Austroalpine Basement Research Articles

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.

Tracing the high pressure stage in the polymetamorphic Texel Complex (Austroalpine basement unit, Eastern Alps): P–T–t–d constraints

Integration of new mineral chemical, geochronological and structural data from the Texel Complex yielded information on (re)crystallization and deformation processes in metapelites, eclogites and tonalitic orthogneisses during eclogite facies metamorphism. Maximum P–T conditions reached 1.2 to 1.4 GPa and 540–620 °C in the Upper Cretaceous. In tonalitic orthogneisses and metapelites, substantial garnet growth took place prior to eclogite facies metamorphism and Sm–Nd data indicate the presence of pre-Cretaceous mineral relics. In contrast, complex garnet-growth and -resorption processes are inferred for eclogites, which produced characteristic atoll microstructures and occurred close to the pressure peak of a single, coherent high pressure event. Garnet Sm–Nd data indicate eclogite facies crystallization at 85 ± 5 Ma. While eclogites retained information on the maximum burial stage, matrix phases in metapelites and orthogneisses were intensely recrystallized during the amphibolite facies metamorphic decompression. All the meso- and macro-scale deformation structures formed during the high pressure event and subsequent exhumation. The major mylonitic foliation is represented by the high pressure phases but was refolded during amphibolite facies exhumation. A biotite-whole-rock Rb–Sr age of 70–80 Ma indicates that cooling below about 300 °C occurred in the Upper Cretaceous.

Characterization of polymetamorphism in the Austroalpine basement east of the Tauern Window using garnet isopleth thermobarometry

AbstractGarnet in metapelites from the Wölz and Rappold Complexes of the Austroalpine basement east of the Tauern Window typically shows two distinct growth zones. A first garnet generation usually forms the cores of garnet porphyroblasts and is separated by a prominent microstructural and chemical discontinuity from a second garnet generation, which forms rims of variable width. Whereas the rims were formed during the Eo‐Alpine metamorphic overprint, the garnet cores represent remnants of at least two pre‐Eo‐Alpine metamorphic events. The pressure and temperature estimates obtained from garnet isopleth thermobarometry applied to the first growth increments of the pre‐Eo‐Alpine garnet cores from the Wölz and Rappold Complexes cluster into two distinct domains: (i) in the Wölz Complex, incipient growth of the first‐generation garnet occurred at 4 ± 0.5 kbar and 535 ± 20 °C, (ii) in the Rappold Complex, incipient growth of the oldest garnet cores took place at 5.3 ± 0.3 kbar and 525 ± 15 °C. The Eo‐Alpine garnet generation started to grow at 6.5 ± 0.5 kbar and 540 ± 10 °C. According to radiometric dating, the low‐pressure garnet from the Wölz complex was formed during a Permian metamorphic event. The first‐generation garnet of the Rappold Complex is probably of Variscan age.

Reaction textures in Al-rich metabasite; implication for metamorphic evolution of the eastern border of the Middle Austroalpine basement units

Al-rich metabasite from the Speik Complex in the Eastern Alps contains coexisting kyanite, staurolite, garnet and calcic amphibole. Compositional zoning and inclusions trails in minerals suggest a prograde metamorphic history to high-pressure amphibolite facies conditions. The formation of kyanite and staurolite was controlled by bulk rock composition, mainly by high Al content in some metabasites. Textural relations and thermodynamic calculations indicate isothermal decompression from 1.1–1.2 GPa/580 °C to 0.5 GPa/580 °C that resulted in formation of Ca-rich plagioclase and new amphibole replacing garnet and locally corundum substituting for clinozoisite and kyanite. The replacement of amphibole by biotite and chlorite and plagioclase by margarite, and the formation of prehnite and pumpellyite were due to isobaric cooling to very low-grade conditions.

