The metamorphic field gradient in the eclogite type locality, Koralpe region, Eastern Alps

The eclogite type locality in the Eastern Alps (the Koralpe and Saualpe region) is the largest region in the Eastern Alps that preserves high‐pressure metamorphic rocks from the Eo‐Alpine orogenic event of the Cretaceous age. Thermobarometric data from the metapelitic gneisses in the region indicate that a metamorphic field gradient across the region can be divided into three parts. The northern part shows continuously increasing P–T from 10 ± 1.5 to 14 ± 1.5 kbar and 500 ± 68 to 700 ± 68 °C over a distance of 40 km. The continuous increase in P–T indicates that no major tectonic boundaries were active in this part during the Eo‐Alpine orogeny. Small discontinuities in the pressure gradient of the northern part can be correlated with more localized deformation. The central part exposes amphibolite–eclogite facies rocks with 15 ± 1.5 kbar and 700 ± 68 °C over about 20 km length. The southern part shows decreasing P–T conditions from 15 ± 1.5 to 10 ± 1.5 kbar and 700 ± 68 to 600 ± 63 °C over a distance of 10 km beyond which conditions remain roughly constant for the remainder of the profile. Overall, the field gradient is characterized by: (i) an increase in age with decreasing metamorphic grade and (ii) a T/P ratio that is lower than common metamorphic geotherms. The age–grade relationship is consistent with the timing relationship along piezothermal arrays predicted by simple models for regional metamorphism. However, the T/P ratio of the field gradient is inconsistent with such an interpretation. These inconsistencies indicate that the profile is not simply an obliquely exposed crustal section. We suggest that the exhumation of the transect is best explained with a two dimensional model of an extruding wedge, as has recently been suggested as a typical scenario for other large scale compressional orogens.     

Contrasting thermomechanical evolutions in the Southalpine metamorphic basement of the Orobic Alps (Central Alps, Italy)

In the Southern Alps a progressive metamorphic zonation, with an increase in the geothermal gradient from NE to SW, has been widely proposed. However, recent investigations have shown that the greenschist metamorphic imprint of the low-grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions. On the other hand, in the medium-grade zone, a later low-pressure, high-temperature (LPHT) metamorphic event has also been proposed. In an attempt to resolve these different interpretations, new petrological and partly new structural data have been obtained for two sectors of the Orobic Alps, traditionally attributed to different metamorphic zones. Thermobarometric determinations, supported by microstructural analysis, indicate the following different pressure-retrograde paths in each sector: (1) in the Val Vedello basement (VVB) rocks, a first metamorphic imprint characterized by P = 7–9 kbar and T = 570–610°C was followed by a greenschist retrogression ( P ≤ 4 kbar and T ≤ 500° C); (2) in the Lario basement (LB) rocks, the first detectable metamorphic stage, characterized by mineral assemblages indicating P = 7–9 kbar and T = 550–630° C, was followed by a LPHT event, synkinematic with F2 extensional deformation. A greenschist retrogression marks the final uplift of these rocks.
Reinterpretation of the available geochronological data indicates a diachronism for the two thermomechanical evolutions. In the light of these data, we interpret the retrograde P–T–t path of the VVB rocks as a pre-Permian post-thickening uplift and the retrograde P–T–t evolution of the LB rocks as a Permo-Mesozoic uplift related to the extensional tectonic regime of the Tethyan rifting.     

Eoalpine and mesoalpine tectonics in the Southern Alps

According to structural and stratigraphic data, eoalpine and mesoalpine tectonics can be clearly recognized in the Southern Alps and distinguished from the better known neoalpine deformation.The eoalpine phase (Late Cretaceous) was generated by N-S compression. It deformed basically the central-western Southern Alps producing overthrusts to the west and flower structures by sinistral transpression in the Giudicarie Belt. Deposition of the coeval flysch successions was controlled by the trend of this belt. This tectonic pattern persisted until Middle Eocene.The mesoalpine phase (Eocene) was generated mainly by a ENE-WSW compression and produced thrust geometries with N-S or NW-SE direction in the eastern part of the Southern Alps. The coeval Eocene Flysch also followed this trend, filling the foredeep basin. This deformation is considered to be the front of the Dinarids, which began to be deformed since Late Cretaceous until at least Early Oligocene.The neoalpine tectonics inherited the eoalpine and mesoalpine structures and produced the major part of the deformation accounting for the present structural framework of the Southern Alps.

