P-T evolution of metasediments from the Eclogite Zone, south-central Tauern Window, Austria

Petrologic data on the paragenesis of (I) kyanite-zoisite marbles and (II) garnet-chloritoid quartz-mica schists are presented with the goal of providing constraints on the pressure-temperature evolution of the Eclogite Zone, Tauern Window, Austria. The peak metamorphic assemblages in the two rock types are: (I) kyanite + zoisite + dolomite + quartz; zoisite + muscovite + dolomite + calcite + quartz; and (II) garnet + chloritoid + kyanite + muscovite + quartz + epidote ± dolomite ± Zn-staurolite. The estimated peak metamorphic conditions are 19 ± 2 kbar, 590 ± 20°C.

Secondary alteration of the kyanite-zoisite marbles was accomplished in two stages. The early stage resulted in the production of margarite, paragonite, secondary muscovite and chlorite and the later stage resulted in the formation of sudoite (a di/trioctahedral Mg---Al layer silicate) and kaolinite. The early alteration is bracketed at conditions between 3 and 10 kbar, 450–550°C and the later alteration between 200 and 350°C, P 3 kbar.

The P-T path is characterized by maximum burial to approximately 19 kbar (60–70 km) (at≈590°C), followed by nearly isothermal decompression to approximately 10 kbar (30 km), and then more gradual decompression with cooling to approximately 3 kbar (10 km). Alteration was apparently accomplished by the influx of H₂O-rich fluids, with the composition of the fluid locally buffered by the mineral assemblage.     

Thermal history of the Sonnblick Dome,south-east Tauern Window,Austria: Implications for heterogeneous uplift within the Pennine basement

The Sonnblick Dome is one of several domal structures affecting the interface between basement and cover within the Pennine Zone of the Tauern Window in the eastern Alps. Rb-Sr isotopic data, comprising 19 biotite and 22 white mica ages from variably deformed granitic gneisses, provide new evidence of the thermal and tectonic history of the dome and its relationships with other parts of the south-east Tauern Window. White mica ages generally cluster between 26 and 30 Ma although there are values up to 82 Ma, which appear to reflect incomplete equilibration during Tertiary metamorphism under low amphibolite facies conditions; six closely spaced samples from an intensely sheared gneiss lamella are more tightly grouped between 26 and 27.6 Ma and provide the best estimate of the age of syntectonic crystallization. Biotite ages are systematically younger, ranging from 19 to 23.5 Ma, reflecting closure during post-metamorphic cooling. Sonnblick Dome and the Hochalm Dome approximately 20 km further east, where closure of Rb-Sr in biotite did not occur until 16.5 Ma; the metamorphic peak here is also probably younger, possibly as late as 22 Ma. The Sonnblick Dome was formed before 27 Ma and the deformation style had changed to extension before biotite closure by 19 Ma. In contrast, rapid updoming in the Hochalm Dome was previously dated at 16.5 Ma and the differences in thermal history can be linked to differences in deformation history. Overall the geochronological data from the south-east Tauern Window demonstrate the heterogeneity of thermal history on a geographical scale of 10 km and emphasize the importance of tectonic displacements in controlling temperature within orogenic belts.     

High-Pressure Metamorphism in the SW Tauern Window, Austria: P-T Paths from Hornblende-Kyanite-Staurolite Schists