Fracture-driven intrusion and upwelling of a mid-crustal pluton fed from a transpressive shear zone--The Rieserferner Pluton (Eastern Alps)

The Rieserferner Pluton was emplaced at a depth of 12–15 km into steeply dipping, greenschist-facies mylonitic rocks of the Austroalpine basement, just south of the Tauern Window (Eastern Alps). Intrusion occurred during north-south–directed shortening and east-west horizontal extension in front of the rigid Southern Alpine Indenter. The regional strain field is transpressive, with a strong coaxial component, and is characterized by east-west stretching. Tonalitic melt ascended through a feeder channel preserved in the steep southern part of the Rieserferner Pluton, within and adjacent to the steep mylonitic foliation of a major shear zone, the Defereggen-Antholz-Vals (DAV) Line. Melt ascent was perpendicular to the north-south shortening direction in the country rocks. Magma emplacement involved melt-induced hydro-fracturing to form a subhorizontal, tabular pluton that protruded northward from the DAV Line into the previously folded country rocks. During the late stage of emplacement, buoyant upwelling of the partly recrystallized magma induced doming of the pluton roof as well as vertical ductile shortening of the directly overlying country rocks. Magma pressure therefore locally exceeded the lithostatic pressure. Thermal modeling constrains the maximum time for doming and solidification of the pluton to have been 32,000 yr. The wavelength of the domes in the pluton roof indicates the viscosity contrast between country rock and partly crystallized tonalite in the pluton to have been at least 100:1.

Zircons in rocks of the Austroalpine and Southalpine complexes of the Eastern Alps: differences and analogies

Zircon crystals have been separated from gneisses, micaschists and phyllites of the Austroalpine and Southalpine tectonic Units of the Eastern Alps (16 and 7 samples respectively). The studied zircon crystals from the Austroalpine Unit are preferentially euhedral and show oscillatory zoning, suggesting a magmatic origin. Those from the Southalpine Unit are detrital grains of magmatic origin and show pitted surfaces and rounded edges, suggesting a long transportation during the sedimentary cycle. Electron microprobe analyses (438) for major (Si, Zr, Hf, P), minor (Y), trace elements (Th, U) and HREE (Gd, Dy, Er, Yb) have been carried out on selected zircons. Using long counting times, determination of REE contents as low as 50 ppm is possible. The zircon crystals are relatively homogeneous in composition except for the HfO2 content, which is strongly enriched in the rims of zircons from the Austroalpine basement. The HfO2 content is generally lower in zircons from the Southalpine basement. Xenotime-type substitution (Y, REE)3++P5+=Zr4++Si4+ is common, but also other (non-xenotime-type) substitutions has been observed.

Ultrahigh‐pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps

AbstractNew evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.

Host rock compositional controls on zircon trace element signatures in metabasites from the Austroalpine basement

Zircon populations of Neoproterozoic and early Paleozoic age occur in metabasites of a high-pressure amphibolite-facies unit of the Austroalpine basement south of the Tauern Window. The host rocks for these zircons are eclogitic amphibolites of N-MORB-type character, hornblende gneisses with volcanic-arc basalt signature, and alkaline within-plate-basalt amphibolites. Bulk rock magmatic trace element patterns were preserved during amphibolite-facies high-pressure and subsequent high-temperature events, as well as a greenschist-facies overprint. Positive Ce and negative Eu anomalies and enrichment of HREE in normalized zircon REE patterns, as analysed by LA-ICP-MS, are typical for an igneous origin of these zircon suites. Zircon Y is well correlated to HREE, Ce, Th, U, Nb, and Ta and allows discrimination of compositional fields for each host rock type. Low Th/U ratios are correlated to low Y and HREE abundances in zircon from low bulk Th/U host rocks. This is likely a primary igneous characteristic that cannot be attributed to metamorphic recrystallization. Variations of zircon/host rock element ratios confirm that ionic radii and charges control abundances of many trace elements in zircon. The trace element ratios—presented as mineral/melt distribution coefficients—indicate a selectively inhibited substitution of Zr and Si by HREE and Y in zircon which crystallized from a N-MORB melt. Correlated host rock and zircon trace element concentrations indicate that the metabasite zircons are not xenocrysts but crystallized from mafic melts, represented by the actual host rocks.

