physical erosion surfaces on the cores. (3) Cross-cutting fracture fillings or piebald replacement domains reworking older zircon. (4) Rare examples of ancient metamorphic partial lead loss have been identified.

Metamorphic zircon is seen in cathodoluminescence (CL) images as fairly homogeneous domains, CL-brightness controlled by U content. In high-U grains (>200 ppm U), CL is not useful, however backscattered electron images may still reveal metamorphic domains. Most of the visual and textural characteristics of metamorphic zircon can also be developed in igneous processes, for example where older zircon is incorporated as xenocrysts in new magma. To confirm a metamorphic origin, mineral-chemical criteria based on microbeam analyses have to be used.

Metamorphic zircon almost always has Th/U ratio less than 0.1 in contrast with igneous zircon in which Th/U is normally greater than 0.4. Zircon grown during pegmatite formation also has low Th/U. Other trace elements such as REE also show some significant variations, which together with textural and geological evidence, allow metamorphic domains in zircon to be confidently identified for dating. The controls of these criteria are investigated in terms of formation temperature, paragenesis, water pressure, ionic radius and oxidation state.

L14 : 2A/02 : H2

Growth, Dissolution and Morphology of Zircon Crystals in High-Grade Metapelites ­ Implications for U-Pb Geochronology

Gerhard Vavra (gerhard.vavra@uni-tuebingen.de)

Institut fuer Mineralogie, Universitaet Tuebingen, Wilhelmstrasse 56, Germany

The phase relationships of zircon with other minerals during metamorphism are not well understood, but its behavior (growth, dissolution and morphology) is partly documented by empirical studies in metamorphic terrains.

Observations of the internal growth morphologies of crystals (cathodoluminescence) in a progressively metamorphosed rock sequence (Ivrea Zone) show that zircons in metapelites and metagraywackes grow during prograde upper amphibolite to granulite facies metamorphism, with a change of the crystal habit from prismatic to stubby and isometric. The prograde growth of zircon in the metapelites cannot be related to known metamorphic mineral reactions, such as the breakdown of biotite and concomitant formation of garnet, because garnet contains about an order of magnitude more Zr than biotite (Reinhardt et al. 1998). It is more likely that the mechanism of prograde zircon growth is Ostwald ripening: The matrix (probably interstitial anatectic melt) is supersaturated with respect to the larger detrital grains, so these grow larger, and is undersaturated with respect to the smaller (dust-size) detrital grains which grow smaller and eventually disappear. The succession from prograde growth to partial dissolution observed in many of the zircon crystals in metapelites probably reflects the attainment of ripeness. The small grains have disappeared and any further anatectic melt batches must subsequently become undersaturated also with respect to the larger grains.

Zircons in restitic metapelites and associated leucosomes have distinct growth morphologies: stubby and isometric habits with unstable planar crystal faces in the restites and normal prismatic habits with stable planar crystal faces in the leucosomes. This reflects the very different growth environments: high supersaturation caused by Ostwald ripening during prograde anatectic melting in the restites and low supersaturation during cooling and crystallization of the leucosome.

The observed relationships between zircon evolution and high-grade metamorphism help to interpret radiometric U-Pb data from metapelites. Since Ostwald ripening is an irreversible process, the prograde zircon growth in metapelites dates the earliest occurrence of a high-grade metamorphic event. Later high-grade events that encounter an already ripened grain size population will not cause any further prograde growth and, thus, may not be detected by U-Pb dating. Prograde zircons in restites and retrograde zircons in associated leucosomes are suited to bracket the peak of high-grade metamorphism and anatexis. The necessary analytical precision in combination with spatial resolution within crystals demands, however, further improvements of U-Pb dating techniques.

Reinhardt J, Vavra G, Reich S, Werner O & Horn A, Terra Nostra, 98/2, 123-126, (1998).

L14 : 2A/03 : H2

SIMS U-Pb Dating of Zircons From Migmatite Khondalites and Enderbite From Lapland Granulite Belt, Finland

Pekka Tuisku (pekka.tuisku@oulu.fi)¹ &

Hannu Huhma (hannu.huhma@gsf.fi)²

¹ Dept. of Geosciences, Univ. of Oulu, 90570 Oulu, Finland

² Geol. Surv. Finland, Box 96, 02151 Espoo, Finland

Zircons and monazites were separated from leucosome, melanosome (neosome) and mesosome (palaeosome) fractions from migmatitic garnet-sillimanite bearing khondalites of the Lapland granulite belt (LGB). Few hundreds of zircon grains were studied by SEM for their internal structure and U- and Pb-isotopes of representative grains were analysed by NORDSIM ionprobe. In addition, a conformable enderbite vein in a blastomylonitic migmatite was analysed.

Zircons in the palaeosome have euherdral-zoning pattern sometimes with igneous resorption discordances. Narrow homogeneous rims are grown around some grains, often separated from the zoned core by a shell of tiny inclusions. Contrary, the leucosome is characterized by homogeneous round shaped zircons, which may contain euhedrally zoned core, but the rim/core-volume ratio is generally much larger than in palaeosome. The zircons in melanosome have characteristics of both palaeosome and leucosome zircons but zoned grains are more prominent than homogeneous. Surprisingly, the zircons in enderbite are mostly homogeneous and rounded showing no zoning. However, zoned grains with or without homogeneous rim are also found.

The homogeneous zircon grains and rims in all rock fractions form a quite concordant and concentrated population having Pb-Pb and U-Pb ages around 1905-1880 Ma (± 4 to 15 Ma). In migmatitic khondalites they are interpreted to have grown during granulite facies metamorphism and anatexis around 1905-1880 Ma. As homogeneous zircon grains are even more prominent in enderbite than in orthoclase-granitic leucosomes of the migmatites, we preclude, that they are metamorphic "sensu stricto" but should rather be considered "igneous". They form a concordant population at 1906 ± 4 Ma. The homogeneous nature of these grains (as well as grains in melted parts of migmatites) probably reflects their stable crystallization conditions and long time of the crystallization process in the lower crust. Further evidence is that some of these grains show features similar to igneous resorption textures.

Zoned grains are mostly concordant or slightly discordant and give ages from 1950 to 2900 Ma and are interpreted to represent detrital grains in sedimentary precursor of the khondalite. Their rounded shapes and discordance of zoning provides textural evidence.

Analysis of SEM images is in accordance with SIMS data that homogeneous zircon grain and overgrowth populations in migmatitic khondalites are enriched in heavy elements U, Pb and Th relatively to the population of zoned grains (or zoned grains are depleted). Medians are 211, 92, 114 and 177, 81, 110 ppm respectively. The concentrations of U, Pb and Th in the population of zoned grains are scattered but they are much more concentrated in homogeneous grains. According to these features, detrital grains could represent different igneous source rocks varying from acid to intermediate and the grains most probably suffered partial lead loss either before or during granulite-facies metamorphism. Because the tie line and concordia run almost parallel between 1900 and 2500 Ma, it is very difficult to say the actual age of detrital grains, even if the grains seemingly are concordant.

The zoned "detrital" grains in enderbite are similar to those in khondalite in every respect and prove country rock or subducting slab inheritance and/or contamination. The U and Pb concentrations in homogeneous grains of enderbite are ten times higher than in khondalite (up to 2600 ppm). The probable reason of this peculiar feature is contamination of enderbitic magma by U-enriched carbon bearing layers of (meta)sedimentary country rocks.

L14 : 2A/04 : H2

Dating Metamorphic Events & Deformation: Shrimp U-Pb Zircon Examples from the Strangways Metamorphic Complex, Arunta Inlier, Australia

Andreas Möller (amoeller@mail.uni-mainz.de)¹,

Richard A. Armstrong²,

Bas J. Hensen¹ &

Ian S. Williams²

¹ School of Geology, University of New South Wales, Sydney, NSW 2052, Australia

² Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia

Selection of structurally controlled rocks and textural control by in-situ dating with the ion-microprobe facilitates the correlation of deformation and/or metamorphic events with the results of U-Pb zircon dating which are otherwise problematic due to inheritance or complex growth histories in zircons. Two Proterozoic phases of high grade metamorphism between 1770 and 1730 Ma have previously been reported for the Strangways Metamorphic Complex (SMC), whereas the exhumation of the granulites by thrusting along low-grade shearzones has been placed into the lower Carboniferous to Devonian Alice Springs Orogeny (ASO, 300-400 Ma). As a new approach to resolve the timing of the high-grade events, zircons from structurally defined orthopyroxene-bearing leucosomes in mafic rocks were analysed with the SHRIMP ion-microprobe. These leucosomes crystallised during the peak of metamorphism, and U-Pb ages of newly crystallised zircons place age constraints on metamorphism and deformation. Two leucosomes, a layer parallel (M1, D1) vein and a second crosscutting (M2, D2) vein were analysed and provide unambiguous evidence for a single high grade metamorphic episode followed by cooling at ca. 1715 Ma. The two metamorphic (M1, M2) events of earlier workers could not be temporally resolved by SHRIMP. They occurred within 10-15 Ma of each other and the second event apparently disturbed U-Pb systematics of zircons which grew during the first event at ca. 1730 Ma. We conclude that M1-M2 and the correlated periods of intense deformation (D1-D2) are part of a single tectonothermal event, and that the age of the main high-grade metamorphic event in the SMC has to be revised to 1715-1730 Ma. Previous, older estimates for granulite metamorphism in the SMC (1730 to 1770 Ma) were based on U-Pb zircon dating of deformed granitic intrusive rocks. Those results may represent inherited zircon populations or pre-metamorphic intrusion ages. Textural relationships of minerals in a staurolite bearing shearzone and in-situ dating of complex zircons reveal a phase of new zircon growth at 443 ±6 Ma (prior to the generally accepted age of the ASO) and followed by development of the St-Bt-Chl assemblage in which monazites have an age of about 385 Ma. The zircon growth event is contemporaneous with high grade metamorphic events in the Harts Range further to the east. The SMC thus has a quite different metamorphic history from the neighbouring Harts Range where upper amphibolite facies metamorphism during the Palaeozoic caused widespread resetting or recrystallisation of monazite. The results also indicate that good structural and textural control is essential to decipher discrete metamorphic or deformation events in complex metamorphic terranes.