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Zusammenfassung Die eo- und mesoalpidische Tektonik in den Südalpen ist strukturell und stratigraphisch erkennbar und von den bekannteren neoalpidischen Bewegungsbildern deutlich zu unterscheiden.Die eoalpidische Phase (Obere Kreide) entspricht einer N-S Kompression, die sich in den zentralen bis westlichen Südalpen bemerkbar macht. Im westlichen Bereich treten Überschiebungen auf, das Judikarische Gebiet wird hingegen durch die von linkslateralen Transpressionen bewirkten Verwerfungsbündel gekennzeichnet, die auch die altersgleichen Flysch-Ablagerungen kontrollieren. Diese tektonische Phase ist bis zum Eozän aktiv.Die mesoalpidische Phase (Eozän) ist auf eine ENE-WSW Kompression zurückzuführen. Sie verursachte in den östlichen Südalpen N-S bis NW-SE gerichtete Überschiebungsbilder. In gleichorientierten vorozeanischen Becken kommt der eozäne Flysch vor. Dieses Deformationsbild kennzeichnet schon in der Oberen Kreide die Dinariden-Front.Die neoalpidische Tektonik vererbte die eo-mesoalpidischen Gefüge-Elemente und verursachte im wesentlichen den gegenwärtig erkennbaren Gefügeplan der Südalpen.

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Résumé Dans les Alpes Méridionales, des données stratigraphiques et structurales permettent de reconnaître les tectoniques éoalpine et mésoalpine et de les distinguer des déformations néoalpines, mieux connues.La phase éoalpine (Crétacé supérieur) a été engendrée par une compression N-S. Celle-ci a affecté surtout la partie centre-occidentale des Alpes Méridionales en produisant des charriages à l'ouest et des structures de décrochement sénestres dans la chaîne des Giudicarie, dont l'orientation a déterminé la sédimentation du flysch concommittant. Cette tectonique s'est poursuivie jusqu' à l'Eocène.La phase mésoalpine a été engendrée surtout par une compression ENE-WSW; elle a produit, dans la partie est des Alpes Méridionales, des charriages de direction N-S à NW-SE. Le flysch éocène a suivi cet alignement structural, en remplissant le bassin de l'avant-fosse. Cette déformation est considérée comme le front des Dinarides, dont la formation a commencé dés le Crétacé supérieur.La tectonique néoalpine a hérité des structures éoalpines et mésoalpines et est responsable de la plus grande part de la structure actuelle des Alpes Méridionales.

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Pseudosection modelling for a selected eclogite body from the Koralpe (Hohl), Eastern Alps

In the Kor-, Saualpe and Pohorje regions of the Eastern Alps eclogite bodies occur within metapelitic gneisses. The bodies are between 1 meter and several hundreds of meters in size and some of them were defined by Haüy (1822) as the type locality for the rock type “eclogite”. A growing body of petrological work has documented the metamorphic evolution of the metapelites surrounding the eclogites. However, few phase diagrams have been constructed for the eclogite bodies themselves. Here we use recently available activity models for amphiboles to present new thermodynamic pseudosections for the Hohl locality of the Koralpe eclogites. We show that this eclogite reached peak conditions in a narrow PT field obliquely oriented in PT space between 16.5 and 20.5 kbar and 620°C to 720°C and that its metamorphic evolution was likely to have occurred under water saturated conditions. We conclude that eclogite and the surrounding metapelites have certainly undergone the same metamorphic peak in Eo-alpine time. Comparison of our results with different PT estimates on the eclogite from Pohorje, suggest that a PT gradient from Koralpe to Pohorje is likely.     

Contrasting P-T Paths in the Eastern Himalaya, Nepal: Inverted Isograds in a Paired Metamorphic Mountain Belt