Refining the timing of eclogite metamorphism: a geochemical, petrological, Sm-Nd and U-Pb case study from the Pohorje Mountains, Slovenia (Eastern Alps)

High-pressure metamorphism in the Pohorje Mountains of Slovenia (Austroalpine unit, Eastern Alps) affected N-MORB type metabasic and metapelitic lithologies. Thermodynamic calculations and equilibrium phase diagrams of kyanite–phengite-bearing eclogites reveal PT conditions of >2.1 GPa at T<750°C, but within the stability field of quartz. Metapelitic eclogite country rocks contain the assemblage garnet + phengite + kyanite + quartz, for which calculated peak pressure conditions are in good agreement with results obtained from eclogite samples. The eclogites contain a single population of spherical zircon with a low Th/U ratio. Combined constraints on the age of metamorphism come from U/Pb zircon as well as garnet–whole rock and mineral–mineral Sm-Nd analyses from eclogites. A coherent cluster of single zircon analyses yields a 206Pb/238U age of 90.7±1.0 Ma that is in good agreement with results from Sm-Nd garnet–whole rock regression of 90.7±3.9 and 90.1±2.0 Ma (eNd: +8) for two eclogite samples. The agreement between U-Pb and Sm-Nd age data strongly suggests an age of approximately 90 Ma for the pressure peak of the eclogites in the Pohorje Mountains. The presence of garnet, omphacite and quartz inclusions in unfractured zircon indicates high-pressure rather than ultrahigh pressure conditions. The analysed metapelite sample yields a Sm-Nd garnet–whole rock scatterchron age of 97±15 Ma. These data probably support a single P-T loop for mafic and pelitic lithologies of the Pohorje area and a late Cretaceous high-pressure event that affected the entire easternmost Austroalpine basement including the Koralpe and Saualpe eclogite type locality in the course of the complex collision of the Apulian microplate and Europe.

Argon and fission track dating of Alpine metamorphism and basement exhumation in the Sopron Mts. (Eastern Alps, Hungary): thermochronology or mineral growth?

The crystalline basement rocks of the Sopron Mountains are the easternmost and most isolated outcrops of the Austroalpine basement of the Eastern Alps. Ar/Ar and K/Ar dating of phengitic mica indicates that the Eoalpine high-pressure metamorphism of the area occurred between 76 and 71 Ma. Short-lived metamorphism is characterised by fluid-poor conditions. Fluid circulation was mostly restricted to shear zones, thus the degree of Alpine overprint has an extreme spatial variation. In several metamorphic slices Variscan mineral assemblages have been preserved and biotite yielded Variscan and Permo-Triassic Ar ages. Different mineral and isotope thermometers (literature data) yielded temperatures of 500–600 °C for the peak of Alpine metamorphism in the Sopron Mountains, but muscovite and biotite do not show complete argon resetting. Thus, we consider this crystalline area as a well constrained natural test site, which either indicates considerably high closure temperatures (around 550 °C) for Ar in muscovite and biotite in a dry metamorphic environment, or which is suitable for testing the widely applied methods of temperature estimations under disequilibrium conditions. Apatite fission track results and their thermal modeling, together with structural, mineralogical and sedimentological observations, allows the identification of a post-metamorphic, Eocene hydrothermal event and Late Miocene-Pliocene sediment burial of the crystalline rocks of the Sopron Mountains.