L14 : 2A/05 : H2

Constraints on the Diffusivity of Pb in Monazite, its Closure Temperature, and its U-Th-Pb Systematics in Metamorphic Terrains, from a TIMS and SIMS Study

Randall Parrish (r.parrish@nigl.nerc.ac.uk)¹ &

Martin Whitehouse (martinw@nrm.se)²

¹ Univeristy of Leicester and, NERC Isotope Geosciences Laboratory, BGS, Keyworth, Notts, NG12 5GG, UK

² Swedish Museum of Natural History, SE-104 05, Stockholm, Sweden

The rententivity of Pb in monazite is poorly known but highly relevant to calculation of its closure temperature (Tc). Knowledge of the Tc of monazite must be better understood to interpret its complex U-Th-Pb systematics, especially in metamorphic rocks, where it is increasingly used to date metamorphism and/or cooling in the upper amphibolite facies. The early estimates of Tc were between 700-725°C, but were based on simplistic interpretations of isotopic systematics. Recently the study of Spear and Parrish (1996) has revised this estimate upwards significantly to over 750°C on the basis of preservation of 1.45 Ga isotopic signature in monazite in a metamorphic rock subjected to younger metamorphism at P-T conditions of 820±30C and 8±1 kb at about 75 Ma ago. We have taken a more detailed look at monazite from this particular sample by making U-Th-Pb ion microprobe measurements on ca. 20 µ m spots using the Nordic Cameca 1270 instrument (NordSIMS facility). Three polished grains of monazite have been used to construct U-Th-Pb age maps, using a traditional calibration approach with a monazite standard which is 1876 Ma old. Although there is clearly considerable new growth of monazite in separate grains and in portions of the 'rims' of at least two of the three grains analysed, the three crystals preserve ²⁰⁸Pb/²³²Th and ²⁰⁶Pb/²³⁸U ages as old as 1.38 Ga, with all of the analyses lying on a discordia between 1.45 Ga and ca. 75 Ma, with discordance of individual points falling between 5-100%. One of the crystals appears to consist entirely of an older grain with minimal new growth, and the data conform to the pattern expected of diffusion of Pb toward the rim, with a fractional loss from the crystal of approximately 0.4 during the younger 820°C event. Although knowledge of the duration of the younger event is required to calculate a diffusion coefficient with the data, preliminary calculations clearly imply that the notional estimates of Tc of 700-725°C often cited are too low by at least 50°C. A review of existing published data and these new observations suggests strongly that given normal monazite crystal quality and chemistry, monazite should not lose all of its Pb during conditions of amphibolite facies to lower granulite facies of metamorphism. The concept of 'thermally reset' monazite ages, or of monazite 'cooling ages' should therefore be abandoned henceforth.

Spear, F & Parrish, R, J Petrology, 37, 733-765, (1996).

L14 : 2A/06 : H2

EMP Dating of Low-Th Monazite and the Discrimination of Caledonian and Variscan Metamorphism in the Aar Massif, Swiss Alps

Kenneth Livi (klivi@jhu.edu) &

Sakiko Olsen (olsen@gibbs.eps.jhu.edu)

Dept. Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA

The use of the electron microprobe (EMP) as a geochonology tool has dramatically increased in the past few years. Much success has been met with its application to monazite crystals that contain high amounts of Th oxide (commonly >5 wt%). As the amount of Th and age decreases, the precision and accuracy decrease as potential systematic errors increase. We present results that yield geologically meaningful ages for monazites that are both young (300-450 Ma) and contain low amounts of Th oxide (2-5 wt%). Analytical parameters that increase in importance for these samples are: 1) the proper positioning of backgrounds, 2) monitoring of absorption edges of major peaks, 3) care in modeling peak overlap corrections, 4) corrections for non-linear backgrounds, 5) jumps and "holes" in spectrometer sensitivity, and 6) bulk composition ZAF corrections. Other considerations are dead-time corrections for high-intensity peaks, and contamination build-up.

Samples from the Lauterbrunnen migmatites (LM), part of the Innertkirchen-Lauterbrunnen Crystalline Zone (ILC) in the Aar Massif, were selected to date the age of migmatization. U-Pb zircon age from the LM samples is ~300 Ma, but equivalent migmatites at Innertkirchen region 30 km away (IM) yielded zircon ages of ~450 Ma (Schaltegger, 1993). Compositions of most monazites analyzed follow trends where Ca, Th, Y and HREE exchange for LREE. Observed zonation patterns can be complex and do not correlate with calculated ages. EMP dates of monazite within the LM cluster around 357 ± 40 Ma. Within the equivalent IM, monazite ages of 475 ± 60 Ma are within error of associated zircon ages of ~450 Ma, with the exception of one sample which yielded a preliminary EMP age of 656± 118 Ma. A similar age of ~ 620 Ma by U-Pb zircon analyses has been reported from the eastern Aar Massif (Schaltegger, 1993). Thus, EMP ages of monazite and U-Pb ages of zircon are in relative agreement within the ILC. These results show that EMP dates of low-Th monazite can be used to differentiate events occurring during the Caledonian and Variscan orogenies important to European geology.

L14 : 2A/09 : H2

Single Grain Chemical Dating of Young Monazite by XRF-MiniProbe

Martin Engi (engi@mpi.unibe.ch)¹,

Andrij Cheburkin (EMMA_Analytical@compuserve.com)² &

Nadim Scherrer (scherrer@mpi.unibe.ch)¹

¹ Min.-petr. Inst., University of Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland

² EMMA Analytical, Elmvale, Ontario L0L 1P0, Canada

Monazite is useful for dating different stages of medium to high grade metamorphism and deformation because it tends to recrystallize readily when strained, and its Th-U-Pb system remains closed up to temperatures >700^oC. As monazite hardly incorporates common lead, chemical dating is possible. For "old" (>100 Ma) monazite, standard electron microprobe analysis (for Th, U, and Pb) permits single grain dating; for younger samples the small Pb K<alpha>-peak cannot be measured with sufficient accuracy due to the rather high background. This limitation was overcome by a new mini-XRF instrument, the design features and performance of which are presented for a number of applications.

The instrument used consists of a conventional 2 kW Mo X-ray tube (thin slit), a focused LiF monochromator, a rotating stage holding the sample (mounted on 4µ m Prolene film) on a petrographic microscope, and a Si(Li) detector. U/Pb and Th/Pb ratios are measured using Pb L<alpha> (10.5 keV); Th L_l (11.12 keV) and U L_l (11.62 keV) lines. For single grains as small as 50 µ m the detection limits for U, Th and Pb are on the order of 10 ppm for analysis times of 10 minutes, the time required to obtain ages to within 3-8% accuracy for each grain. These errors are less for longer analytical periods. More importantly, statistical analysis of age data for populations of single grains is useful in dating polymetamorphic samples.

For a suite of metamorphic samples from the Central Alps, for which monazite had been previously dated by one or more isotopic methods, chemical monazite ages were measured using this XRF method. Comparison shows excellent agreement for recrystallized samples as young as 15 Ma; where isotopic ages were discordant, chemical dating indicates bimodal (or more complex) distributions of the single grain age data. Problems due to single grain zoning will be shown.

Being non-destructive, simple and inexpensive, this new analytical method thus offers several advantages, particularly in deciphering and interpreting polymetamorphic samples from "young" orogens.

L14 : 2A/10 : H2

REE Zonation in Garnets as a Monitor of Accessory Phase Stability in Igneous and Metamorphic Systems

Christophe Iain Prince (c.i.prince@open.ac.uk)¹,

Detlef Gunther (detlef@erdw.ethz.ch)²,

Nigel B. W. Harris (n.b.w.harris@open.ac.uk)¹ &

Derek Vance (d.vance@open.ac.uk)¹

¹ Department of Earth Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA, England

² D-ERDW, ETH Zentrum, CH-8092 Zurich, Switzerland

Garnet is used extensively for PT determinations and major-element zoning in this mineral provides prograde thermobarometric information (at least up to upper-amphibolite facies conditions). Although age information can be extracted from garnets it is often difficult to obtain. Conversely accessory phases such as monazite and zircon yield high precision age information but it is often difficult to relate to the PT history of the rock. The work presented here demonstrates that garnet zonation provides a monitor of accessory phase behaviour during crystallisation of igneous and prograde metamorphic assemblages, thus enhancing the utility of garnet-bearing assemblages for accessory phase chronometry.

In-situ REE element concentration profiles have been obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Two samples have been studied from Himalayan assemblages; a kyanite-zone pelitic schist and a deformed leucogranite. In both cases garnets retain major-element zonation and REE concentrations are broadly symmetrical with the HREE concentration decreasing from core to rim. For the metamorphic garnet the rimward decrease in the HREE is extremely sharp with a concomitant increase in the LREE concentrations. For the igneous garnet a large increase in both HREE and LREE is observed at the rim. The overall rimward decrease in HREE concentration in both the metamorphic and the igneous garnets suggest that strong partitioning of HREE into the growing garnet depletes the remainder of the bulk rock in these elements. However, the changes in the core of the metamorphic garnet and at the outer rim of the igneous garnet require additional controls. Chondrite-normalised plots show distinct changes in REE profiles with garnet growth indicating changes to the bulk REE partition coefficients (KdREE). We propose that these changes result from the stability of apatite and monazite in the igneous and metamorphic systems respectively.

The integration of thermobarometric and accessory phase stability information obtained from garnet zonation with the precise age information obtained from accessory phases can thus, potentially, provide tight constraints on the petrogenetic history of the rocks. However, further work is required to (i) obtain trace-element concentrations which can be unequivocally linked to individual accessory phase behaviour and (ii) improve the sensitivity of LA-ICP-MS to allow more precise analyses of the LREE which occur in low abundances in garnet.

L14 : 2A/11 : H2

Monazite-Group Minerals in Granulite-Facies Metabasites from the Ivrea Verbano Zone, N. Italy: The Role of Phosphate Minerals in High Grade Fluid Flow

Daniel E. Harlov (dharlov@gfz-potsdam.de) &

Hans-Juergen Foerster (forhj@gfz-potsdam.de)

GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, F.R. Germany

Composite populations of monazite-group minerals have been discovered in thin layers of granulite-facies metabasites (Opx-Gt gneisses) from the Val Strona de Omegna region of the Ivrea-Verbano Zone (IVZ), N. Italy, which is part of the pre-Alpidic basement of the southern Alps. The base of the IVZ consists of a succession of granulite-facies metagabbros of basaltic origin. This Mafic Formation is overlain by a sequence of interlayered metapelites and metabasites which range in metamorphic grade from granulite to middle amphibolite facies. It is commonly postulated that thermal input from the magmatic underplating responsible for the Mafic Formation (ca. 300 Ma) induced granulite facies metamorphism in the overlying rocks (Franz and Harlov, 1998). Monazite is widespread in the granulite-facies metabasites and is hosted by a variety of major and accessory minerals including Gt, Plg, Qtz, Ap and Ilm and, in general, is distinguished by a large compositional diversity in terms of lanthanide ((sum)REE₂O₃ = 34.5 - 70.1 wt%) and ThO₂ concentrations (from below the microprobe detection limit to 35 wt%). Monazite inclusions in Gt contain up to 30.1 mol% ThSiO₄. Monazites this rich in the huttonite component are known from only half a dozen other occurrences worldwide. In distinct contrast, we also observe tiny monazite microcrystals (less than 1 - 2 µm) inside the apatites and somewhat larger grains on the Ap surfaces that are strongly enriched in REE's but extremely poor in both Th and U. These specific compositions are in accordance with those obtained from previous studies of monazite inclusions in Ap from various high-grade metamorphic rocks (e.g. Pan and Fleet, 1993). Generally, the formation of monazite as a primary (igneous) mineral is incompatible with the basaltic nature of its host rock, in which major minerals, such as Amph, Pyx and Gt commonly account for the bulk of the lanthanides and actinides. In the case of the IVZ metabasites, crystallization of monazite should be secondary and is most likely linked to contact-metamorphic overprinting and associated high-grade fluid flow which has been suggested to have affected the metabasites during emplacement of the Mafic Formation and subsequent granulite-facies metamorphism (cf. Franz and Harlov, 1998). We propose that breakdown of primary monazite-(Ce) in the interlayered metapelites during granulite-facies metamorphism mobilized the REE's and Th, which were then transported by high-grade metamorphic fluids into the metabasite layers to form the Th-rich minerals of the monazite-huttonite series as well as induce monazite formation in the apatites via incongruent dissolution (cf. Wolf and London, 1995).

Franz L, Harlov DE, J. Geol, 106, 455-472, (1998).