The hornblende garbenschist horizon of the Lower Schieferhulleseries (LSH) in the SW Tauern Window, Austria, contains theassemblage hornblende + kyanite + staurolite + garnet + biotite+ epidote + plagioclase + ankerite + quartz + rutile + ilmenite,with either chlorite or paragonite present in all samples. Theseassemblages are divariant in the system SiO₂-Al₂O₃-TiO₂-Fe₂O₃-MgO-FeO-MnO-CaO-Na₂O-K₂O-H₂O-CO₂.Garnet-biotite geothermometry yields temperatures of final equilibrationof {small tilde}550 °C, and garnet-plagioclase-kyanite-quartzgeobarometry indicates pressures of 6–8 kb for the matrixassemblage and 9–10 kb for plagioclase inclusions in garnet.Quantitative modelling of zoned garnet, hornblende, and plagioclaseindicates growth and equilibration along a decompression pathfrom {small tilde}530 °C, 10 kb to {small tilde}550 °C,7 kb. Fluid inclusion data constrain the uplift path to havepassed through a point at {small tilde} 375 °C, 1.5 kb. These data permit the construction of a relatively completeP-T loop for metamorphism associated with the Alpine orogeniccycle in the LSH of the SW Tauern Window. The maximum pressureconditions ({small tilde}10 kb at 530 °C) recorded alongthis loop are considerably higher than previous estimates of5–7 kb for the region. Simple overthrust models developedfor the Tauern Window cannot account for pressures of this magnitude;a more likely scenario involves partial subduction of the rocksto a depth of {small tilde}35 km, followed by prolonged heatingin response to decay of the subduction isotherms. Initial upliftappears to have been rapid and occurred along a nearly isothermalpath. Significant cooling did not occur until the rocks werewithin {small tilde}5 km of the surface. Detailed tectonic modelsfor the evolution of the Tauern Window must be able to accountfor the quantitative features of the P-T loop.     

A Clinopyroxene Paragenesis of Albite--Epidote--Amphibolite Facies in Meta-Syenites from the South-East Tauern Window, Austria

Mineral assemblages and textures are described from clinopyroxene-bearingmeta-syenites and related rocks from a small area in the PenninicBasement Complex of the south-east Tauern Window. Evidence from mineral textures, mineral compositions and geobarometryindicate that the clinopyroxene, a sodic salite, crystallizedas part of an equilibrium albite-epidote-amphibolite faciesparagenesis in the 35–40 Ma meso-Alpine metamorphic event.Phase relations in co-facial quartz + albite + K-feldspar +sphene-bearing meta-syenites and meta-granites are examinedusing a projection from these minerals onto the plane (A1₂O₃+ Fe₂O₃)-CaO-(MgO + FeO + MnO). The projection demonstratesthat salitic clinopyroxene can only be a stable phase in suchrocks if the bulk-rock Al/Na + K ratios are low. This is confirmedby comparing the whole-rock analyses of clinopyroxene-bearingmeta-syenites with those of clinopyroxene-free meta-syenitesand meta-granites. Mineral assemblages in a variety of lithologies from the south-eastTauern Window are used to construct a generalized AKM diagramfor magnesian albite + epidote + quartz-bearing rocks of thealbite-epidote-amphibolite facies. Thermochemical calculations indicate that the meta-syeniteswere metamorphosed at temperatures close to 500 C and at a pressureof 6+2 –4 kb. Fluids in equilibrium with meta-syeniteand meta-granite mineral assemblages had X_H2O values of 0–95,assuming X_H2O + X_CO2O= 1.0.     

Staurolite in garnet amphibolite from Sölden, Ötztal old crystalline basement,Austria

Summary Textural and compositional relations of coexisting staurolite, hornblende, garnet and kyanite in a garnet amphibolite of undoubted igneous origin are reported. The bulk chemistry of the staurolite bearing rock is determined and compared with the composition of those amphibolites of the same locality, which contain no staurolite. The important difference seems to be the CaO-content. The staurolite bearing rock has 24.41 molecular percent CaO whereas the neighbouring amphibolites have a higher value ranging from 27 to 31 molecular percent. The Mg-value (100 Mg/Mg+Fe) of the analysed staurolite is 31.77, similar to the staurolite analysis 41001 ofGibson (1978), thus probably indicating the similarity of the host rocks. This study unequivocally demonstrates the formation of staurolite in metabasic rocks as has been reported byGibson (1978).