ULTRAHIGH-PRESSURE METAMORPHISM OF GARNET PERIDOTITES FROM POHORJE MTS. (EASTERN ALPS, SLOVENIA)

New evidence for ultrahigh-pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet peridotites of Pohorje Mts. in Slovenia. In this area, an eo- Alpine UHPM has been recently documented in the eclogites (Janak et al. 2004). These eclogites are closely associated with metaultrabasites - predominantly serpentinised dunite and harzburgite with garnet peridotite remnants. The country rocks of eclogites and metaultrabasites are amphibolites, orthogneisses, paragneisses and micaschists. All these rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. At least four stages of recrystallization have been identified in the garnet peridotite based on an analysis of reaction textures and mineral compositions. Stage I is a high-temperature protolith assemblage of olivine + orthopyroxene + clinopyroxene + Cr-spinel. Aluminous pyroxenes occur as inclusions in garnet, chromian spinel is preserved in the matrix. Stage II – an ultrahigh-pressure stage is defined by matrix assemblage garnet + olivine + orthopyroxene + clinopyroxene + Cr-spinel. Garnet contains up to 67 mol% of pyrope, olivine has 90 mol% of forsterite, orthopyroxene is low in Al2O3 (~0.8 wt%) and spinel has a Cr* ~ 50. Stage III – a decompression stage is manifested by formation of kelyphitic rims of high-Al orthopyroxene, aluminous spinel and pargasitic hornblende replacing garnet. Due to retrogression, garnet shows a decrease in MgO. Stage IV – is represented by formation of tremolitic amphibole, chlorite, serpentine and talc. P-T estimates based on geothermobarometric calculatios a) Fe-Mg exchange between garnet, olivine and orthopyroxene thermometers, b) the Al-in-orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at ~820-900oC and 3-3.5 GPa. This is consistent with previous estimation of very high P-T conditions in metaultrabasites by Hinterlechner-Ravnik et al. (1991) and the associated eclogites (Janak et al. 2004). These results suggest that the mantle fragment (garnet peridotite) and the crustal fragment (eclogite) in the Pohorje Mts. both experienced a common UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of a continental slab which incorporated peridotites from an overlying mantle wedge.

Neoproterozoic to Early-Palaeozoic magmatic evolution in the Gondwana-derived Austroalpine basement to the south of the Tauern Window (Eastern Alps)

In the Austroalpine Basement to the south of the Tauern Window, distinct suites of metabasites occur with orthogneisses in pre-Early-Ordovician units. Tholeiitic and alkaline within-plate basalt-type metabasites are associated with acid meta-porphyroids in the post-Early-Ordovician Thurntaler Phyllite Group. According to their correlated trace element abundances, metabasite zircons crystallized with their host rocks. Protolith Pb–Pb zircon ages, whole-rock Ta/Yb–Th/Yb and oxygen, Sr, Nd isotope data define two principal evolution lines. An older evolution at elevated Th/Yb typical of subduction-related magmatism, started by 590-Ma N-MORB-type and 550–530 Ma volcanic arc basalt type basic suites, which mainly involved depleted mantle sources. It finished with mainly crustal-source 470–450-Ma acid magmatites. An other evolution line by tholeiitic and 430-Ma alkaline within-plate basalt-type suites in both pre- and post-Early-Ordovician units is characterized by an intraplate mantle metasomatism and enrichment trend along multicomponent sources. These magmatic evolution lines can be related to a plate tectonic scenario that involved terranes in a progressively mature Neoproterozoic to Ordovician active margin, and a subsequent Palaeo-Tethys passive margin along the north Gondwanan periphery.