Pan Y, Fleet ME, Mineral. Mag, 57, 697-707, (1993).

Wolf MD, London D, Amer. Mineral, 80, 765-775, (1995).

L14 : 2A/12 : H2

Inclusions and the Role of Trace Element Partitioning in the Retentivity of Mineral Chronometers During Metamorphism

Jan D. Kramers (kramers@mpi.unibe.ch)¹,

Valérie Chavagnac (vchava@mit.edu)² &

Igor M. Villa¹

¹ Min. Pet. Inst, (Isotope Geology), Erlachstrasse 9a, 3012 Bern, Switzerland

² Dept. Earth, Atm. & Planet.. Science,, Mass. Inst. of Technology, Cambridge, MA 02139, U.S.A.

Mineral chronometers are reset by isotope exchange with the medium surrounding them. The frequent assumption that this medium is effectively an infinite sink or source is probably true if transport is by advection in a fluid on grain boundaries. If a chronometer grain is entirely enclosed (armoured) in another mineral, or if fluid is absent (e.g. in high grade metamorphism or if a melt is nearby) it becomes important to consider trace element transport in the surrounding minerals.

If transport is treated as volume diffusion in such a case, the effective diffusivity is governed by DxK_D (D = diffusion coefficient, K_D = trace element partition coefficient): Close to the interface between the dateable mineral A and its surrounding phase B, concentration gradients for radiogenic isotopes are related by  C_A/ x = K_D C_B/ x. If the radiogenic isotope is strongly compatible in phase A (K_D >>1), then exchange is impeded by phase B unless the diffusion coefficient in phase B is much greater. If K_D <1, diffusivity in B will rarely be a limiting factor. An example of the effect of a high K_D is a relic apatite Sm-Nd age of 2.4 Ga obtained in a migmatite in the Mahalapye Complex, Botswana, in the Central Zone of the Limpopo Belt, which underwent high grade metamorphism at 2.0 Ga. This appears to contradict the low retentivity predicted for apatite on the basis of high ionic porosity (Dahl, 1997). In low K_D minerals with a low internal diffusivity, as is the case for Nd in garnet, a strong isotope ratio gradient of results within the grain. This may explain the widely varying estimates of the 'closure temperature' of the Sm-Nd systematics in this mineral. In low K_D minerals with a high diffusivity, as for Sr in biotite, both factors combine to facilitate resetting.

A special case are U-Th-Pb chronometer minerals, as radiogenic Pb in these is almost certainly tetravalent (Frei et al., 1997) and is therefore similarly compatible to the parent U and Th isotopes. This would explain the extreme retentivity of U-Th-Pb systematics in minerals dated by them, frequently withstanding high grade metamorphism or incorporation into a melt.

Chavagnac, V., Ph.D. thesis Univ. Rennes and Bern, 405 pp, (1998).

Dahl, P., Earth Planet. Sci. Lett., 150, 277-290, (1997).

Frei, R. and 7 others, Geochim. Cosmochim. Acta, 61, 393-414, (1997).

Geochronology of Heterogeneous Minerals: 1. Polymetamorphic Amphiboles

Igor M Villa (igor@mpi.unibe.ch)

Isotopengeologie, Erlachstrasse 9a, 3012 Bern, Switzerland

Amphibole is an index mineral of a great number of metamorphic terranes. It is largely stable under retrograde evolution and is an excellent qualitative thermobarometer. Due to the numerous substitutions in its chemical formula, practically any retrograde recrystallization is petrologically clearly distinguishable from the pristine "peak amphibole".

As heterochemical overgrowths retain the record of the previous history, dating each amphibole generation separately is an attractive problem. ³⁹Ar/⁴⁰Ar stepwise heating of amphiboles has long been known to frequently yield discordant age spectra. A new approch that allows extracting age information from such spectra is based on the observation [1] that Ar is released in vacuo in narrow, discrete pulses that correspond to structure breakdowns. It is straightforward to extrapolate the ionic porosity approach of Dahl [2] to explain why amphiboles with different bond lengths/strengths break down at different crucible temperatures.

Deconvolving discordant spectra in terms of sequential breakdown of different amphibole generations requires diagnosing and identifying them. This is done by three-isotope correlation diagrams constructed from ³⁷Ar, ³⁸Ar and ³⁹Ar, hereafter referred to as Ca/Cl/K plots [4]. In these plots, data points are always enclosed by a polygon, whose N vertices represent discrete mixing endmembers. N=2 is the rare case of a binary amphibole mixture. Artificial binary mixtures also behave as predicted [3]. This approach has been recently applied to reconstruct the metamorphic evolution of amphibolites [4]. Their independently known geological history validates the approach completely.

In the Malenco serpentinites [5], electron microprobe and BSE mapping are a necessary complement to the ³⁹Ar/⁴⁰Ar stepheating, as (1) each analysed rock contains at least three chemically and texturally resolved amphiboles that are intergrown on the 5-10 µm scale; (2) the amphibole generations petrographically identified have Ca/Cl/K ratios determined by EMP that match those inferred from ³⁷Ar/³⁸Ar/³⁹Ar plots; (3) the coexisting amphiboles of each rock can be assigned a characteristic Na(M4) value, i.e. a pressure. The ³⁷Ar/³⁸Ar/³⁹Ar/⁴⁰Ar* correlations allow to date the Na(M4)-rich and -poor amphibole generations. i.e. a P-T-t segment.

The very retentive behaviour of amphiboles supports the conjecture [6] that the rate of Ar loss is limited by the migration of lattice defects, whose mobility is in turn controlled by that of structure-forming cations such as Si, Al, Ca, etc. In amphibole this ensures that every generation will have a different Cl/Ca/K signature and at the same retain its K/Ar age.

Lee JKW, Chem Geol, 106, 133-170, (1993).

Dahl PS, Geochim Cosmochim Acta, 60, 3687-3700, (1996).

Villa IM et al, Contrib Mineral Petrol, 126, 67-80, (1996).

Villa IM & Boriani AC, Schweiz Mineral Petrogr Mitt, 77, 381-401, (1997).

Villa IM, Hermann J, Müntener O & Trommsdorff V, Contrib Mineral Petrol (subm)

Villa IM, Terra Nova, 10, 42-47, (1998).

L14 : 2A/14 : H2

Geochronology of Heterogeneous Minerals: 2. K-Feldspars from the Aar Metagranite, ³⁹Ar/⁴⁰Ar Thermochronometers or Hygromorphometers?

Nathalie Challandes (Nathalie.Challandes@geol.unine.ch)¹ &

Igor M. Villa (igor@mpi.unibe.ch)²

¹ Institut de Géologie, Université de Neuchâtel, Switzerland

² Isotopengeologie, Universität Bern, Switzerland

The Aar Massif metagranite belongs to the External Crystalline Massifs of the Swiss Alps and is deformed by a main Tertiary heterogeneous ductile deformation. This high-temperature (430-450°C) deformation leads to the occurrence of anastomosed shear zones surrounding lenses of weakly deformed rocks.

In a progressively deformed profile near a shear zone we selected a sequence of three rocks over a total physical distance of 100 m: a weakly deformed metagranite, an orthogneiss and a mylonite. K-feldspars (Kfs) were separated, irradiated and analysed by ³⁹Ar/⁴⁰Ar stepwise heating. Rb/Sr analyses on whole rocks (WR) and Kfs were by bulk dissolution; additionally, Kfs Aar 10 was leached stepwise, yielding a fraction L soluble in hot HCl and a residue R.

The least deformed sample, Aar 10, gives a staircase-shaped age spectrum, such as are frequently interpreted as "diffusion profiles" (fig. 1). The two more deformed samples, Aar 12 and Aar 17, have flat age spectra which define the same age as the initial heating steps of Aar 10.

Two interpretations are possible: (1) following "domain" theory, heating by friction induced Ar loss from an unmodified, homogeneous Kfs. The "domains" inferred using Fick's Law constrain the thermal history of Aar 10. As paleotemperatures are independently known, Aar 10 could only have spent ¾0.95 years at peak T. The unreasonable slip rate discredits this approach. (2) Petrographic observation shows all Kfs, especially Aar 10, to be a heterogeneous mixture. This is easily diagnosed in three-element correlation diagrams (fig. 2), which demonstrate the coexistence of a young, Cl-poor and Ca-rich Kfs overgrowing an older, Ca-poor Kfs generation.

Even more conclusive proof is provided by the Sr isotopic record. WR of deformed rocks have higher ⁸⁷Sr/⁸⁶Sr, demonstrating open-system behaviour and mass transfer. The ⁸⁷Sr/⁸⁶Sr ratio of Kfs Aar 10-L is 0.7355±3, while that of R is 0.7328±3. The alteration signature in L is clear.

The "Kfs separates" thus consist of distinct mineral phases that were formed diachronically and that retain their characteristic chemical signature. They are termed hygromorphometers because their morphology (comminution and recrystallization in the shear zone) is favoured or even controlled by the availability of water, as attested by the petrography of the rocks. Their Ar/Ar age spectra have nothing to do with Fickian diffusion.

Session L14:2B

L14 : 2B/21 : H2

Quantitative Models of Metamorphic Reaction Mechanisms and Textures Using Differential and Irreversible Thermodynamics

C. T. Foster (tom-foster@uiowa.edu)

Geology Department, University of Iowa, Iowa City, IA, USA

Textures in metamorphic rocks provide a record of the P-T-X conditions a rock has experienced because they are produced by reaction mechanisms that develop in response to changing environments during metamorphism. Thus, development of quantitative techniques to reliably interpret metamorphic textures will considerably improve our understanding of metamorphism by increasing the resolution of P-T-X-t information that can be extracted from metamorphic rocks.

To gain insight about processes that control texture development, the reaction mechanisms produced by various P-T-X-t paths have been quantitatively modeled using differential and irreversible thermodynamics, allowing rock textures to be calculated for the paths. The technique employs three different types of calculations: 1) the whole-rock reactions and mineral modes along the path are calculated using differential thermodynamics, 2) nucleation patterns are determined using a method that relates overstepping to domains of local equilibrium in heterogeneous rocks, and 3) reaction mechanisms are simulated using irreversible thermodynamics to model material transport between reaction sites. The method permits calculation of a sequence of textures that show the spatial distribution of minerals in a rock at each time step in its history. It also identifies the reaction mechanisms that produced the textures and quantifies the chemical potential gradients that control material transport between reaction sites for each time step.

Important features that strongly affect the types of textures that develop are: 1) the way the overall reaction is influenced by the P-T-X history and bulk composition of the rock; 2) the nucleation pattern, which is influenced by rates of reaction overstepping, scales of effective transport and compositional heterogeneity; 3) controls on material transport along grain boundaries; 4) the availability of components from external sources; and 5) deformation. Variation of these parameters produces a wide array of distinctive and easily recognized textures in typical metamorphic rocks. Identification of these patterns on a regional scale in metamorphic terranes should allow detailed constraints to be placed on the temporal and spatial variation of P-T-X in four dimensions, providing new insights into the processes that control metamorphism.

L14 : 2B/22 : H2

Construction and Assessment of Metamorphic Histories

Graham J. Potts (gpotts@liv.ac.uk)¹ &

Steven M. Reddy (sreddy@lithos.curtin.edu.au)²

¹ Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, U.K.