Petrology and phase equilibria of rocks from two profiles inEastern Nepal from the Lesser Himalayan Sequences, across theMain Central Thrust Zone and into the Greater Himalayan Sequencesreveal a Paired Metamorphic Mountain Belt (PMMB) composed oftwo thrust-bound metamorphic terranes of contrasting metamorphicstyle. At the higher structural level, the Greater HimalayanSequences experienced high-T/moderate-P metamorphism, with ananticlockwise P–T path. Low-P inclusion assemblages ofquartz + hercynitic spinel + sillimanite have been overgrownby peak metamorphic garnet + cordierite + sillimanite assemblagesthat equilibrated at 837 ± 59°C and 6·7 ±1·0 kbar. Matrix minerals are overprinted by numerousmetamorphic reaction textures that document isobaric coolingand re-equilibrated samples preserve evidence of cooling to600 ± 45°C at 5·7 ±1·1 kbar.Below the Main Central Thrust, the Lesser Himalayan Sequencesare a continuous (though inverted) Barrovian sequence of high-P/moderate-Tmetamorphic rocks. Metamorphic zones upwards from the loweststructural levels in the south are: Zone A: albite + chlorite + muscovite ± biotite; Zone B: albite + chlorite + muscovite + biotite + garnet; Zone C: albite + muscovite + biotite + garnet ± chlorite; Zone D: oligoclase + muscovite + biotite + garnet ± kyanite; Zone E: oligoclase + muscovite + biotite + garnet + staurolite+ kyanite; Zone F: bytownite + biotite + garnet + K-feldspar + kyanite± muscovite; Zone G: bytownite + biotite + garnet + K-feldspar + sillimanite+ melt ± kyanite. The Lesser Himalayan Sequences show evidence for a clockwiseP–T path. Peak-P conditions from mineral cores average10·0 ± 1·2 kbar and 557 ± 39°C,and peak-metamorphic conditions from rims average 8·8± 1·1 kbar and 609 ± 42°C in ZonesD–F. Matrix assemblages are overprinted by decompressionreaction textures, and in Zones F and G progress into the sillimanitefield. The two terranes were brought into juxtaposition duringformation of sillimanite–biotite ± gedrite foliationseams (S₃) formed at conditions of 674 ± 33°C and5·7 ± 1·1 kbar. The contrasting averagegeothermal gradients and P–T paths of these two metamorphicterranes suggest they make up a PMMB. The upper-plate positionof the Greater Himalayan Sequences produced an anticlockwiseP–T path, with the high average geothermal gradient beingpossibly due to high radiogenic element content in this terrane.In contrast, the lower-plate Lesser Himalayan Sequences weredeeply buried, metamorphosed in a clockwise P–T path anddisplay inverted isograds as a result of progressive ductileoverthrusting of the hot Greater Himalayan Sequences duringprograde metamorphism. KEY WORDS: thermobarometry; P–T paths; Himalaya; metamorphism; inverted isograds; paired metamorphic belts     

Eoalpine tectonics of the Eastern Alps: implications from the evolution of monometamorphic Austroalpine units (Schneeberg and Radenthein Complex)

Monometamorphic metasediments of Paleozoic or Mesozoic age constituting Schneeberg and Radenthein Complex experienced coherent deformation and metamorphism during Late Cretaceous times. Both complexes are part of the Eoalpine high-pressure wedge that formed an intracontinental suture and occur between the polymetamorphosed Ötztal–Bundschuh nappe system on top and the Texel–Millstatt Complex below. During Eoalpine orogeny Schneeberg and Radenthein Complexes were south-dipping and they experienced a common tectonometamorphic history from ca. 115 Ma onwards until unroofing of the Tauern Window in Miocene times. This evolution is subdivided into four distinct tectonometamorphic phases. Deformation stage D1 is characterized by WNW-directed shearing at high temperature conditions (550–600°C) and related to the initial exhumation of the high-pressure wedge. D2 and D3 are largely coaxial and evolved during high- to medium-temperature conditions (ca. 450 to ≥550°C). These stages are related to advanced exhumation and associated with large-scale folding of the high-pressure wedge including the Ötztal-Bundschuh nappe system above and the Texel–Millstatt Complex below. For the area west of the Tauern Window, F2/F3 fold interference results in the formation of large-scale sheath-folds in the frontal part of the nappe stack (formerly called “Schlingentektonik” by previous authors). Earlier thrusts were reactivated during Late Cretaceous normal faulting at the base of the Ötztal–Bundschuh nappe system and its cover. Deformation stage D4 is of Oligo-Miocene age and accounted for tilting of individual basement blocks along large-scale strike-slip shear zones. This tilting phase resulted from indentation of the Southern Alps accompanied by the formation of the Tauern Window.     