Oligocene trondhjemitic dikes in the Austroalpine basement of the Pfunderer Berge, Sudtirol - level of emplacement and metamorphic overprint

Trondhjemitic to tonalitic porphyric dikes in the Austroalpine basement of the Pfunderer Berge, Sudtirol, are genetically related to the adjacent Oligocene intrusions, the Rieserferner pluton to the east, and the Rensen pluton to the west, both dated at about 30 Ma. Their phase assemblage, mineral composition, and microstructure are investigated to gain insight into the level of emplacement, and the conditions and time scales of metamorphic overprint and deformation. For samples with uniform whole rock chemistry, the phenocryst assemblage and microstructure of the dikes change from east to west. In the east, the dikes contain phenocrysts of igneous garnet, but no muscovite. In the west, the dikes contain phenocrysts of muscovite, but no igneous garnet, while fine-grained post-magmatic garnet has formed at subsolidus conditions. The igneous garnet in the eastern dikes crystallised at temperatures of about 800 ± 50°C, as indicated by biotite-garnet exchange thermometry. The eastern dikes underwent only slight subsolidus deformation at temperatures near 300°C and the original magmatic microstructure is largely preserved. In contrast, the western dikes are foliated and show intense subsolidus deformation of quartz in the dislocation creep regime. The composition of the garnet-phengite-plagioclase-biotite assemblages of the fine-grained matrix indicates crystallisation at temperatures of about 500 to 600°C and pressures of about 0.5 to 0.6 GPa, which corresponds to the minimum pressure required to stabilise the igneous muscovite at about 700°C. Most likely crystallisation time scales in magma chambers on the order of a few hundred years are obtained from kinetic modelling, with dike emplacement being possibly triggered by a magma mixing event. In contrast, subsolidus cooling of some dikes emplaced at deeper levels spanned millions of years. The results show that the eastern dikes were emplaced at a shallower crustal level and within a cooler country rock, compared to the western dikes - the amount of uplift and erosion since the contemporaneous intrusion at about 30 Ma increasing from about 10 km in the Pfunderer Berge to about 15 to 25 km in the Rensen area in the west.

Geochemistry and metamorphic evolution of the Pohorje Mountain eclogites from the easternmost Austroalpine basement of the Eastern Alps (Northern Slovenia)

Kyanite-rich and quartz-rich eclogites occur as lenses within amphibolite-facies quartzo-feldspathic gneisses in the Pohorje Mountains, Northern Slovenia, that form the easternmost Austroalpine basement. Major and trace elements indicate that the kyanite-rich eclogites were derived from plagioclase-rich gabbroic cumulates, whereas the quartz-rich eclogites represent more fractionated basaltic compositions. Both varieties are characterized by a LREE-depleted N-MORB type REE signature. Geothermobarometry and P– T pseudosections indicate that eclogites equilibrated at 1.8–2.5 GPa and 630–700 °C, consistently with the lack of coesite and with equilibration conditions of the chemically similar eclogites from the adjacent basement units at Koralpe and Saualpe type localities. Decompression reaction textures include (i) clinopyroxene–plagioclase intergrowths after omphacite, (ii) replacement of kyanite by corundum–plagioclase–spinel±sapphirine symplectites, (iii) breakdown of phengite to biotite–plagioclase sapphirine symplectites. The results of this study indicate that Koralpe, Saualpe and Pohorje high-pressure rocks represent former MORB-type oceanic crust that was subducted in the course of the late Cretaceous (approximately 100 Ma ago) collision between the European and the Apulian plates.

Ordovician meta-pegmatite garnet (N-W Ötztal basement, Tyrol, Eastern Alps): preservation of magmatic garnet chemistry and Sm–Nd age during mylonitization