² School of Applied Geology, Curtin University, Perth WA 6001, Australia

An accurate, integrated metamorphic/deformation history is an essential requirement of any investigation of the rates and mechanisms of metamorphic processes. However, as we shall demonstrate, such histories are inherently ambiguous. In recent years the relationships between porphyroblasts and foliations have been examined in considerable detail in order to establish unequivocal geometrical criteria with which the relative ages of these features may be determined. To address the systematics of metamorphic/deformation histories we take the conclusions of this work as our starting point and assume that the relative ages of porphyroblasts and foliations can be established unequivocally. The systematics of metamorphic/deformation histories have been investigated using basic combinatorial mathematics. Using this approach, for a set of porphyroblasts and foliations, the number of possible histories can be established together with the number of relationships that must be observed to substantiate them. Using this information, the ambiguous nature of all metamorphic/deformation histories will be demonstrated together with the impact of unobserved relationships. A procedure will be described for the construction of metamorphic histories that is systematic, repeatable and rapid and within which ambiguities can be easily recognised. The effects of these ambiguities upon the interpretation of porphyroblast growth are highlighted using several case studies.

L14 : 2B/23 : H2

Non-Lithostatic Pressure Build-Up at Onset of Continental Convergence and its Implications for pTt-Paths

Katja Petrini (katja@erdw.ethz.ch)¹ &

Yuri Podladchikov (yuri@erdw.ethz.ch)²

¹ Inst. Min. Petr., Sonneggstr.5, 8092 Zurich, Switzerland

² Inst. Geology, Sonneggstr.5, 8092 Zurich, Switzerland

The aim of this study is to test the validity of the common belief that pressure estimates obtained from geobarometry are related to depth estimates by the simple relationship p = <rho>gz. For this purpose we calculate the pressure distribution of a layered lithosphere in a compressive tectonic setting using a 2-D finite element code with coupled mechanical and thermal response. The model rheology is elasto-plastic or thermally activated power law creep. Our results show that during convergence pressures significantly higher than the pressure resulting from the overburden (lithostatic pressure) are achieved throughout the entire lithosphere. The brittle upper crust and uppermost mantle attain pressures twice lithostatic values (p = 2<rho>gz); in the viscous lower crust and the lower upper-mantle the pressure gradient relaxes, but the pressures are still significantly higher than lithostatic. Our results suggest that care should be taken in the interpretation of pressure estimates inferred from metamorphic mineral assemblages and, conversely, when calculating pTt-paths from tectonic models. For example, we show that high p/T ratios are not necessarily caused by low temperature gradients but can be due to high-pressure gradients achieved by horizontal shortening of the normal continental lithosphere. The model shows that tectonic stresses can increase the thermodynamic pressure rock experiences without an increase in burial depth. Thus, the model provides a mechanically and thermodynamically justified alternative to the conventional models of Barrovian metamorphism as a response only to burial. Furthermore it greatly simplifies the problem of formation and exhumation of high and ultra-high pressure rocks.

L14 : 2B/24 : H2

Chlorite-Phengite Thermobarometry

Olivier Vidal (vidal@cristal.ens.fr),

Teddy Parra &

Romain Bousquet

E.N.S., Lab. Géologie, 24, rue Lhomond, Paris, France

Trioctahedral chlorites and phengites are potentially valuable indicators of the P-T conditions of metamorphism, because 1) they are widespread in rocks with a wide range of composition, 2) it has been shown experimentally that their composition vary with varying the P-T conditions of metamorphism, and 3) their composition does not completely reequilibrate with changing the P-T conditions during the metamorphic history i.e. several generations of chlorites and phengites involved in the same parageneses but showing various compositions can be found in different portions of the same hand sample and eventually of the same thin section. This last observation is of particular importance, because it suggests that identifying the relative time of growth of chlorites and phengites will provide informations on the evolution of the P-T conditions through time. Unfortunately, no activity model accounting for the observed compositional variability in chlorites and phengites is yet available. Indeed, published activity models allow at best the Tschermack and Fe-Mg substitutions to modelized, but they do not account for the di/trioctahedral (DT) substitution in chlorite (responsible for octahedral vacancies and different Al^iv vs Al^vi contents) and the pyrophyllitic (PR) substitution in phengites (responsible for interfoliar vacancies and and different Al^iv vs A^vi contents). A revue of the composition of phyllosilicates in samples metamorphosed at P-T conditions ranging from 350°C-5 kbar to 750°C-30 kbar indicates that the DT and PR substitutions are both significant and cannot be neglected at T < 450°C. The aim of the present study was to derive 1) new on-site activity models for chlorites and phengites and 2) thermodynamic data for daphnite, 14 A chlorite of amesite composition and celadonite. The amesite and celadonite data were extracted using published experimental results on the location of 6 equilibria involving chlorite and 4 equilibria involving phengite in MASH along with Berman's (1988) thermodynamic data for clinochlore and muscovite. P-T dependant WAlMg interaction terms were necessary to get consistency among the different sets of experiment obtained over a large range of P-T conditions (450-850°C, 6-35 kbar). WAlFe terms and daphnite thermodynamic data were estimated using more than hundred natural samples metamorphosed at well-known P-T conditions, including the Fe-Mg partitioning data between Chl-Ctd used to calibrate the Chl-Ctd thermometer of Vidal et al. (1999). Using these data, we show that chlorite-phengites assemblages are valuable indicator of the P-T conditions and allow continuous determination of P-T paths in rocks devoid of low variance assemblages.

Berman, RG, . Journal of Petrology, 29, 445-522, (1988).

Vidal O, Goffé B, Parra T, Bousquet R, Journal of metamorphic geology, 17, 25-40, (1999).

L14 : 2B/25 : H2

Phengite Thermobarometry ­ How Accurately can we Determine Metamorphic Conditions?

Guy Simpson (simpson@erdw.ethz.ch),

Alan Thompson (alan@erdw.ethz.ch) &

Jamie Connolly (jamie@erdw.ethz.ch)

IMP, Sonneggstr. 5, ETH Zurich, Switzerland

Several studies have extracted thermodynamic data for the Mg- and Fe-Tschermak end-member components of phengitic muscovite on the basis of experimental studies carried out over the last 30 years. This data permits calculation of stability fields of high-variance assemblages, which can be used to constrain metamorphic pressures and temperatures. There are, however, several different sets of thermodynamic data and solution models for celadonite and other phyllosilicate end-member components. To test whether calculations based on the available thermodynamic data match experimental and field constraints, a series of calculations were performed on various multivariant assemblages involving combinations of phengite, chlorite, biotite, K-feldspar, quartz and water in KFMASH using THERMOCALC (and the data set of Holland and Powell, 1998) and PERPLEX (using data and solution models obtained by Massonne and Szpurka, 1997). Four multivariant equilibria are potentially useful as thermobarometers in low-medium grade metamorphic rocks of granitic and pelitic composition. At phengite compositions close to the muscovite end-member, calculations (irrespective of the data source and solution models) show reasonable agreement with respect to experimental and field-based data. All calculations display progressively larger discrepancies with experimental and field data as phengite compositions approach the celadonite end-member. Fortunately, because celadonite-rich compositions (i.e. > 50 mol.% celadonite) are unusual in nature, the actual differences resulting from the choice of data and solution models are anticipated to be significantly less than 5 kb for any given temperature and composition.

Holland TJB & Powell R, Journal of Metamorphic Geology, 16, 309-343, (1998).

Massonne H-J & Szpurka Z, European Journal of Mineralogy, 1, 229-250, (1997).

L14 : 2B/26 : H2

New Garnets for Old: Cautionary Tales from Young Mountain Belts

Tom Argles (t.w.argles@open.ac.uk),

Christophe Prince,

Gavin Foster &

Derek Vance

Department of Earth Sciences, Walton Hall, Milton Keynes, UK

Major and trace elements in garnet are commonly used to characterize P-T-t paths in orogenic belts. They are often used without age constraints to obtain P-T estimates alone. However, some garnets in the cores of young orogenic belts may have grown not during the most recent orogeny, but much earlier. Such 'old' garnets can preserve original major element growth zonation, and thus information on their host rock's early history. Among a number of important implications for orogenic studies, they strike a cautionary note for thermobarometry.

During investigations into the prograde phase of the Alpine-Himalayan Tertiary orogeny, garnets with anomalously old Sm-Nd ages were discovered in three different areas. They occurred in similar parageneses to garnets which grew during Tertiary prograde metamorphism related to collisional thickening and burial. Garnets with homogenized major elements from the Garwhal Himalaya and Nanga Parbat give ages >500 Ma, while a sample from the Betic Cordillera with distinctive growth zonation gives an age of 235±1.3 Ma.

While the Garwhal date coincides with a recognized magmatic and metamorphic event, the 235 Ma age from the Betics at first sight appears one of the more unlikely times for garnet growth. However cores and rims are entirely distinct in this sample, which with the quality and reproducibility of the analyses (on core separates) suggests the date to be a genuine record of core growth, with a hiatus before resorption and rim development. Preliminary research suggests initial growth may be linked to early crustal extension during Pangaea break-up.

The robust nature of garnet and its low diffusivity for many elements - confirmed by these results - have placed much of the burden of orogenic research onto its shoulders. The complex history of polymetamorphic orogenic internal zones is often difficult to unravel, and garnet has become invaluable in structural, metamorphic and chronometric studies. The significance of garnet dating lies in its integration of chronometric with structural and metamorphic information (e.g. Mezger et. al., 1989; Vance et. al., 1998), so the discovery of such old garnets unlocks a wealth of potential information on events previously obscured by the most recent tectonic and metamorphic reworking. At the same time, there are serious implications for metamorphic petrology in terms of assumed garnet-matrix equilibrium, progressive metamorphism, and specifically garnet-based thermobarometry.

Mezger K, Hanson GN & Bohlen SR, Contributions to Mineralogy and Petrology, 101, 136-148, (1989).

Vance D, Strachan RA & Jones KA, Geology, 26, 927-930, (1998).

L14 : 2B/29 : H2

Peak Temperatures and Thermal Histories in Ultrahigh-Temperature (UHT) Metamorphism

Simon L Harley (sharley@glg.ed.ac.uk)

Dept of Geology and Geophysics, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh, Scotland, UK

Ultrahigh-temperature (UHT) crustal metamorphism is generally best documented by mineral assemblages in metapelites that are constrained by FMAS and KFMASH petrogenetic grids. Sapphirine + quartz is stable only at >1040°C in reduced rocks, and orthopyroxene + sillimanite + quartz is restricted to pressures greater than 8 kbar at T > 900°C. New Fe-Mg exchange thermometry and thermometry based on Al in orthopyroxene coexisting with garnet now allows further evaluation of the peak or near-peak temperatures in UHT terrains. The Ganguly et al. (1996) garnet-orthopyroxene Fe-Mg thermometer yields temperature estimates in the range 900-1100°C for several UHT case studies, but closes at lower temperatures in other cases. Thermometry based on the high Al₂O₃ contents (8-12 wt%) of orthopyroxene coexisting with garnet, in sillimanite- and sapphirine-bearing assemblages that are constrained from the grids to be >1000°C, support the UHT nature of the areas for which reliable data are available. For example, the FAS-system orthopyroxene Al-geothermometer (Aranovich and Berman, 1997) yields temperature estimates of 960±50°C for granulites from the Napier Complex, Rauer Islands, Sri Lanka Eastern Ghats and similar UHT areas.