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Staurolith im Granatamphibolit von Sölden, Ötztaler Altkristallin, Österreich

Zusammenfassung In einem Granatamphibolit eindeutig magmatischen Ursprungs werden Gefüge und Zusammensetzung der Paragenese Staurolith-Homblende-Granat-Disthen beschrieben. Der Pauschalchemismus dieses Gesteins wird mit dem der Nachbargesteine ohne Staurolithe verglichen, dabei wirkt sich der Hauptunterschied im Gehalt von CaO aus, der im Gestein mit Staurolith 24.42 Mol.-% CaO beträgt, während die Nachbargesteine einen deutlich höheren (27–31%) Gehalt haben. Der Mg-Gehalt (100 Mg/Mg+Fe) der Staurolithe beträgt 31.77 und ist somit ähnlich dem Wert vonGibson (1978), wodurch die Ähnlichkeit des Vorkommens belegt wird In der vorlieenden Arbeit wird die metamorphe Bildung von Staurolithe in einem Metabasit, wie bereits vonGibson (1978) beschneben eindeutig belegt.

     

Understanding the pre-Variscan and Variscan basement components of the central Tauern Window, Eastern Alps (Austria): constraints from single zircon U-Pb geochronology

New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting out the time dimension among the various Variscan and pre-Variscan basement components that were strongly overprinted by Alpine orogeny. Single-grain isotope dilution (ID-TIMS) U-Pb zircon geochronology of three Basisamphibolit samples yield protolith formation ages of 351±2, 349±1 and 343±1 Ma. Laser ablation ICP-MS and ID-TIMS U-Pb detrital zircon dating of the Biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362±6 Ma and 368±17 Ma. Paragneisses from the Zwölferzug yield maximum sedimentation ages from 345±5 Ma (ion microprobe data) to 358±10 Ma. Zircons from gabbroic clasts and detrital zircons from a meta-agglomerate from the Habach Phyllite give an upper intercept age of 536±8 Ma and a near-concordant age of 506±9 Ma, respectively. Hence, apart from the Habach Phyllite, the maximum sedimentation ages of the metasediments investigated range from Upper Devonian to Lower Carboniferous. Consequently, the Basisamphibolit, the Biotitporphyroblastenschiefer, and the paragneisses of the Zwölferzug form parts of the Variscan basement series. The Basisamphibolit (351-343 Ma) is distinct both in space and time of formation from the Zwölferzug garnet amphibolite (c. 486 Ma), which forms part of the pre-Variscan basement.     

Geothermobarometry in metasediments of the southern Grossvenediger area (Tauern Window, Austria)

Metasediments in the southern Grossvenediger area (Tauern Window, Austria) were studied along a cross-section through rocks of increasing metamorphic grade from the margin of the Tauern Window in the south to the base of the Upper Schieferhülle, including the Eclogite Zone, in the north. In the southern part of the cross-section there is no evidence for a pre-late Alpine metamorphic history in the form of high-pressure relics or pseudomorphs. Mineral assemblages are characterized by the stability of tremolite + calcite, biotite + calcite and biotite + chlorite + calcite. In the northern part a more complete Alpine metamorphic evolution is preserved. Primary high-pressure assemblages are dolomite + quartz, tremolite + zoisite, zoisite + dolomite + quartz + phengite I and probably tremolite + dolomite + phengite I. Secondary, post-kinematic assemblages [tremolite + calcite, talc + calcite, phengite II + chlorite + calcite (+ quartz), biotite + chlorite + calcite, biotite + zoisite + calcite] formed as a result of the dominant late Alpine metamorphic overprint. The occurrence of biotite + zoisite + calcite is confined to the northernmost area and defines a biotite–zoisite–calcite isograd. P–T estimates based on standard thermobarometric techniques and on stability relationships of tremolite + calcite + dolomite + quartz and zoisite give consistent results. P–T conditions of the main Tertiary metamorphic overprint were 525° C, P= 7.5 ± 1 kbar in the northern part of the cross-section. The southern part was metamorphosed at lower temperatures of 430–470° C. The Si-content of phengites from this area is almost as high as that of phengites from the Eclogite Zone (Si^max= 3.4 pfu). Pressures > 10 kbar at 420° C are suggested by phengite barometry according to Massone & Schreyer (1987). In the absence of high-pressure relics or pseudomorphs, these phengites, which lack late Alpine re-equilibration, are the only record that rocks of the southern part probably also experienced an early non-eclogitic high-pressure metamorphism.     