Meta-pegmatites and meta-pegmatite-hosted garnet from three localities in the western/northern Ötztal basement, Tyrol (Eastern Alps), have been analysed for major and selected trace element chemistry and for Sm–Nd isotope systematics. The garnets have frequently retained a euhedral shape, even though they are now set in a strongly foliated matrix consisting of dynamically recrystallized quartz and feldspar. They are characterized by high Fe and Mn contents (up to 29 mol% spessartine) and a simple major element zoning with decreasing MnO and increasing FeO from core towards the rim, suggesting a single-phase crystallization and primary, magmatic chemistry. Chondrite-normalized REE patterns display a strong enrichment of HREE relative to LREE, and pronounced negative Eu anomalies. Based on ID-TIMS analysis, neodymium concentration in all but one of the optically clean garnet separates varies between 0.12 and 5.5 ppm, at concomitant high Sm/Nd ratios (1.2–4.9). LA-ICP-MS analysis revealed very strong within-grain variation both for Nd concentration (<0.1–6.3 ppm) and Sm/Nd ratio (<1–27.4). Garnet-whole rock or garnet-feldspar Sm–Nd isochron ages range between 445±3 and 473±3 Ma, with a distinct concentration of the age data in the 455–462 Ma time interval. These ages indicate pegmatite formation in different areas of the Ötztal basement during Middle to Late Ordovician time, related to the widespread generation of crustal melts towards the end of the long-lived Early Palaeozoic magmatic event. The presence of granites and associated pegmatites is a distinctive feature mainly of the north-western parts of the Austroalpine basement in the Eastern Alps and typical for the so-called “Caledonian” era.

Seismic image of the deep crust at the eastern margin of the Alps (Austria): indications for crustal extension in a convergent orogen

A 39-km-long deep seismic reflection profile recorded during two field campaigns in 1996 and 2002 provides a first detailed image of the deep crust at the eastern margin of the Eastern Alps (Austria). The ESE–WNW-trending, low-fold seismic line crosses Austroalpine basement units and extends approximately from 20 km west of the Penninic window group of Rechnitz to 60 km SSE of the Alpine thrust front. The explosive-source seismic data reveals a transparent shallow crust down to 5 km depth, a complexly reflective upper crust and a highly reflective lowermost crust. The upper crust is dominated by three prominent west-dipping packages of high-amplitude subparallel reflections. The upper two of these prominent packages commence at the eastern end of the profile at about 5 and 10 km depth and are interpreted as low-angle normal shear zones related to the Miocene exhumation of the Rechnitz metamorphic core complex. In the western portion of the upper crust, east-dipping and less significant reflections prevail. The lowermost package of these reflections is suggested to represent the overall top of the European crystalline basement. Along the western portion of the line, the lower crust is characterised by a 6–8-km-thick band of high-amplitude reflection lamellae, typically observed in extensional provinces. The Moho can be clearly defined at the base of this band, at approximately 32.5 km depth. Due to insufficient signal penetration, outstanding reflections are missing in the central and eastern portion of the lower crust. We speculate that the result of accompanying gravity measurements and lower crustal sporadic reflections can be interpreted as an indication for a shallower Moho in the east, preferable at about 30.5 km depth. The high reflectivity of the lowermost part of the lower crust and prominent reflection packages in the upper crust, the latter interpreted to represent broad extensional mylonite zones, emphasises the latest extensional processes in accordance with eastward extrusion.

Tectonometamorphic Evolution of the Austroalpine Nappe Complex in the Central Eastern Alps—Consequences for the Eo-Alpine Evolution of the Eastern Alps