Elemental mapping of Al zoning in orthopyroxene coexisting with other aluminous phases provides key insights into pre- and post-peak thermal histories of UHT terrains. In the Rauer Islands early orthopyroxenes formed with garnet are zoned from cores with only ca. 8 wt% Al₂O₃ to rims with up to 10.5 wt% Al₂O₃. This zoning reflects heating through 90±20°C in the interval 950-1050°C at 9-12 kbar, and corresponds with the likely temperature interval over which melts have been generated in these gneisses. In contrast, in the Napier Complex orthopyroxene that coexists with sapphirine and quartz zones from over 12 wt% Al₂O₃ in cores down to only ca. 9 wt% Al₂O₃ in rims and recrystallised grains. These compositional and textural features reflect the reaction

2MgAl₂SiO₆ = Mg₂Al₄SiO₁₀ + SiO₂ (A).

The zoning in the orthopyroxene reflects cooling through 80°C within the stability field of sapphirine + quartz, and leads to a new estimate of at least 1120±20°C for the peak temperature of regional metamorphism in this UHT terrain. Thermodynamic modelling of reaction (A) indicates that present estimates for the entropy of high-T sapphirine may be too high by 15-18% compared with entropy estimates that are consistent with reconnaissance MAS system experiments.

Aranovich LYa & Berman RG, Am Mineral, 82, 345-353, (1997).

Ganguly J, Cheng W & Tirone M, Contrib Mineral Petrol, 126, 137-151, (1996).

L14 : 2B/30 : H2

Structural-Metamorphic Evolution of Garnet-Epidote-Biotite Gneiss from the Moine Supergroup, Scotland: Mineral Zonation, Reaction History, Garnet Modelling and Geotectonic Implications

Armin Zeh (kris046@rzbox.uni-wuerzburg.de)¹ &

Ian L. Millar (ILM@BAS.AC.UK)²

¹ Mineralogisches Institut, Am Hubland, D-97074 Wuerzburg, Germany

² NERC Isotope Geoscience Laboratory, Nicker Hill, Keyworth, Nottingham NG 12 5 GG, United Kingdom

Highly garnetiferous lenses within metapelitic gneisses of the Glenfinnan Group of the Moine Supergroup, Scotland, contain a few large and numerous small garnets, associated with zoned epidote and plagioclase, in a biotite matrix. The large garnets show four zones (AI to AIV), small garnets only three or less. The zonation patterns of the distinct garnet types clearly indicate successive garnet formation with increasing nucleation densities from 10 Grt/dm³ to about 400 Grt/dm³.

The shape and internal texture of garnet A testify that garnet zones AI and AIV grew under static conditions, whereas the formation of garnet zones AII and AIII were accompanied by deformation. Furthermore, mineral inclusions and zonation patterns indicate successive garnet formation in four distinct assemblages (all: +biotite +plagioclase +quartz): (1) garnet AI/AII + chlorite + muscovite + epidote ± ilmenite ± titanite, (2) garnet AIIIa + muscovite + epidote + titanite, (3) garnet AIIIb + muscovite + epidote + magnetite + titanite, and (4) garnet AIV + muscovite + quartz + magnetite + titanite.

Geothermobarometric calculations and Gibbs method P-T path, P-T-X-M and garnet growth modelling point to fast growth of garnet zone AI during heating from 550 to 560°C at pressures of about 6.0 kbar. In contrast to garnet AI, the syntectonic formation of garnet zone AII was accompanied by a nearly isothermal compression from 6 to 8.5 kbar (at about 560°C), indicating crustal stacking and thickening. Subsequently, garnet AIII growth took place during isobaric heating from about 575 to 630°C (8.5 kbar), which possibly results from post-stacking relaxation of the crustal steady-state heat flow. Further garnet AIII growth was accompanied by an isothermal pressure decrease of 3-4 kbar, indicating post-stacking exhumation, may be due to an orogenic collapse+erosion. Compositional zoning of garnet AIV indicates further heating, reaching maximum conditions of 670°C (< 5.5 kbar). The source of this heating is not clear, at this stage. Geochronological data from these rocks are complex, but suggest that the P-T evolution described above took place during the Neoproterozoic Knoydartian orogeny, rather than during the Caledonian orogeny.

L14 : 2B/31 : H2

Structural Geology and Thermobarometry of the Piz di Groven Region, Val Calanca, Southern Switzerland

Roger Ruetti (Roger.Ruetti@etu.unil.ch)

Section des Sciences de la Terre, Université de Lausanne, CH-1015 Lausanne-Dorigny, Switzerland

Structural geology and PT conditions of an area of about 9 km² belonging to the Pennine nappes, Central Alps, are presented. The study area is located in the Val Calanca in southern Switzerland and is made up of the Simano and Adula nappes. Both nappes are built by gneissic and pelitic rocks with some layers of amphibolites. An ultramafic body is situated in the upper part of the Adula nappe. The region underwent amphibolite facies metamorphism and the study area sits on the sillimanite-in isograd. The study area shows a homogeneous structural orientation of its rocks dipping 20 to 30° to the NE. Four major deformation phases have been recognized by this study: The deformation phase D1 is identified in one sample by white mica relicts in quartz and feldspar. The second phase of deformation D2 represents the dominant structures of the study area: Fold axes in the Simano nappe give a NW to N direction, whereas in the Adula nappe they show greater variation from NW to NE. The main schistosity is recorded along a great circle in the Simano nappe, in the Adula nappe the variation is considerably smaller. The fold surfaces dip into a NE direction for both nappes. Stretching lineations of both nappes point toward a NNW direction with bigger dipping amounts recorded in the Adula nappe. The deformation phase D3 observed only rarely in both nappes. This phase crenulates D2. The folds are oriented in a N-S direction. The fourth phase of deformation shows fractures at all scales throughout the region. They are oriented roughly in a E-W direction. Cataclastic rocks and kakirites, displaying a green schist facies paragenesis, witness the dynamics of this late phase of deformation. The limit of the two nappes as recognized by this study is formed by a strongly probably isoclinal folded contact of paragneisses of the Simano nappe and the two-mica orthogneiss at the base of the Adula nappe. A thermobarometric study was undertaken on two samples of pelitic rocks of each of the nappes. The parageneses of these rocks allowed the application of the GARB thermometer and the GASP barometer to define the PT conditions recorded by these rocks. The strongly altered state of these samples sets the limits to the results obtained by this study. With the calibration of (Hodges & Spear, 1982) the following PT conditions are obtained: for the Simano nappe 613° C ± 50° C and 5 to 7 kbars. For the Adula nappe the figures are 744°C ± 50°C and 9 to 11 kbars.

Hodges KV & Spear FS, Amer. Mineral, 67, 1118-1134, (1982).

L14 : 2B/32 : H2

Complex P-T-t History of the Lepontine Domain as Recorded by Garnet-Amphibolite from the Simano Nappe

Valerie Grandjean (vgrandj@mpi.unibe.ch)¹,

Tivadar M. Toth (m.toth@geo.u-szeged.hu)² &

Martin Engi (engi@mpi.unibe.ch)¹

¹ MPI, University of Bern, Baltzerstrasse 1, Bern, Switzerland

² Inst. Min. Geochem. Petrol., Jozsef Attila University, Szeged, Hungary

It is widely accepted that the Adula / Cima Lunga nappe (ACL), after subduction to >60 km, reversed direction in the lower Eocene and was then rapidly obducted and emplaced in the Lepontine nappe stack. The P-T-t history of nappes beneath the ACL is still unclear. We present pertinent results for the Simano nappe.

Garnet-amphibolite retains evidence of several stages of the P-T evolution. These were deciphered by combining careful petrography, EMP data and thermodynamic models (DOMINO / THERIAK, de Capitani 1994). Stable assemblages, including the mode of coexisting phases, were computed for fixed bulk compositions and were compared with the microtextural evidence observed in the samples. Isopleths for solid solutions present as porphyroblasts were also calculated using these programs. The slopes of the isopleths constrain segments of the P-T path; the spacing of isopleths can be related to the observed and measured zoning in phases such as garnet, amphibole and plagioclase. By combining the computed reaction sequence and isopleth geometry, the variation in pressure and temperature corresponding to local assemblages could be documented. In some samples, early prograde growth of porphyroblasts of horblende, garnet and plagioclase lead to compositionally differentiated domains. Each of these evolved more or less isochemically in the subsequent P-T path of the samples. The different stages of this evolution are sometimes observed in one and the same sample, with each domain corresponding predominantly to one specific stage.

Despite attaining temperature in excess of 650^oC (at ~28 Ma in the South), interpretable evidence of an earlier (pre-collisional?) decompression history remains intact. However, it is at present unclear whether all of the documented stages are of Alpine age. Mineral chronometry to date some of these stages is currently underway.

De Capitani C, Ber Deutsche. Miner. Ges, 1, 48, (1994).

L14 : 2B/33 : H2

Restite-Melt Back Reactions: Implications for P-T Paths and Inferred Tectonic Settings

Leo M. Kriegsman (leo.kriegsman@utu.fi)¹ &

Bas J. Hensen (B.Hensen@unsw.edu.au)²

¹ Department of Geology, University of Turku, FIN 20014 Turku, Finland

² School of Geology, University of New South Wales, Sydney 2052, NSW, Australia

Most high-grade terranes show evidence for dehydration melting of hydrous minerals resulting in the formation of migmatites, in which quartzofeldspathic layers or pods (leucosome) alternate with restitic domains (mesosome). The proximity of leucosome and restite indicates that part of the melt crystallized in situ and therefore some back reaction between restite and melt is to be expected (Powell, 1983; Ashworth & Brown, 1990). Textures that may result from back reaction in migmatites are: (i) biotite-sillimanite-rich melanosome; (ii) biotite rims between garnet and leucosome; (iii) amphibole rims between pyroxenes and leucosome; (iv) biotite-rich patches with relics of ferromagnesian minerals; and (v) complex coronas separating ferromagnesian minerals from leucosome.

Back reaction between restite and melt may be more widespread than commonly believed and has important implications. In particular, geobarometers involving quartz need to be applied with caution in migmatites. If quartz is eliminated during progressive partial melting the silica activity drops below unity, which affects calculated pressures. Thermodynamic calculations (Kriegsman & Hensen, 1998) indicate that inferred pressures may require corrections of upto 3 kbar, which may amount to 100% of the original value. As a result, some "clockwise" P-T paths (in petrological diagrams with pressure axis upwards) and "counter-clockwise" P-T paths claimed for migmatites in the literature may need a reevaluation. In addition, the dP/dT slopes of most reactions steepen with decreasing silica activity (op. cit.). Hence, many corona textures in migmatites, e.g. those rimming garnet, do not require substantive changes in pressure, but may result from initial cooling after the peak of metamorphism.

The reinterpretation of P-T paths has significant implications for the inferred tectonic histories and settings of some migmatite terranes, notably if the P-T paths claimed for those areas involve pressure changes. Tectonic processes like erosional and extensional unroofing, and gravitational collapse, are commonly inferred from a calculated decrease in pressure. However, if pressure changes turn out to be negligible when back reaction is taken into account, the inferred tectonic settings no longer apply. Our data suggest that crustal processes previously assumed to have operated at 10 km depth may in fact have taken place at twice that depth. The relevance for geophysical models, using petrological data as input, is obvious.

Powell R, Geological Society of London Journal, 140, 629-633, (1983).