Evolution of metamorphic fluids in shear zones: The record from the emeralds of Habachtal, Tauern Window, Austria

Fluid inclusions in emeralds from the Habachtal, Central Tauern Window, have been studied by microthermometry. Results allow a detailed reconstruction of trapping history and evolution of the metamorphic fluids during the Middle Alpine Tauernkristallisation metamorphic event and some of the subsequent cooling period. Five different types of fluid inclusions, corresponding to at least five trapping periods, have been distinguished. In general, the earliest primary (type 1) inclusions, which occur as negative crystals or thin long tubes, are represented by low salinity ( < 10 wt. % NaCl equivalent) aqueous fluids with or without CO₂ with up to X_CO2 ≈ 0.04. Later primary type 2 inclusions are distinguished by different morphologies and distribution patterns. Lower salinity CO₂-free brines and CO₂-bearing denser inclusions with higher CO₂ contents (up to X_CO2 ≈ 0.11) are characteristic of this stage. The type 2 inclusions may also occur as pseudosecondary arrays. The effects of necking have been studied, and found to be considerable in the type 1 primary inclusions. This mechanism has occasionally resulted in the appearance of almost pure CO₂ fluids. The possibility of fluid immiscibility has been examined, and rejected, for the apparent “coexistence” of primary brine and CO₂-bearing inclusions. Instead, mixing of fluids which fluctuated between two different compositions is proposed. The fluctuation was probably due to the sequence of hydration reactions during the Tauernkristallisation. Maximum trapping pressures (3.6 kbar) obtained for stage 1 of the Tauernkristallisation are thought to represent a situation where sublithostatic fluid pressures exested in shear zones during the crystallisation period of many of the emerald cores and coexisting biotite and actinolite. Maximum fluid pressures of 7 kbar were obtained from the type 2 inclusions. This is similar to pressure estimates obtained from mineral equilibria. At least four phases of deformation are indicated by the trapping history. A pressure-temperature-time path for the Tauernkristallisation and the subsequent cooling/uplift period has been constructed for the Habachtal area, using the maximum pressure estimates obtained in this work together with previously existing data. In the cooling period, fluid pressures lower than the lithostatic load again prevailed. This difference, about 1–2 kbar, was probably due to late stage fracturing and/or the development of an open system. At least two more phases of minor deformation and three more stages of entrapment have been defined for this period. During this time, fluids gradually evolved towards more CO₂-poor, and less saline compositions. The present work shows that the possibility of fluctuations in fluid pressures must be considered seriously when attempting to define the PT cooling path from fluid inclusions in metamorphic rocks, especially those in shear zones. Postulations of retrograde PT paths based on fluid inclusions alone may result in pressure estimates which are too low.     

Pre-Alpine history of the pennine zone in the Tauern Window,Austria: U-Pb and Rb-Sr geochronology

Zircon ages from major lithologies of the Zentralgneis suggest that much of the Variscan magmatism in the Tauern Window is older than previously suggested. In the southeast Tauern Window a tonalite has been dated at 314±7 m.y. and a granodioritic biotite augen gneiss at 313±10 m.y. Two granodiorites from the Granatspitzkern yielded zircon data consistent with a similar age. These zircon data require re-interpretation of some previously published Rb-Sr whole rock ages and raise the possibility that Alpine metamorphism caused more widespread disturbance of Rb-Sr whole rocks than commonly supposed. Rb-Sr data on fabric-forming white micas from two banded gneisses give ages close to 220 m.y., indicating the foliation in these rocks is pre-Alpine and has not been greatly affected by Alpine recrystallisation.     