Thermobarometric constraints on pressure variations across the Plattengneiss shear zone of the Eastern Alps: implications for exhumation models during Eoalpine subduction

Forward and inverse mineral equilibria modelling of metapelitic rocks in the hangingwall and footwall of the Plattengneiss, a major shear zone in the Eastern Alps, is used to constrain their tectonometamorphic evolution and assess models for their exhumation. Forward (pseudosection) modelling of two metapelitic rocks suggests a steep clockwise P–T path with a near‐isothermal decompression segment from a pressure peak at ~18–19 kbar and 670 °C to the metamorphic peak at 680–720 °C and 11–13 kbar. A subsequent decrease to 600–645 °C and 8–9 kbar is inferred from the late growth of staurolite in some samples. Conventional thermobarometric calculations (inverse modelling) on 18 samples with the inferred peak assemblage garnet + plagioclase + muscovite + biotite + quartz + rutile ± ilmenite ± kyanite are associated with large 2σ uncertainties, and absolute pressures calculated for all samples are statistically indistinguishable. However, calculations constraining relative pressure differences (ΔP) between samples sharing a common mineral assemblage are associated with much smaller uncertainties and yield pressure differences that are statistically meaningful. Although the overall pattern is complicated, the results suggest a pressure gradient of up to 3 kbar across the shear zone that is consistent with volume loss and a model of exhumation related to slab extraction for the Plattengneiss shear zone.     

P-T data of Ky-St-Grt gneisses hosting HP amphibolites and eclogites from the Austroalpine Polinik complex, Kreuzeck Mountains, Eastern Alps, Austria

This study focuses on metapelites of the Polinik complex in the Kreuzeck Mts. southeast of the Tauern Window, Eastern Alps, where kyanite — staurolite — garnet gneisses host eclogites and high pressure (HP) amphibolites of the Austroalpine basement. The stable mineral assemblage is garnet — staurolite — biotite — kyanite — quartz. Estimated metamorphic conditions from conventional geothermobarometry are 654±30 °C and 0.9±0.08 GPa, and Average P-T values calculated by THERMOCALC, are 665±15 °C at 0.77±0.09 GPa. Formation of the present mineral association in gneisses is related to the exhumation (D2) stage of hosted eclogites/HP amphibolites within a lateral strike-slip zone.     

Contrasting serpentinization processes in the eastern Central Alps

Stable isotope compositions have been determined for serpentinites from between Davos (Arosa-Platta nappe, Switzerland) and the Valmalenco (Italy). D and ¹⁸O values (–120 to –60 and 6–10, respectively) in the Arosa-Platta nappe indicate that serpentinization took place on the continent at relatively low temperatures in the presence of limited amounts of metamorphic fluids that contained a component of meteoric water. One sample of chrysotile has a ¹⁸O value of 13 providing evidence of high W/R ratios and low formation temperature of lizardite-chrysotile in this area. In contrast, relatively high D values (–42 to –34) and low ¹⁸O values (4.4–7.4) for serpentine in the eastern part of the Valmalenco suggest a serpentinization process that took place at moderate temperatures in fluids that were dominated by ocean water. The antigorite in the Valmalenco is the first reported example of continental antigorite with an ocean water signature. An amphibole sample from a metasomatically overprinted contact zone to metasediments (D=-36) indicates that the metasomatic event also took place in the presence of ocean water. Lower D values (–93 to –60) of serpentines in the western part of the Valmalenco suggest a different alteration history possibly influenced by fluids associated with contact metamorphism. Low water/rock ratios during regional metamorphism (and metasomatism) have to be assumed for both regions.     

Exhumation of the Tauern window,Eastern Alps

The exhumation of metamorphic domes within orogenic belts is exemplified by the Tauern window in the Eastern Alps. There, the exhumation is related to partitioning of final orogenic shortening into deep-seated thrusts, near-surface antiformal bending forming brachyanticlines, and almost orogen-parallel strike-slip faults due to oblique continental plate collision. Crustal thickening by formation of an antiformal stack within upper to middle crustal portions of the lower lithosphere is a prerequisite of late-stage orogenic window formation. Low-angle normal faults at releasing steps of crustal-scale strike-slip faults accomodate tectonic unloading of synchronously thickened crust and extension along strike of the orogen, forming pull-apart metamorphic domes. Initiation of low-angle normal faults is largely controlled by rock rheology, especially at the brittle-ductile transitional level within the lithosphere. Several mechanisms may contribute to uplift and exhumation of previously buried crust within such a setting: (1) Shortening along deep-seated blind thrusts results in the formation of brachyanticlines and bending of metamorphic isograds; (2) oversteps of strike-slip faults within the wrench zone control the final geometry of the window; (3) unloading by tectonic unroofing and erosional denudation; and (4) vertical extrusion of crustal scale wedges. Rapid decompression of previously buried crust results in nearly isothermal exhumation paths, and enhanced fluid circulation along subvertical tensile fractures (hydrothermal ore and silicate veins) that formed due to overall coaxial stretching of lower plate crust.     