Ashworth JR & Brown M, High-temperature and crustal anatexis: London, Unwin Hyman, 1-18, (1990).

Kriegsman LM & Hensen BJ, Geology, 26, 1111-1114, (1998).

L14 : 2B/34 : H2

PTt Evolution of the Svecofennian Metamorphic Belt in Connection with Plutonic Activity and Tectonics: SE Finland and N Ladoga Region

Viktor Glebovitsky (vg@vg.1404.spb.edu)

Emb.2, St Petersburg, Institute of Precambrian Geology and Geochronology, 199034, Russia

The study was carry out near the boundary between the Archaean Fenno-Karelian craton and the Palaeoproterozoic Svecofennian accretion orogen where the structural evolution are described and which is a classic terrain of andalusite-sillimanite type zonality. The metamorphic grade varies from greenschist to granulite facies (Korsman et al., 1984; Glebovitsky et al, 1985). Some features of the mineral assemblages and mineralogical thermo-barometers allow the PT conditions to be estimated and the prograde path of evolution to be inferred, which is supported by study of fluid and melt inclusions in the minerals. The initial anatectic migmatites appear in the muckovite stability zone, and the partial melting become more and more intensive in granulite facies. The homogenisation method, i.e. melt inclusions in a leucosome of the early migmatites, was used to determine temperatures in the zone where migmatization had commenced. The lowest temperature averaged 680°C and the highest was nearly to 770°C. For the pressure estimation a criometry of CO₂ inclusions was used. If to take into account only the most dense inclusions a pressure in the metamorphic zonality increases from 3.5 to 5.5 kb that is in a good agreement with the data, obtained by calculations of mineral equilibria, including the TWEEQU method. It should be assumed that during beginning of the retrograde stage temperature did not fall significantly at decompression, as the melt was at least preserved and likely appeared in the complex. All these data provide a clock-wise path to be inferred.

The age limits for metamorphic cycle were established by dating of the plutonic rocks. There exist new geological, petrologic, geochemical and geochronological data on the main sinmetamorphic granitoid complexes which include norite-enderbite (Kurkijoki complex, 1871±6 Ma), diorite-tonalite-trondhjemite (Lauvatsaari-Impiniemi complex, 1864±13 Ma) and tonalite-granodiorite (Tamhanko complex, ca 1860 Ma) assemblages and compose the new crust generated around the active continental margin. During the late metamorphic stage the Tervus K-granite complexes was formed (1856±7 Ma) in connection with collision the new crust and continental margin. Systematics of the available Sm-Nd isotope date and calculation the TDM model ages which are of 2.3-2.4 Ga provide the conclusion that a magma of all these complexes was emplaced from juvenile Proterozoic sources and contaminated by metasedimentary rocks contained the Archaean clastic material.

Session L14:3A

L14 : 3A/01 : H2

Fluid Infiltration in Eclogites at 2 GPa: Evidence from Cs, Rb and Ba Systematics in Phengite and Amphibole

Thomas Zack (tzack@gwdg.de)¹,

Steven F. Foley (sfoley@gwdg.de)¹ &

Toby Rivers (trivers@sparky2.esd.mun.ca)²

¹ Mineralogisch-Petrologisches Institut, Universitaet Goettingen, Goldschmidtstr. 1, 37077 Goettingen, Germany

² Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X5, Canada

We have investigated a suite of eclogite parageneses from Trescolmen (Adula nappe, Swiss Alps), where eclogite boudins are surrounded by kyanite-garnet mica schists (Heinrich, 1986; Meyre et al., 1997). The eclogites record a complex history of fluid infiltration under high pressure conditions. Petrographic evidence for fluid infiltration includes: (1) tremolitic to barroisitic amphiboles overgrowing the eclogitic foliation; (2) replacement of kyanite by paragonite; and (3) veins of talc (partially replaced by tremolitic amphibole) in Mg-rich eclogite.

In order to constrain the scale of fluid infiltration, we measured the abundances of mobile elements in hydrous phases by laser ablation ICP-MS. Variation in the Ba concentration of amphibole coexisting with phengite ranges from 0.9-8 ppm and is strongly coupled to both the A-site occupancies of the amphiboles and to the Ba content of coexisting phengite. A positive linear correlation between the A-site occupancy in the amphibole and the D(Amp/Phe) for Ba supports mineral chemical evidence of equilibration between these two phases.

Amphibole from samples without phengite has the same range of Ba concentration, however, although concentrations at least an order of magnitude higher were predicted from protolith constraints. The relatively uniform Ba contents of all amphiboles in the Trescolmen eclogites are therefore interpreted as a result of buffering by an infiltrating fluid. The Ba content of the fluid itself was controlled by the composition of nearby phengite; in the case of the phengite-absent eclogite samples, it was probably buffered by the surrounding metapelite.

On the other hand, significantly different Cs/Rb ratios of phengite from the eclogite (0.016-0.035) and the surrounding metapelite (0.05) indicate that these elements were not externally buffered. The values are comparable to altered basalt and pelagic sediments respectively.

Fluid infiltration modelling (Nabelek, 1987) consistent with these results constrains fluid/rock ratios to the range 0.002-0.02. The lower limit is the minimum ratio necessary to homogenize Ba concentrations in all amphiboles, and the upper limit still leaves the primary Cs/Rb heterogeneities of eclogitic and metapelitic phengites intact. The data thus provide evidence for the scale of fluid circulation under eclogite conditions. It is inferred that in subducting crust, fluids resulting from dehydration reactions leave the reaction sites and migrate into the overlying mantle wedge.

Heinrich CA, Journal of Petrology, 27, 123-154, (1986).

Meyre C, de Capitani C & Partzsch JH, Journal of Metamorphic Geology, 15, 687-700, (1997).

Nabelek PI, Geochimica et Cosmochimica Acta, 51, 1765-1769, (1987).

L14 : 3A/02 : H2

The Role of Internal Versus External Processes in Controlling Lower Crustal Fluid Chemistry; An Example from the Caledonides of Western Norway

Henrik Svensen (hsvensen@geologi.uio.no) &

Bjørn Jamtveit

Dept. of Geology, University of Oslo, PO BOX 1047 Blindern, N-0316 Oslo, Norway

Eclogite formation from dry granulites in the Caledonian mountain range in western Norway was a widespread process. It required fluid infiltration at depths greater than 50 km and local hydration processes in the granulites. Brine inclusions trapped in omphacites from eclogites formed during hydration of granulites contain a range of solid phases that constrain the brine chemistry. The solid phases include gypsum, halite, calcite, Fe-Cu-As sulphides and three lead-bearing minerals, one of them containing bromine. Some of these are daughter crystals, whereas others may contain components from the host mineral and were formed during retrograde reactions between trapped brine and omphacite. The lead bearing minerals have not previously been found in fluid inclusions, and signifies extreme lead and bromine enrichment in the eclogite pore-fluid present during trapping of the inclusions. Bulk halogen analyses of fluid inclusions from 4 eclogite localities in the western Gneiss region show that there is a trend towards decreasing Cl/Br weight ratios and increasing Br concentrations, with the Pb-bearing inclusions being at the end of this trend. A bromine and lead enrichment in brines present during hydration can be explained by a lowering of the water activity caused by consumption of H₂O, leaving elements incompatible with the silicate structures in the residual solution. The Cl/Br ratios are lowered due to favourable substitution of Cl relative to Br into micas and amphiboles as the H₂O activity is lowered. Iodine does not seem to follow the trend of Bromine enrichment. The extent of local control on the chemistry of infiltrating fluids is a function of the extent of hydro-silicate formation and the volumes of fluids infiltrating. Assuming a uniform fluid source, the fluids with the lowest Cl/Br ratios were most strongly affected by local hydration processes and represents the last residual fluids before fluid absent conditions.

L14 : 3A/03 : H2

Fluid Composition Estimation for Granulitic and Eclogitic Rocks

Sergei Simakov (simakov@vap.usr.pu.ru)

Geol.Dep., St.-Petersburg Univ., Russia

By nowadays there are many fluid inclusions data of granulitic and pelitic rocks (Turet & Dietvorst, 1983). But there is a lack of reliable oxygen barometers for this rocks.

Model of garnet-clinopyroxene oxygen barometer for granulitic and eclogitic rocks was developed on the basis of the reaction:

2Ca₃Fe₂Si₃O₁₂ + 2FeSiO₃ + 4SiO₂ = 6CaFeSi₂O₆ + O₂

The model was checked on Carroll and Wyllie (1989) experimental data for tonalite-peridotite system synthesised at 900-1030° C, 15 kbar and nearly QFM buffer. By this method oxygen fugacity and equilibrated fluid compositions in the C-O-H system were calculated for Alpine, Norwegian, Indian and another granulites, pelitic shifts and eclogitic rocks. Calculated fluid compositions mainly closed to yearly extracted. This method was applied to diamond-bearing and -free Kokchetave eclogites. By the calculations diamond-free rocks have mainly CO₂ fluid meanwhile diamond-bearing ones - mainly H₂O-CH₄ fluid.

Turet J & Dietvorst P, J. Geol. Soc, 140, 635, (1983).

L14 : 3A/04 : H2

Oxygen Isotope Composition of Quartz-Al₂SiO₅ Veins as Indicators of the Temperature-Fluid History of High-Grade Metamorphic Rocks

Benita Putlitz (benita@vms.huji.ac.il)¹,

Alan Matthews (alan@vms.huji.ac.il)¹ &

John W. Valley (valley@geology.wisc.edu)²

¹ Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel

² Department of Geology and Geophysics, University of Wisconsin, Madison, WI, USA

It has long been recognized that the oxygen isotope composition of veins within metamorphic rocks preserve a record of the fluid history. Less well studied, however, is their potential for oxygen isotope thermometry, particularly so in quartz-rich veins from high temperature rocks. The oxygen isotope fractionation between quartz and minerals resistant to retrograde-requilibration (such as Al-silicates and garnet) theoretically should provide accurate thermometry of the event during which the vein formed. Accurate temperatures correspondingly enable us to more correctly estimate the isotopic compositions of fluids and to better pinpoint the (relative) time of vein formation and to verify structural information.

We demonstrate the power of such methods through laser probe studies of the oxygen isotope compositional variations of quartz-Al₂SiO₅ veins and their pelitic host rocks located in kyanite-sillimanite zone of Naxos (Greece). Oxygen isotope temperatures (Sharp 1995) for quartz-kyanite-sillimanite pairs in various vein samples range from 630^oC to 680^oC and agree well with the petrologically-deduced peak metamorphic temperatures of between 620^oC (sillimanite-in) and 700^oC (melt-in) (Buick & Holland, 1989). Lower apparent temperatures of 584^oC to 592^oC would be estimated from quartz-kyanite pairs of the host rocks and 430^oC and 377^oC for quartz-biotite pairs (Bottinga & Javoy, 1975), which indicates that the host rocks samples have undergone substantial isotopic re-equilibration due to diffusional exchange of quartz and biotite. Yet, the similarity of isotopic compositions of veins (e.g.,  ¹⁸O quartz = 13.09 to 13.51 permil and  ¹⁸O kyanite = 10.3 to 10.9 permil) and their host rocks (e.g.,  ¹⁸O quartz= 13.86 permil and  ¹⁸O kyanite = 10.8 permil) suggest that the fluids involved in vein formation were in isotopic equilibrium with the host rocks, thus favoring a mechanism of vein formation by local segregation at peak metamorphic temperatures. Our data demonstrate that vein mineral assemblages, such as quartz-Al silicates, can be used as effective thermometers in high temperature rocks where retrograde equilibration is a major problem. Further examples of quartz-Al-silicate and quartz-garnet veins from high-grade rocks in the Cyclades and the southern Alps will be presented.