Alpine metamorphism in the south-east Tauern Window, Austria: 2. Rates of heating, cooling and uplift

Abstract Existing geochronological data are reviewed and new Rb-Sr, K-Ar and ³⁹Ar–⁴⁰Ar ages are presented, including a suite of 33 mica ages from a 20 km north–south tunnel section. These data are discussed in relation to the thermal history from the overthrusting of the Autroalpine nappes c. 65 Myr ago to the present. The earliest phase of metamorphism, involving lawsonite crystallization, is associated with emplacement of these nappes. Subsequently, temperatures in the rocks beneath rose, at a mean rate of 3–6°C/Myr, until the climax of metamorphism.
At high structural levels, published data indicate an age > 35 Myr for the metamorphic climax. In contrast, a new ³⁹Ar–⁴⁰Ar step-heating age of 23.8 ± 0.8 Myr on amphibole, from near the base of Peripheral Schieferhülle, closely approximates the age of metamorphism and provides the first clear indication that the climax of metamorphism occurred later at deeper structure levels. Following the climax, near-isothermal uplift and erosion reduced pressure to c. 1 kbar before white mica closure at 19 Myr; this implies uplift at >3 mm/yr.
Along the tunnel section, white mica K-Ar ages vary systematically from 24 Myr to 16.5 Myr with position relative to a late 4 km amplitude dome whereas biotite Rb-Sr ages are uniform at 16.5 Myr across the whole profile; doming is thus dated at 16.5 Myr with transient uplift rates >5 mm/yr. At other times uplift rates were <1 mm/yr.     

Jadeite-gneiss from the Eclogite Zone,Tauern Window,Eastern Alps,Austria: Metamorphic,geochemical and zircon record of a sedimentary protolith

In the Eclogite Zone of the Tauern Window, a layer of strongly retrogressed leucocratic jadeite-bearing gneiss is intercalated between mafic kyanite-eclogites. The jadeite-gneiss consists of garnet + quartz + paragonite ± phengite ± zoisite + zircon + rutile + apatite + pyrite. Kyanite, jadeite or omphacite are exclusively present as inclusions in garnet. Retrogressive hydration during exhumation led to a breakdown of matrix jadeite to form pseudomorphs of calcic amphibole + albite. Peak metamorphic conditions derived from the primary gneiss assemblage are 2.0–2.4 GPa at approximately 640 °C. Major, trace element and isotopic compositions of the jadeite-gneiss are consistent with a siliciclastic sedimentary protolith. Zircon morphology and zonation patterns reveal a complex history. The presence of fracture-truncated zircons suggests a detrital origin, whereas most internal structures and Th/U ratios are characteristic of zircons from magmatic rocks. In situ LA-ICP-MS and SHRIMP U–Pb geochronology and zircon geochemisty provide evidence of at least three magmatic events in the provenance area. These were dated at 466 ± 2 Ma, 437 ± 2 Ma and 288 ± 9 Ma. Older ages ranging from 503 to 691 Ma are preserved in the cores of some zircon grains, suggesting derivation from peri-Gondwanan sources. Surprisingly, no firm evidence of the Tertiary high-pressure metamorphic event and subsequent retrograde overprint was seen in any of the studied zircons. However, some zircons show resorbed surfaces suggesting corrosion by a superficial fluid phase undersaturated in zirconium and one extensively altered porous zircon yielded highly discordant ²⁰⁶Pb/²³⁸U ages in the range 325–109 Ma documenting partial recrystallization by dissolution–reprecipitation of a highly reactive grain.     

Crustal stacking and extension recorded by tectonic fabrics of the SE margin of the Tauern Window, Austria

Structural and metamorphic analyses show that Alpine deformation in the Austroalpine-Pennine contact zone around the margin of the SE Tauern Window can be divided into two main stages: (i) early crustal thickening associated with prograde metamorphism; and (ii) a younger history of ductile flow that added to cumulative displacement of the upper units to the NW quadrant but was associated with substantial subvertical attenuation of the contact zone, and most probably of the overriding Austroalpine plate as well. During the history of this region strain localization progressively shifted down section. Radiometric ages constrain the early deformation to be older than 75 Ma. The onset of contact-zone attenuation and upper-plate extension was after this date but before 35 Ma (before major involvement of European basement in the collisional orogen), and associated with both retrograde metamorphism and a degree of non-coaxiality less than simple shear. Estimates of thinning in the contact zone and on a regional scale are in good agreement and indicate vertical attenuation of approximately 40%. These results suggest that pre-collisional tectonic thinning of the Austroalpine domain may be more widespread and significant than generally recognized.     