Oceanic mafic rocks in the Eastern Alps

The Eastern Alps in Austria have been interpreted as a pile of thrust sheets resulting from the collision of two continental masses. The only remains of the ocean-floor which may once have separated these continents could be the highly deformed greenschists, metasediments and serpentinites found in the lower thrust sheets. To test this hypothesis, a total of sixty mafic rocks from the Großglockner, Mooserboden, Fusch, Hochtor, Matrei Zone and Strobl localities have been analysed for the stable trace elements, Ti, Zr, Y, Nb and Cr, and the less stable elements K, Rb, and Sr. Visual and statistical comparison of the stable elements with known magma types reveals that five of the sample groups classify clearly as tholeiitic ocean-floor basalts, while one group, the Fusch locality, classifies as within-plate (probably ocean island) basalts. It is suggested that the tectonic units containing such rocks comprise a mélange of disrupted oceanic crust, upper mantle and seamounts, pelagic sediments and continental margin sediments. The rocks may have formed in a large ocean basin, rather than a marginal basin behind an island arc.     

Structure of the lithosphere beneath the Eastern Alps (southern sector of the TRANSALP transect)

The interpretation of the seismic Vibroseis and explosive TRANSALP profiles has examined the upper crustal structures according to the near-surface geological evidences and reconstructions which were extrapolated to depth. Only the southern sector of the TRANSALP transect has been discussed in details, but its relationship with the whole explored chain has been considered as well. The seismic images indicate that pre-collision and deep collision structures of the Alps are not easily recognizable. Conversely, good records of the Neo-Alpine to present architecture were provided by the seismic sections.Two general interpretation models (“Crocodile” and “Extrusion”) have been sketched by the TRANSALP Working Group [2002]. Both illustrate the continental collision producing strong mechanical interaction of the facing European and African margins, as documented by giant lithosphere wedging processes. Arguments consistent with the “Extrusion” model and with the indentation of Adriatic (Southalpine) lithosphere underneath the Tauern Window (TW) are:

– According to the previous DSS reconstructions, the Bouguer anomalies and the Receiver Functions seismological data, the European Moho descends regularly attaining a zone south of the Periadriatic Lineament (PL). The Moho boundary and its geometry appear to be rather convincing from images of the seismic profile;

– the Tauern Window intense uplift and exhumation is coherent with the strong compression regime, which acted at depth, thus originating the upward and lateral displacement of the mobile and ductile Penninic masses according to the “Extrusion” model;

– the indentation of the Penninic mobile masses within the colder and more rigid Adriatic crust cannot be easily sustained. Wedging of the Adriatic stiffened lower crust, under high stresses and into the weaker Penninic domain, can be a more suitable hypothesis. Furthermore, the intrusion of the European Penninic crustal wedge underneath the Dolomites upper crust is not supported by any significant uplifting of the Dolomites. The total average uplift of the Dolomites during the Neogene appears to be 6−7 times smaller than that recognized in the TW. Markedly the northward dip of the PL, reaching a depth of approximately 20 km, is proposed in our interpretation;

– finally, the Adriatic upper crustal evolution points to the late post-collision change in the tectonic grow-up of the Eastern Alps orogenic chain. The tectonic accretion of the northern frontal zone of the Eastern and Central Alps was interrupted from the Late Miocene (Serravallian–Tortonian) onward, as documented by the Molasse basin evolution. On the contrary, the structural nucleation along the S-vergent tectonic belt of the eastern Southern Alps (Montello–Friuli thrust belt) severely continued during the Messinian and the Plio–Pleistocene. This structural evolution can be considered to be consistent with the deep under-thrusting and wedge indentation of the Adriatic lithosphere underneath the southern side of the Eastern Alps thrust-and-fold belt.

Similarly, the significance of the magmatic activity for the construction of the Southern Alps crust and for its mechanical and geological differentiation, which qualified the evolution of the thrust-and-fold belt, is highlighted, starting with the Permian–Triassic magmatism and progressing with the Paleogene occurrences along the Periadriatic Lineament and in the Venetian Magmatic Province (Lessini–Euganei Hills).     