Bottinga Y & Javoy M, Rev.Geophys.Space.Phys., 13, 401-418, (1975).

Buick IS & Holland TJB, Geol. Soc. London. Spec. Publ, 40, 365-371, (1989).

Sharp ZD, Am. J. Sci, 295, 1058-1076, (1995).

L14 : 3A/05 : H2

REE Distribution in Marbles: A Witness of Fluid Flow During Granulite-Facies Metamorphism

Philippe Boulvais

(philippe.boulvais@univ-rennes1.fr)¹,

Serge Fourcade (fourcade@univ-rennes1.fr)¹,

Gérard Gruau (gruau@univ-rennes1.fr)¹,

Michel Cuney (cuney@cregu.cnrs-nancy.fr)² &

Bernard Moine (fax: 05 62 26 75 89)³

¹ Géosciences- Bât.15, Campus de Beaulieu, 35042 Rennes cedex, France

² CREGU et GDR CNRS-CREGU 77, BP 23, Vandoeuvre, France

³ Laboratoire de Minéralogie, UMR 067 CNRS-Université P. Sabatier, 39 allée Jules Guesdes 31000 Toulouse, France

Marbles are widely used to unravel the fluid regime during high-grade metamorphic recrystallization on the basis of their mineralogy and/or stable isotopic composition. However, concerning the latter approach, processes such as diagenesis, decarbonation with isotopic exchange between carbonate and silicate phases as well as various fluid-rock interactions during the prograde path may introduce significant O and C isotopic variations. These variations may obscure the isotopic effects of subsequent syn-metamorphic fluid-rock interactions. Therefore, other tracers (like trace elements distribution) may be useful to reveal syn-metamorphic fluid infiltration within marbles.

Here we focus on the Tranomaro Panafrican area (Madagascar), where syn-granulitic fluid flow gave rise to the widespread formation of pyroxenites (skarns) at the expense of marbles. Some skarns are enriched in U, Th, REE, Zr..., indicating that these elements were mobile during the metasomatic event. Marbles of the area display a large variation in  ¹⁸O (from +19 to +6.5‰) and a more restricted  ¹³C range (from +1.4 to -2.5‰). Boulvais et al. (1998) proposed that the  ¹⁸O variations were mostly acquired during a pre-metamorphic event (dolomitization), preventing the syn-metamorphic fluid regime to be isotopically studied on the regional scale. The fluid/marble interaction that gave rise to pyroxenites was studied in a metre-scale profile, where an U-Th mineralized pyroxenite and its marble protolith are associated. Using this reference, REE characteristics of infiltrated marbles are defined and compared to those of marbles of the whole area.

In the small-scale profile, the formation of pyroxenite (Di+Cal) is due to the influx of dissolved SiO₂ in the marble (Cal+Dol+Fo+Spl). In the first reaction steps, dolomite disappears to form calcite+forsterite. This assemblage subsequently disappears to form diopside with some residual calcite. U, Th, and REE contents increase in the marble towards the pyroxenite (e.g., the Nd content increases in the marble from 16 up to 137 ppm at the contact, then decreases down to 84 ppm in the centre of the pyroxenite body). Besides an overall increase of REE abundances, a negative Eu anomaly develops towards the contact.

On the regional scale, the Nd content of marbles varies between 7 and 120 ppm, many marbles having a Nd content higher than 25 ppm (this is the maximum value calculated for impure marbles evolved in closed system since sedimentation). Moreover, for increasing Nd contents, a negative Eu anomaly develops, reaching a value nearly identical to that measured in metasomatic pyroxenites (Eu/Eu* = 0.25), which mimics what is observed among infiltrated marbles in the small-scale profile. Thus, in favourable areas, REE patterns of marbles may be helpful to decipher fluid infiltration on the regional scale. Unradiogenic Nd isotopic composition of these marbles demonstrates the local crustal source of the elements.

Boulvais P, Fourcade S, Gruau G, Moine B, Cuney M, Chemical Geology, 150, 247-262, (1998).

L14 : 3A/06 : H2

Oxygen Isotope Patterns Associated with the Metasomatic Alteration of Hornblendite Xenoliths in a Granulite-Facies Marble from the Ivrea Zone, N-Italy

Rainer Abart (rainer.abart@kfunigraz.ac.at)

Institute for Mineralogy-Crystallography and Petrology, Karl-Franzens-University Graz, A-8010 Graz, Austria

The Ivrea-Verbano Zone at the Swiss Italian border represents a segment of the intermediate and lower crust comprised of amphibolite to granulite facies metasedimentary and magmatic rocks. The metasediments are primarily comprised of metapelites and subordinate metacarbonates.

In the Val Fiorina, about 11 km southeast of Domodossola, centimeter to meter sized fragments of hornblendites were incorporated into highly deformed calcitic marbles in the course of tectonic processes during granulite facies metamorphism. At the hornblendite-marble interface a succession of metasomatic mineral zones developed: (1) unaltered hornblendite comprised of pargasitic amphibole, almandine-pyrope garnet and clinopyroxene (2) 2 to 5 centimeter wide monomineralic clinopyroxene zone (3) 4 to 8 centimeter wide grandite-garnet clinopyroxene zone (4) 4 to 8 centimeter wide scapolite-clinopyroxene zone (5) the calcitic marble. The succession of mineral zones reflects the degradation of an initially sharp geochemical front at the hornblendite-marble interface. The principal geochemical contrasts were in the carbon dioxide content of the pore-fluid and in the availability of silica.

The oxygen isotope composition of the marble is 17 to 18 per mill relative to SMOW. The hornblendite bulk rock oxygen isotope composition is 8 to 9 per mill. This implies that an oxygen isotope gradient of 7 to 10 per mill is accomodated in the about 15 cm wide metasomatic alteration zone. The geometry of the oxygen isotope pattern is characterized by a 3 per mill step at the boundary between the unaltered hornblendite and the monomineralic clinopyroxene zone. Another 1.5 per mill step in the oxygen isotope composition occurs at the boundary between the monomineralic clinopyroxene zone and the grandite-garnet clinopyroxene zone. Within the clinopyroxene zone the oxygen isotope compositions gradually increase from 12 per mill in the innermost to 14.3 per mill in the external clinopyroxene zone. In the grandite-garnet clinopyroxene zone garnet and clinopyroxene have oxygen isotope compositions of 15 per mill and 15.6 per mill, respectively. According to the calibration of Chiba et al. (1989) these compositions are in equilibrium with the surrounding carbonates at 700^oC.

The coincidence of discontinuities in the oxygen isotope pattern at mineral-zone boundaries suggests that oxygen isotope exchange was enhanced during metasomatic mineral reactions. The gradational change in oxygen isotope compositions within the clinopyroxene zone indicates a certain degree of transport control on oxygen isotope alteration. The geometry of the oxygen isotope front within the clinopyroxene zone may be understood in terms of a moving boundary diffusion problem (Crank, 1955). Diffusive transport within the clinopyroxene zone was controlled by the diffusivity of the oxygen bearing species. The movement of the boundary was caused by progressive replacement of the unaltered hornblendite. This situation allows to derive quantitative relations between the rate of propagation of the metasomatic replacement front and the effective oxygen diffusivity within the clinopyroxene zone.

Chiba, H., Chacko, T., Clayton, R.N., Goldsmith, J.R., GCCA, 53, 2985-2995, (1989).

Crank, J, The mathematics of diffusion, Clarendon Press, (1955).

L14 : 3A/09 : H2

Fluid Transport in Deforming Overpressured Rock Masses

Bruce Hobbs (b.hobbs@per.dem.csiro.au) &

Alison Ord

CSIRO Exploration & Mining, Private Bag, Wembley 6014, Australia

In the absence of a driving force which maintains a lithostatic fluid pressure gradient, a column of overpressured fluid in a rock mass with interconnected porosity will flow until a hydrostatic fluid pressure gradient develops. The time scale for this to occur is short compared to geological time scales. This means that the top of the column becomes overpressured, and the base of the column underpressured, with respect to lithostatic pressure. Given a high enough column of interconnected fluid, the top will ultimately deform to relieve the overpressure, by generating new porosity, and the base will deform to close up porosity. The density difference between the fluid and the country rock can drive this fluid compartment upwards once the compartment exceeds a critical height. Connolly has demonstrated that waves of these higher porosity packages can propagate upward, in a rock mass of intrinsically low permeability, from a devolatilization front. The situation examined by Connolly arises in a rock mass which is isotropically stressed, the process being driven by buoyancy induced stresses which in turn produce porosity ahead of the rising porosity wave. The propagation of these waves is controlled by the rheological distribution in the rock mass.

In a low permeability rock mass which is deforming under conditions where the deformation induces new porosity (and related permeability) an additional (but related) process arises in addition to these porosity waves. In regions where the effective stress induces deformation, higher permeability regions are developed which can themselves behave as porosity packages. If these exceed a critical height they move upwards under the influence of buoyancy. These porosity packages depend only on deformation for their development and not upon porosity generated at a devolatilization front. The structure within these deformation induced porosity packages depends upon the stress field in the package. We propose here that the vein systems common in many overpressured metamorphic terrains constitute the structure within the deformation induced porosity packages.

We contrast here the following two situations:

* Darcy flow in a rock mass which is deforming by a shortening deformation in which folding takes place together with the development of shear zones. The constitutive behaviour is elasto-plastic with deformation induced dilatancy (including the ability to hydrofracture once the effective stress induces yield). Fluid flow is focussed, the focussing being controlled essentially by deformation induced changes in pore-pressure and permeability rather than by the rheology of the material.

* Flow which takes place through the propagation of relatively high permeability packets, the permeability distribution being controlled by deformation induced permeability increases and decreases once the material yields. The distribution of yielding regions is controlled by the externally imposed deformation and the (heterogeneous) distribution of effectives stress. Fluid flow may or may not be focussed but the overall control on fluid flow is now the rheology and stress state of the material rather than the initial porosity and permeability distribution.

The geological consequences of these two contrasted modes of fluid transport is that under conditions of Darcy flow, fluid is expected to be focussed into rocks of high intrinsic permeability such as sandstones which are commonly strong relative to shales. However, under conditions where fluid flow occurs by porosity package migration, fluid is expected to be focussed into rocks of low strength such as shales or schists which have low intrinsic permeability.

We apply these concepts to several overpressured regions such as the Yilgarn, Broken Hill and the Ballarat-Bendigo region in an attempt to relate the observed structures with fluid transport mechanisms.

L14 : 3A/10 : H2

The Nature and Significance of the Isograd Surface in Infiltration-Metamorphism

Gregory M. Dipple (dipple@eos.ubc.ca)

Dept. Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada

Isograd surfaces in infiltration-metamorphism differ from the smooth surfaces typical of thermally driven metamorphism. Spatially heterogeneous fluid flow produces a heterogeneous distribution of reactant and product mineral assemblages during infiltration-driven reaction. In this instance, isograds are highly irregular surfaces. Permeability, fluid pressure, thermal structure, protolith composition, reaction kinetics, and the distribution of fluid sources and sinks may each influence the shape of the surface. In some cases, one or two of these controls dominates and isograd geometry can be inverted to yield information on that control.