Oxygen and hydrogen isotope studies on minerals from alpine fissures and their gneissic host rocks,Western Tauern Window (Austria)

Fourteen cogenetic quartz-biotite pairs from gneissic wall rocks, and 22 quartz, 16 calcite, and 8 biotite samples and 1 sample of albite from fissure-filling veins in the Western Tauern Window were analyzed for their oxygen isotope composition. The δ¹⁸O values show the following ranges: (a) quartz, +6.0 in fissure in amphibolite to +10.3 in fissures in granite gneisses; (b) biotite, +2.5 to +6.7; and (c) calcite, +7.0 to +8.9. The δ¹⁸O value of albite is +7.1. Only a small variation in the hydrogen isotope composition of biotite was detected. δD values of 7 biotites from gneisses and fissure fillings varied from −54 to −59. There is no significant difference in the hydrogen isotope composition of fissure biotite and biotite from the host rock. This indicates that a common water source of probably deep-seated origin existed, with no detectable contribution from isotopically light meteoric water. Oxygen isotope fractionations between coexisting quartz and biotite of 3.5 to 7.0‰ indicate equilibrium temperatures of 640 ° to 450 ° C, respectively, using the fractionation curve of Hoernes and Friedrichsen (1978). The highest temperatures of equilibration are for the rocks at the Alpenhauptkamm, i.e., the central part of the Tauern Window. Successively lower temperatures are found to the north and to the south of the Alpenhauptkamm along a traverse through Penninic units of the Tauern Window. The metamorphism of the host rocks and the filling of fissures has occurred at the same temperature in a given sample locality.     

Syn‐metamorphic folding in the Tauern Window,Austria dated by Th–Pb ages from individual allanite porphyroblasts

High‐precision ²³²Th–²⁰⁸Pb dates have been obtained from allanite porphyroblasts that show unambiguous microstructural relationships to fabrics in a major syn‐metamorphic fold in the SE Tauern Window, Austria. Three porphyroblasts were analysed from a single garnet mica schist from the Peripheral Schieferhülle in the core of the Ankogel Synform, one of a series of folds which developed shortly before the thermal peak of Alpine epidote–amphibolite facies metamorphism: allanite grain 1 provided two analyses with a combined age of 27.7 ± 0.7 Ma; grain 2, which was slightly bent and fractured during crenulation, provided two analyses with a combined age of 27.7 ± 0.4 Ma; a single analysis from grain 3, which overgrew an already crenulated fabric, gave an age of 28.0 ± 1.4 Ma. The five ²³²Th–²⁰⁸Pb ages agree within error and define an isochron with an age of 27.71 ± 0.36 Ma (95% confidence level; MSWD = 0.46). The results imply that the crenulation event was in progress in a short interval (<1 Ma) c. 28 Ma, and that the Ankogel Synform was forming at this time. The thermal peak of regional metamorphism in the SE Tauern Window was probably attained shortly after 28 Ma, only c. 5 Ma after eclogite facies metamorphism in the central Tauern Window. Metasediment may contain allanite porphyroblasts with clear‐cut microstructural relationships to fabric development and metamorphic crystallization; for such rocks, ²³²Th–²⁰⁸Pb dating on microsamples offers a powerful geochronological tool.     