Metamorphism and metallogeny in the Eastern Alps

The two Alpine orogenic phases of the Eastern Alps, in the Cretaceous and in the Tertiary, were both accompanied by the formation of mineral deposits. However, subduction-related magmatic belts as well as the typical “Andean” ore deposits are missing. Therefore, the role of metamorphism in East Alpine metallogeny was tentatively explored for more than 60 y, although for a long time without tangible results. Microthermometric, geochemical and isotopic investigations of fluid inclusions from selected Alpine mineral deposits presented allow a preliminary confirmation of the involvement of metamorphic fluids in their origin. Deposits which were formed immediately after the first, Cretaceous orogeny, were produced at high pressures by fluids of very high salinity and high density, and with an isotopic composition of the water falling into the metamorphic field. These fluids are best understood as products of metamorphic de-volatilization of rocks of the subducted South Pennine domain. In contrast to this, the deposits formed after the second, Tertiary orogeny, originated at relatively low pressures from fluids with an appreciable content of CO₂ and of low to moderate salinities. Isotopic compositions of this carbon indicate a deep crustal or even mantle source for CO₂, while the water is isotopically more heterogeneous and may have mixed sources, both surficial and metamorphic. Tectonic control of these mineralizations is late-orogenic trans-tensional faulting, which exposed hot metamorphic rocks to fluid convection along brittle structures. These deposits conform best to the model of metamorphogenic metallogenesis by retrograde leaching, although ponded metamorphic fluids and mantle volatiles may also have been involved. Received: 4 August 1998 / Accepted: 5 January 1999     

Garnet Sm–Nd data from the Saualpe and the Koralpe (Eastern Alps, Austria): chronological and P–T constraints on the thermal and tectonic history

Garnets from recrystallized, staurolite- and kyanite-bearing mica schists from the central Saualpe basement, representing the host rocks of the type-locality eclogites, give concordant Sm–Nd garnet–whole-rock isochron ages between 88.5±1.7 and 90.9±0.7 Ma. The millimetre-sized, mostly inclusion-free grains show fairly homogeneous element profiles with pyrope contents of 25–27%. Narrow rims with an increase in Fe and Mn and a decrease in Mg document minor local re-equilibration during cooling. According to phengite geothermobarometry, peak metamorphic conditions at 90 Ma were close to 20  kbar and 680  °C and similar to those recorded by the eclogites. The garnet rims record about 575  °C/7  kbar for the final stages of metamorphism. A phengitic garnet–mica schist, sampled at the immediate contact with the Gertrusk eclogite, gave a garnet–whole-rock Sm–Nd age of 94.0±2.7 Ma.
Garnet porphyroclasts separated from a pegmatite–mylonite of the Koralpe plattengneiss near Stainz are unzoned and show spessartine contents of 15%. Composition and Sm–Nd ages of close to 260 Ma point to a magmatic origin for these garnets.
The garnet data from the Saualpe document an intense Alpine metamorphism for this part of the Austroalpine basement. The mica schists recrystallized during decompression and rapid exhumation, at the final stages of and immediately following a high- P event. The Koralpe data show that high Alpine temperatures did not reopen the Sm–Nd isotope system, implying a closure temperature in excess of c . 600  °C for this isotopic system in garnet.     

Rb/Sr geochronology in the Eastern Alps

New Rb/Sr data on mineral and whole rock samples from in and around the south-east corner of the Tauern Window are presented. Pennine orthogeneisses yield an Rb/Sr whole rock age of 279±9 m.y., while orthogneiss samples from the Altkristallin Sheet near Innerkrems, Carinthia, yield an age of 381±30 m.y. by the same technique. The apparent mineral age break across the margins of the Tauern Window is investigated in an area of good structural and petrofabric control. The post-Palaeozoic history of the Eastern Alps is then discussed in the context of the available Rb/Sr data. It is argued that the bulk of the Katschberg Phyllites are of pre-Mesozoic age; that the major overthrusting movements of the Austroalpine Units were completed by 60–65 m.y.; and that the Peri-Adriatic intrusives can be little older than middle Tertiary.     

A Metamorphosed Early Cambrian Crust-Mantle Transition in the Eastern Alps, Austria