In the east-central part of the Eastern Alps, three major deformation events can be distinguished within the Koralm Complex and adjacent units (Plankogel Complex, Gleinalm Complex, Seckau Crystalline Complex, Paleozoic of Graz). A first deformation event D1 is characterized by the formation of a penetrative foliation and an E-W stretching lineation. Remnants of deformational micro-structures indicate a top-to-the-west sense of shear during this deformation event. Most of the D1-related fabrics were overprinted by subsequent metamorphism. This metamorphic event did affect the presumed tectonic boundary between the Koralm Complex and the Gleinalm Complex below. Particularly, D2 is related to the Plattengneis shear zone, which formed in the uppermost structural sections of the Koralm Complex, characterized by a N-S-oriented stretching lineation. Eclogites in the footwall have been affected by this deformation event, too. This deformation event is associated with pure shear in the central parts of the Koralm Complex, probably with top-to-the-south displacement in the southern parts, and top-to-the-north displacement in the northern parts. Deformation within the Plattengneis and the eclogites below occurred along the decompressional path, indicated by decreasing minimum pressures within the eclogites, and by northward and southward decreasing pressures and temperatures. The Plattengneis shear zone continuously passes into a low-angle normal fault in the northeastern part of the Koralm, forming the contact between the Koralm Complex and the Paleozoic of Graz. Thus, the Plattengneis shear zone primarily formed as an extensional structure and triggered exhumation of the eclogites. D3-related structures are restricted to distinct low-angle normal shear zones along the northern and southern margins of the Koralm Complex, with top-to-the-N/NE and top-to-the-S/SE displacement, respectively. These are related to the juxtaposition of exhumed high-pressure rocks of the Koralm Complex, and medium- to low-grade metamorphic units above. According to this evolution, the Cretaceous collisional process (Eo-Alpine cycle), which formed the present Austroalpine Nappe Complex, may be subdivided into two distinct phases: The first phase is the (ES)E-ward subduction and closure of the Hallstatt-Meliata Basin, resulting in the assembly of the Upper Austroalpine Nappe Complex. After closing of the Hallstatt-Meliata oceanic basin during the Late Jurassic, the Cretaceous orogeny in the Eastern Alps encompasses the collision between (south)easternmost parts of the Austroalpine continental crust and a continental fragment to the east. The second phase involves the southward underplating of the southern Apulian continental margin and resulted in the imbrication of the Middle Austroalpine basement complex. These units were additionally affected by pronounced metamorphim, increasing from greenschist-facies conditions in the northern parts to amphibolite- and eclogite-facies conditions in its southern-most parts. Continuous underplating was accompanied by extension in the internal parts of the orogen, resulting in the formation of an extensional detachment in the lower crust (Plattengneis shear zone), and exhumation of high-pressure metamorphic rocks during the Late Cretaceous. Coeval extension in the upper plate resulted in the formation of the Gosau sedimentary basins. Toward the north, the Plattengneis shear zone continuously climbed toward shallower crustal levels, and passed into a foreland-directed thrust. This thrust is conjectured to have affected the Upper Austroalpine Nappe Complex as well as the formation of distinct out-of-sequence thrusts.

Kinematic reworking and exhumation within the convergent Alpine Orogen

Kinematic data from the internal zones of the Western Alps indicate both top-to-SE and top-to-NW shearing during synkinematic greenschist facies recrystallisation. Rb/Sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 °C. The data indicate an initial phase of accretion and foreland-directed thrusting at ca. 60 Ma followed by almost complete reworking of thrust-related deformation by SE-directed shearing. This deformation is localised within oceanic units of the Combin Zone and the base of the overlying Austroalpine basement, and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike. The timing of deformation in this shear zone spans 9 Ma from 45 to 36 Ma. The SE-directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy, and such structures have been interpreted in the past as backthrusts in response to ongoing Alpine convergence. However, on a regional scale, the top-to-SE deformation is related to crustal extension, not shortening, and is coincident with exhumation of eclogites in its footwall. During this extension phase, deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking. Top-NW kinematics preserved within the Combin Zone show a range of ages. The oldest (48 Ma) may reflect the final stages of emplacement of Austroalpine Units above Piemonte oceanic rocks prior to the onset of extension. However, much of the top-to-NW deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone. 40Ar/ 39Ar data from the region are complicated due to the widespread occurrence of excess 40Ar in eclogite facies micas and partial Ar loss during Alpine heating. Reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 Ma. These ages are slightly younger than Rb/Sr deformation ages and suggest that cooling below ca. 350 °C occurred after juxtaposition of the units by SE-directed extensional deformation. Our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the Alpine Orogen. To constrain the palaeogeographic and geodynamic evolution of the Alps requires that these data be integrated with data from the more external zones of the orogen. Complexity such as that described is unlikely to be restricted to the Western Alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens. Recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens.