Zoisite- and clinozoisite-segregations in metabasites (Tauern Window, Austria) as evidence for high-pressure fluid–rock interaction

High-pressure zoisite- and clinozoisite-bearing segregations are common in garnet- and albite-bearing amphibolites of the Palaeozoic part of the Lower Schieferhülle, south-central Tauern Window, Austria. The zoisite segregations (primary assemblage: Zo+Qtz+Cal) formed during an early to pre-Hercynian high-pressure event (P≫0.6 GPa, T =500–550 °C) by hydrofracturing as a result of protolith dehydration. Zoisite is growth zoned from Fe³⁺-poor cores (Al₂Fe=9 mol%) to Fe³⁺-rich rims (17 mol%), and has high Sr, Pb and Ga contents and LREE-enriched REE patterns, controlling the trace element budget of the segregations. Hercynian deformation at c. 0.7 GPa/600 °C kinked and cracked primary zoisite and enhanced breakdown into secondary zoisite (13 mol% Al₂Fe), clinozoisite (40–55 mol% Al₂Fe), albite (an_<20), calcite and white mica during an Eoalpine high-pressure event at 0.9–1.2 GPa/400–500 °C. The clinozoisite segregations (primary assemblage: Czo+Qtz+Omp+Ttn+Chl+Cal) are mm- to cm-wide, vein-like bodies, cross-cutting fabric elements of the host garnet amphibolite. They formed during the Eoalpine high-pressure event at 0.9–1.2 GPa/400–500 °C. During Alpine exhumation, omphacite was pseudomorphed by amphibole, albite, quartz and clinozoisite. Oxygen isotope data suggest equilibrium between host metabasite and zoisite segregations and indicate an internal fluid source and fluid buffering by the protolith. Mobility of P, Nb and LREE changed the protolith’s trace element composition in the vicinity of the zoisite segregations: Mobilization of LREE is evidenced by decreasing modal amounts of LREE-rich epidote and decreasing LREE contents in LREE-rich epidote towards the segregations, changing the REE patterns of the host metabasite from LREE-enriched to LREE-depleted. Tectonic discrimination diagrams, based on the trace element content of metabasites, should be applied with extreme caution.     

Unravelling the pre-Variscan evolution of the Habach terrane (Tauern Window, Austria) by U-Pb SHRIMP zircon data

The U-Pb SHRIMP age determinations of zircons from the Habach terrane (Tauern Window, Austria) reveal a complex evolution of this basement unit, which is exposed in the Penninic domain of the Alpine orogen. The oldest components are found in zircons of a metamorphosed granitoid clast, of a migmatitic leucosome, and of a meta-rhyolitic (Variscan) tuff which bear cores of Archean age. The U-Pb ages of discordant zircon cores of the same rocks range between 540 and 520 Ma. It is assumed that the latter zircons were originally also of Archean origin and suffered severe lead loss, whilst being incorporated into Early-Cambrian volcanic arc magmas. The provenance region of the Archean (2.64-2.06 Ga) zircons is assumed to be a terrane of Gondwana affinity: i.e., the West African craton (Hoggar Shield, Reguibat Shield). The Caledonian metamorphism left a pervasive structural imprint in amphibolite facies on rocks of the Habach terrane; it is postdated by discordant zircons of a migmatitic leucosome at <440 Ma (presumably ca. 420 Ma). Alpine and Variscan upper greenschist- to amphibolite-facies conditions caused partial lead loss in zircons of a muscovite gneiss ('white schist') only, where extensive fluid flow and brittle deformation due to its position near a nappe-sole thrust enhanced the grains' susceptibility to isotopic disturbance. The Habach terrane - an active continental margin with ensialic back-arc development - showed subduction-induced magmatic activity approx. between 550 and 507 Ma. Back-arc diorites and arc basalts were intruded by ultramafic sills and subsequently by small patches of mantle-dominated unaltered and (in the vicinity of a major tungsten deposit) altered granitoids. Fore-arc (shales) and back-arc (greywackes, cherts) basin sediments as well as arc and back-arc magmatites were not only nappe-stacked by the Caledonian compressional regime closing the presumably narrow oceanic back-arc basin and squeezing mafic to ultramafic cumulates out of high-level magma chambers (496-482 Ma). It also induced uplift and erosion of deeply rooted crystalline complexes and triggered the development of a successor basin filled with predominantly clastic greywacke-arkosic sediments. The study demonstrates that the basement rocks exposed in the Habach terrane might be the 'missing link' between similar units of the more westerly positioned External domain (i.e., Aar, Aiguilles Rouges, Mont Blanc) and the Austroalpine domain to the east (Oetztal, Silvretta).     