Two examples are presented. In the first, the distribution and evolution of permeability during contact metamorphism largely control the geometry of the periclase and wollastonite isograds in marble. The amplitude of the isograd surface can be related to the permeability contrast in the system. In some field areas, the amplitude exceeds 200 m and the implied range in permeability spans nearly 5 orders of magnitude. It is proposed that this permeability contrast is not inherited from the protolith limestone, but rather develops from a reactive-infiltration instability that exaggerates small initial perturbations in permeability. The wavelength of the isograd surface is commonly within the range of cm to m. The lower limit of wavelength is controlled by the efficiency of transverse dispersion within high permeability zones. The upper limit records the scale of lithologic and structural control on permeability.

In the second example, heat flow exerts more control on isograd geometry. The diopside isograd in siliceous dolomitic marble may be highly irregular, however the amplitude is controlled by the thermal structure of the contact aureole. The distribution of diopside cannot be inverted to yield information on permeability contrast during metamorphism. As in the previous example, the wavelength of the isograd is a measure of transverse dispersion and permeability distribution. These examples highlight the utility of carefully documented field isograds in deciphering the nature, evolution and distribution of permeability attending metamorphism.

L14 : 3A/11 : H2

Relations of Contact Metamorphism and Fluid Flow in the Eastern Monzoni Thermal Aureole

Erwin Povoden (erwin.povoden@kfunigraz.ac.at),

Micha Horacek (micha.horacek@kfunigraz.ac.at) &

Rainer Abart (rainer.abart@kfunigraz.ac.at)

Institute for Mineralogy-Crystallography and Petrology, Karl-Franzens-University Graz, A-8010 Graz, Austria

The Monzoni intrusive complex in the western central Dolomites intruded a Permo-Triassic sequence of unmetamorphosed marls and dolomitic rocks during Mid Triassic time. Petrological evidence exhibits contact metamorphic processes in the Monzoni aureole and the lithologic control on fluid flow during contact metamorphism.

A general trend of increasing temperatures from 350^oC at 685 m to 540^oC at 155 m from the intrusive is derived from calcite-dolomite solvus thermometry (Anovitz and Essene, 1987). Metamorphic peak temperatures are obtained at contact distances > 550 m. A flattening of the temperature profile towards the contact strongly suggests significant reequilibration during cooling at distances < 550 m from the contact. Retrogressive formation of dolomite is also identified by mineral textures. Phase petrologic constraints for the thermal structure are derived from a dolomitic lithology of the Cencenighe member, where the progressive formation of talc, tremolite (X_Mg=0.94-0.98), forsterite (X_Mg=0.94) and periclase yields an increase of temperature from < 350^oC at 750 m to > 600^oC at 200 m from the contact. In the marly lithologies of the Val Badia -and Cencenighe members melilite (66 mole% åkermanite) coexisting with clinopyroxene (53 mole% esseneite) and calcite indicates temperatures > 700^oC at 185 m from the contact. The succession of parageneses in the dolomitic lithologies indicates a high degree of internal buffering. In contrast the successive formation of epidote-zoisite (70-100 mole% epidote), actinolite (X_Mg=0.6-0.83), clinopyroxene (X_Mg=0.73-1), andradite-grossular, wollastonite, melilite and monticellite (X_Mg=0.45) in the marly lithologies of the Val Badia -and Cencenighe members and the even more siliciclastic Campil member support significant input of externally derived, water rich fluids. External buffering was most pronounced in the Campil member, where wollastonite first occures at 460 m from the contact, compared to formation of wollastonite only at 204 m in the marls of the Val Badia -and Cencenighe members. Metasomatism is evident from the formation of phlogopite after forsterite in a dolomitic lithology at contact distances < 565 m, propably at peak metamorphic conditions. In some marly lithologies garnet was replaced by the assemblage albite, potassium feldspar, scapolite (meionite component ranging from 35 mole% at 403 m from the contact to 52 mole% at 163 m) and clinopyroxene during late stage metamorphism. This indicates a decrease of water fugacity in these lithologies, whereas serpentinization of periclase and forsterite points out introduction of silica with an aqueous fluid. Bulk rock analyses of dolomitic lithologies point towards increasing silica values from 4.3 wt% in unserpentinized rocks up to 37.4 wt% in strongly altered rocks. Only in some instances periclase is transformed to brucite.

Metamorphic peak temperatures reached values >700^oC. The Monzoni thermal aureole is fairly heterogenous regarding the fluid evolution during contact metamorphism. Fluid evolution is largely lithologically controlled, as evident from the contrasting metamorphic parageneses of different lithologies. Metasomatism affected even previously relatively impermeable lithologies during late stages of metamorphism.

Anovitz, LM, Essene, EJ, J. Pet, 28, 389-414, (1987).

L14 : 3A/12 : H2

Stable Isotope Signatures in the Eastern Monzoni Contact Aureole

Micha Horacek (micha.horacek@kfunigraz.ac.at),

Erwin Povoden (erwin.povoden@kfunigraz.ac.at) &

Rainer Abart (rainer.abart@kfunigraz.ac.at)

Institute for Mineralogy-Crystallography and Petrology, Karl-Franzens-University Graz, A-8010 Graz, Austria

In the western central Dolomites (N-Italy) the Monzoni intrusive complex intruded previously unmetamorphosed Permo-Triassic sediments during Mid Triassic time. The countryrocks comprise a sequence of carbonates with variable siliciclastic input. The lithologies of interest comprehend the units from Mid Triassic age to the Permo-Triassic transition.

The Schlern (= uppermost) unit consists of almost pure marble with relativeliy constant ¹³C and ¹⁸O compositions and shows no significant isotopic variation towards the intrusion. It did not interact with externally derived fluids, nor did it loose significant proportions of its volatile components by decarbonation. The Buchenstein unit comprises an interlayered limestone with chert sequences and shows systematic depletions in the oxygen system towards the intrusion with respect to unmetamorphosed reference samples, with shifts up to 15 per mill. Carbon isotope signatures remain almost unchanged. This suggests interaction with water rich, externally derived and isotopically light fluids. The Contrin unit consists of almost pure limestones and shows no significant isotopic shift towards the aureole. Only where it has been dolomitised during diagenesis an isotopic shift of 2 per mill in the ¹³C signature is observed. The Cencenighe dolomit oolite is a pure dolomite and shows only a minor trend to lower values in the oxygen system in the innermost aureole with a maximum depletion of 4 per mill. In the carbon system a significant shift (similar to the shift in the Contrin unit) coincides with the prograde decay of do to cc + per. At this position ( about 300 m from the intrusive contact) the carbon value shows a 2 per mill decrease. This unit was only slightly permeable to external fluids in the inner aureole. Samples of the Cencenighe and Val Badia marls show significant depletions up to 10 per mill in the oxygen system and also some shifts in the carbon system due to decarbonation reaction. These marls are isotopically controlled by externally derived fluids. The Campiler unit contains the highest siliciclastic input. The ¹⁸O depletion with a shift up to 10 per mill is observed even beyond the outermost metamorphic aureole and shows the influence of externally derived fluids. Significant shifts in the carbon system nicely correlate with the extent of decarbonation reactions.

The aureole scale isotope systematics indicate lithologic control of fluid flow during contact metamorphism. This is also observed on small scale studies (dm to mm scale). Detailled isotope profiles within the marble - marl alternating sequence of the Gastropode oolite unit reveal systematic ¹³C and¹⁸O variations accross individual layers. Pure marbles show ¹⁸O of 22 per mill SMOW and ¹³C up to 4 per mill PDB and appear to have preserved their original isotopic compositions. In contrast, the marly layers show signatures in ¹⁸O down to 8.5 per mill SMOW and ¹³C decreasing to -5.5 per mill PDB which indicates introduction of isotopically light fluids and intense decarbonation.

Fluid flow of externally derived fluids is lithologically controlled and dependent on the siliciclastic content of the rocks. It shows that a channelized external fluid flow through the siliciclastic rich layers existed during dehydration of the intrusive rocks.

L14 : 3A/13 : H2

Metasomatic Alteration in Roof Pendants of the Drammen Granite, Oslo Graben

Anne-Mette Christensen (christ@kfunigraz.ac.at)¹,

Jens Konnerup-Madsen (jenskm@geo.geol.ku.dk)² &

Rainer Abart (rainer.abart@kfunigraz.ac.at)¹

¹ Institute for Mineralogy-Crystallography and Petrology, Karl-Franzens-University Graz, A-8010 Graz, Austria

² Geological Institute, Univesity of Copenhagen, Denmark

The Drammen granite some 30 km southwest of Oslo is a biotite granite which intruded Paleozoic schists and limestones in a shallow crustal level during the Permian. The granite is mushroom shaped with a thickness of about 3 km and a diameter of 20 km. At Myrseter, 4 km north of Drammen, carbonaceous roof pendants are exposed.

The metacarbonates of the roof pendants contain skarns with zoned vesuvianite, grossular-andradite garnet and wollastonite. The skarns are associated with magmatic veins comprised of quartz and zoned microcline. The mafic phase in these veins is clinopyroxene. A peculiarity of the veins is the large abundance of sphene. In an early stage of vein formation, up to 5 millimeter long fibrous quartz crystals grew into the vein interior from the vein-host rock interface. Vein quartz contains an earliest generation of primary fluid inclusions, with a low density vapour-rich aqueous fluid. A slightly younger generation of primary or pseudosecondary inclusions contains saline (about 30 to 35 wt.% NaCl eq.) aqueous inlcusions. Both inclusion types may coexist, indicating boiling during late stage crystallization in the roof of the Drammen Granite (Olsen and Griffin, 1984). The oxygen isotope composition of the zoned vesuvianite in the associated skarns is 4,9 per mill in the core and 5,6 per mill in the rim. At 500^oC the oxygen isotope fractionation between grossular and quartz is about 5 per mill (Mathews, 1994). Taking grossular as a proxy for vesuvianite this would indicate that the skarn minerals were in equilibrium with the Drammen Granite (Jamtveit et al., 1992).

The fibrous quartz at the vein-carbonate interfaces is ascribed to undercooling of the intruding melt. The occurrence of clinopyroxene indicates relatively low water fugacity in the veins as compared to the biotite bearing Drammen Granite: The precipitation of sphene facilitated acommodation of the titanium which is mainly biotite hosted in the Drammen Granite. The calcium required for sphene formation was derived from the calcareous country rocks. The vesuvianite oxygen isotope composition indicates dominance of the magmatic component in the pore fluid. The slight but significant increase in oxygen isotope composition from the vesuvianite core to the rim further suggests that the magmatic fluid was successively diluted by isotopically heavy volatiles derived from the calcareous metasediments. In this respect the vesuvianites from Myrseter differ from the zoned garnets described from the eastern Drammen aureole by Jamtveit and Hervig (1994), which inidcate late stage influx of meteoric waters. The introduction of carbon dioxide and "salting in" in the saline brines that remained from boiling may account for the decreased water fugacity in the vein fluids at Myrseter.

Jamtveit, B, Grorud, HF, Bucher-Nurminen, K, EPSL, 114, 131-148, (1992).

Jamtveit, B, Hervig, RL, Science, 263, 505-508, (1994).

Mathews, A, J. Met. Geol, 12, 211-219, (1994).

Olsen, KI, Griffin, WL, Contrib. Mineral. Petrol, 87, 1-14, (1984).