In the Speik Complex (Eastern Alps, Austria), highly melt-depleted,metamorphosed harzburgites with abundant pods and layers ofchromitite are interlayered with a suite of metamorphosed orthopyroxenites,clinopyroxenites and gabbros. Coarse-grained orthopyroxenitesoccur as centimetre- to metre-wide veinlets and pods, but alsoas intrusive plugs several tens of metres wide. Intimately associatedmetaclinopyroxenite and metagabbro are present as bodies upto several metres thick at a distinct stratigraphic level withinthe complex. In the ultramafic rocks, relict magmatic olivine,orthopyroxene, clinopyroxene and spinel have been overprintedby a metamorphic assemblage of forsterite, diopside, tremolite,anthophyllite, chlorite, serpentine, talc and Cr–Fe-richspinel. Hornblende, epidote, zoisite and chlorite dominate themetamorphic paragenesis in metagabbros, in addition to rarerelicts of clinopyroxene and two phases of Ca-rich garnet. Thepolymetamorphic evolution of the Speik Complex includes rarelypreserved pre-Variscan (400 Ma) eclogite-facies conditions,Variscan (330 Ma) amphibolite-facies conditions (600–700°C,>5 kbar) and Eoalpine (100 Ma) greenschist- to amphibolite-faciesconditions reaching 550°C and 7–10 kbar. Orthopyroxenitesare characterized by high concentrations of SiO₂, MgO and Cr,and by U-shaped chondrite-normalized rare earth element (REE)patterns similar to those of their harzburgite hosts. The REEpatterns of the clinopyroxenites are flat to slightly enrichedin light REE. Metagabbro compositions are variable, but generallycharacterized by low SiO₂ and high mg-numbers (61–78).Their REE patterns all have Gd_N/Yb_N > 1; some samples havelarge positive Eu anomalies implying the original presence ofcumulus plagioclase. In the orthopyroxenites, clinopyroxenitesand some peridotites, Pt, Pd and Re are distinctly enrichedcompared with Os, Ir and Ru, whereas most harzburgites haveunfractionated to slightly fractionated platinum-group element(PGE) patterns with respect to average upper mantle. The Re–Osisotope compositions of the pyroxenites define an errorchronat 550 ± 17 Ma and a supra-chondritic ¹⁸⁷Os/¹⁸⁸Os of0·179 ± 0·003. An isochron age of 554 ±37 Ma with _Nd(i) +0·7 is indicated by the Sm–Ndisotope compositions of whole-rock pyroxenite and gabbro samples,whereas the harzburgites plot on an errorchron of 745 ±45 Ma and _Nd(i) +6. The pyroxenites and gabbros probably representa cogenetic suite of magmatic dykes intruded into uppermost,highly depleted, suboceanic mantle below the crust–mantletransition zone in an oceanic basin close to the northwesternmargin of Gondwana. KEY WORDS: pyroxenite; metagabbro; geochemistry; Re–Os isotopes; Sm–Nd isotopes     

Petrology and retrograde P-T path in granulites of the Kanskaya formation, Yenisey range, Eastern Siberia

The Kanskaya formation in the Yenisey range, Eastern Siberia is a newly studied example of retrogression of granulite facies rocks. The formation consists of two stratigraphical units: the lower Kuzeevskaya group and the upper Atamanovskaya group. Rocks from both of these units show rare reaction textures such as replacement of cordierite by garnet, sillimanite and quartz, silimanite coronas around spinel and corundum, and garnet rims around plagioclase in metabasites, while plagioclase rims around garnet can be seen in associated metapelites. The paragenesis quartz + orthopyroxene + sillimanite is a feature of the Kuzeevskaya group. In many samples, chemical zoning of garnet and cordierite shows an increase in Mg from core to rim as well as the reverse.
Biotite-garnet-cordierite-sillimanite-quartz as well as spinel±biotite-garnet°Cordierite±sillimanite-quartz assemblages were studied using geothermometers and geobarometers based on both exchange and net-transfer reactions (Perchuk & Lavrent'eva, 1983; Aranovich & Podlesskii, 1983; Gerya & Perchuk, 1989). Detailed investigation of 10 samples including 1000 microprobe analyses revealed decompression (first stage) followed by the near isobaric cooling of the granulites. From geological studies, the 7 km total thickness of the sequence closely corresponds to the pressure difference (∼ 2.2kbar) measured by geobarometers in the samples taken from different levels in the sequence. Individual samples yield P-T paths ranging from 100°C/kbar to 140°C/kbar depending on their locations with respect to the large Tarakskiy granite pluton. In places the 100°C/kbar path changed to the 140°C/kbar due to the influence of the intrusion. In a P-T diagram these trajectories are subparallel lines, whose P-T maxima define the Archaean geotherm between 3.1 and 2.7 Ga, determined isotopically. A petrological model for P-T evolution of the Kanskaya formation is proposed.     