Age constraints on the evolution of the Austroalpine basement to the south of the Tauern Window

The Austroalpine basement to the south of the Tauern Window once was part of the northern margin of Gondwana. It includes the “Altkristallin” and the phyllitic Thurntaler Complex. In the Altkristallin (AMU, MPU), suites of arc-related metamafic sequences occur together with calc-alkaline metagranite. SHRIMP U–Pb dating of zircon from calc-alkaline metagranite associated with an eclogitic amphibolite give an age of 470 ± 3 Ma interpreted as the age of protolith emplacement. In the Thurntaler Complex, metaporphyroids occur together with tholeiitic as well as alkaline within-plate basalt-type metabasite. The metaryholites of this association give a crystallization SHRIMP age of 477 ± 4 Ma, which suggests contemporaneity of arc-related and extensional settings in the Austroalpine basement units. The age data demonstrate widespread magmatic activity associated with the Early-Ordovician amalgamation at the end of the 550–470 Ma subduction–accretion–collision cycle. The Pb–Pb and U–Pb systematics of step-wise leached staurolite and kyanite from the peak-metamorphic assemblage of the Altkristallin indicate that (1) step-wise leaching of staurolite and kyanite yields the age of inclusions rather than the host; (2) zircon inclusions in staurolite suggest an Ordovician or older age for the precursor of the staurolite-schists; (3) the weighted average of the ²⁰⁶Pb/²³⁸U data of the various leaching steps yields a Variscan age for the inclusions (ilmenite, biotite, and andesine). Since these inclusions are part of the metamorphic mineral assemblage, this age provides a minimum estimate for staurolite growth, i.e., metamorphism. Thus, the Pb–Pb and U–Pb systematics of staurolite provide evidence for a Variscan metamorphism of the Austroalpine basement, e.g., MPU, AMU and Thurntaler Complex, to the south of the Tauern Window.     

Large-ion lithophile element characteristics of an amphibolite facies to granulite facies transition at Gruinard Bay, North-west Scotland

Abstract Lewisian grey gneisses from Gruinard Bay, North-west Scotland retain mineralogical and geochemical evidence for Scourian horn-blende-granulite facies metamorphism, and they may be used to assess current models of elemental depletion at granulite grade. Their 'immobile'major and trace element geochemistry is indistinguishable from that of Lewisian amphibolite and pyroxene-granulite facies counterparts. The K, Rb, Th and U contents of the Gruinard Bay gneisses are depleted relative to amphibolite facies gneisses, but generally the abundances of these elements are above those of comparable pyroxene granulites. U and Th have reached an advanced stage of depletion, but allanite appears to be crucial in maintaining significantly higher U and Th abundances at Gruinard Bay than in pyroxene granulites. K and Rb loss is less extreme, and depends on the stability of the rock-forming minerals: K-feldspar; biotite; and, amphibole. Early removal of K and Rb has resulted in a small rise in K/Rb, but further preferential Rb loss would have been required to generate the characteristically high K/Rb ratios of Lewisian pyroxene granulites.
The residence of U and Th in the accessory minerals of granulite facies gneisses, which are often correlated with the residua of intracrustal partial melting, renders unlikely their extreme incompatibility required by such models. Even if such phases are ignored, high mineral-melt partition coefficients for silicic melts argue against partial fusion as an efficient depletion mechanism. On the other hand, the advanced stage of U and Th depletion reached in Gruinard Bay gneisses, which were still partly hydrous, severely restricts the role played by CO₂-dominated fluids and a hydrous medium is preferred.