Jurassic strike slip versus subduction in the Eastern Alps

Late Jurassic formations of the Northern Calcareous Alps (NCA) contain ample evidence of synsedimentary tectonics in the form of elongate basins filled with turbidites, debris flows and slumps. Clasts are derived from the Mesozoic of the NCA; they commonly measure tens of metres in diameter and occasionally form kilometre-size bodies. These sedimentologic observations and the presumed evidence of Late Jurassic high-pressure metamorphism recently led to the hypothesis of a south-dipping Jurassic subduction zone with accretionary wedge in the southern parts of the NCA. We present new ⁴⁰Ar/³⁹Ar dates from the location of the postulated high-pressure metamorphism that bracket the age of this crystallization not earlier than 114–120 Ma. The event is therefore part of the well-documented mid-Cretaceous metamorphism of the Austro-alpine domain. Thus, there is currently no evidence of Late Jurassic high-pressure metamorphism to support the subduction hypothesis. The sediment record of the Late Jurassic deformation in the NCA, including the formation of local thrust sheets, is no conclusive evidence for subduction. All these phenomena are perfectly compatible with synsedimentary strike-slip tectonics. Large strike-slip fault zones with restraining and releasing bends and associated flower structures and pull-apart basins are a perfectly viable alternative to the subduction model for the Late Jurassic history of the NCA. However, in contrast to the Eastern Alps transect, where arguments for a Jurassic subduction are missing, a glaucophane bearing Jurassic high-pressure metamorphism in the Meliatic realm of the West Carpathians is well documented. There, the high-pressure/low-temperature slices occur between the Gemeric unit and the Silica nappe system (including the Aggtelek-Rudabanya units), which corresponds in facies with the Juvavic units in the southern part of the NCA. To solve the contrasting palaeogeographic reconstructions we propose that the upper Jurassic left lateral strike-slip system proposed here for the Eastern Alps continued eastwards and caused the eastward displacement of the Silica units into the Meliatic accretionary wedge.     

Balancing lateral orogenic float of the Eastern Alps

Oligocene to Miocene post-collisional shortening between the Adriatic and European plates was compensated by frontal thrusting onto the Molasse foreland basin and by contemporaneous lateral wedging of the Austroalpine upper plate. Balancing of the upper plate shortening by horizontal retrodeformation of lateral escaping and extruding wedges of the Austroalpine lid enables an evaluation of the total post-collisional deformation of the hangingwall plate. Quantification of the north–south shortening and east–west extension of the upper plate is derived from displacement data of major faults that dissect the Austroalpine wedges. Indentation of the South Alpine unit corresponds to 64 km north–south shortening and a minimum of 120 km of east–west extension. Lateral wedging affected the Eastern Alps east of the Giudicarie fault. West of the Giudicarie fault, north–south shortening was compensated by 50 to 80 km of backthrusting in the Lombardian thrust system of the Southern Alps. The main structures that bound the escaping wedges to the north are the Inntal fault system (ca. 50 km sinistral offset), the Königsee–Lammertal–Traunsee (KLT) fault (10 km) and the Salzach–Ennstal–Mariazell–Puchberg (SEMP) fault system (60 km). These faults, as well as a number of minor faults with displacements less than 10 km, root in the basal detachment of the Alps. The thin-skinned nature of lateral escape-related structures north of the SEMP line is documented by industry reflection seismic lines crossing the Northern Calcareous Alps (NCA) and the frontal thrust of the Eastern Alps. Complex triangle zones with passive roof backthrusts of Middle Miocene Molasse sediments formed in front of the laterally escaping wedges of the northern Eastern Alps. The aim of this paper is a semiquantitative reconstruction of the upper plate of the Eastern Alps. Most of the data is published elsewhere.     

The lithospheric density structure of the Eastern Alps

The three-dimensional (3D) lithospheric density structure of the Eastern Alps was investigated by integrating results from reflection seismics, receiver function analyses and tomography. The modelling was carried out with respect to the Bouguer gravity and the geoid undulations and emphasis were laid on the investigations of the importance of deep lithospheric features. Although the influence of inhomogeneities at the lithosphere–asthenosphere boundary on the potential field is not neglectable, they are overprinted by the response of the density contrast at the crust–mantle boundary and intra-crustal density anomalies. The uncertainties in the interpretations are in the same order of magnitude as the gravity field generated by the deep lithosphere.After including the deep lithospheric geometry from the tomographic model it is shown that full isostatic equilibrium is not achieved below the Eastern Alps. However, calculation of the isostatic lithospheric thickness shows two areas of lithospheric thickening along the central axis of the Eastern Alps with a transition zone below the area of the TRANSALP profile. This is in agreement with the tomographic model, which features a change in lithospheric subduction direction.