P-T gradient of Eo-Alpine metamorphism within the Austroalpine basement units east of the Tauern Window (Austria)

¶A petrological study has been undertaken in the Austroalpine basement units, along a SSE-NNW profile across the northern parts of the Saualpe Complex in the Seetal Alps, the Micaschist-Marble Complex (Rappold and Wolz Units) in the Wolz Tauern and the Speik Complex with Permian cover sequences at Hochgrosen Massif in the Rottenmann Tauern. With exception of the Permian cover rocks all units experienced Pre-Alpine as well as Eo-Alpine metamorphism. Relicts of early Variscan eclogite facies metamorphism (397 Ma) are preserved in the Speik Complex in the Hochgrosen Massif. Different compositional zoning and inclusion patterns in garnet porphyroblasts from the Rappold and Wolz Units allow to distinguish two Pre-Alpine metamorphic events: a Permian (∼ 250 Ma) low-P/high-T and an Upper Carboniferous (330 Ma?) medium-pressure metamorphism of amphibolite facies conditions.

Petrogenesis and geotectonic setting of ultramafic rocks in the Eastern Alps: constraints from geochemistry

In the Eastern Alps, a heterogeneous pile of Pre-Mesozoic and Mesozoic nappes was emplaced during several distinct orogenic events. Ultramafic rocks are preserved in: (1) Pre-Mesozoic basement units of the Penninic Tauern Window and in Austroalpine basement complexes, and (2) in Mesozoic units within Penninic windows and in overlying Austroalpine segments. Geochemical data (major, trace and rare earth elements, REE, of bulk rock samples) of ultramafic rocks from 18 regions covering all the major tectonic units are presented. Metamorphic olivine-rich and clinopyroxene-rich ultramafic rocks and serpentinites from the Cambro-Ordovician Lower Schist cover units of the Tauern Window (Stubach Group, “Greiner Series”) are grouped into peridotites (#Mg values=molecular 100×Mg/(Mg+Fe tot), 87–92; <2 wt.% Al 2O 3) and clinopyroxene-rich pyroxenites (#Mg=80–92; 1–3 wt.% Al 2O 3). Major and trace element characteristics indicate a residual origin of most peridotites as restites after 10–20% partial melting. Most probably, the pyroxenites are crystal segregates from a basaltic melt in a mantle transition zone. Small lenses of completely serpentinised ultramafic rocks in the Cambrian Habach Group (#Mg=90–92) also have residual character, although they are strongly metasomatised. The Stubach and Habach Groups developed in a back arc and volcanic arc setting, respectively, along the northern margin of Gondwana. Metamorphic harzburgite and dunite (#Mg=89–90; 0.5–2.4 wt.% Al 2O 3) veined by metagabbroic dikes were sampled in the Austroalpine Silvretta nappe (Hochnörderer). The harzburgites represent restites after 12% partial melting in suboceanic mantle, most probably during the Cambrian. In the Austroalpine basement east of the Tauern Window, metamorphosed ultramafic rocks occur in association with amphibolite and eclogite in the Proterozoic (about 750 Ma) Speik Complex. Harzburgite and dunite (#Mg=89–92; 0.1–0.8 wt.% Al 2O 3) are highly depleted in incompatible elements (2.6–35 ppb Yb) and are interlayered with small amounts of more fertile peridotite (amphibole-rich harzburgite and lherzolite, #Mg=89; up to 3.0 wt.% Al 2O 3), metaclinopyroxenite (#Mg∼85) and hornblendite. Coarse-grained orthopyroxenite (#Mg=89–92; 0.5–0.7 wt.% Al 2O 3; 19–36 ppb Yb) is present as veins and stocks in the harzburgite–dunite sequence. Chondrite-normalised rare earth element patterns of harzburgite–dunite and orthopyroxenite are characteristically U-shaped. The geochemical data indicate that harzburgites and some dunites are products of multiple melting events. They can be modelled as restites after 20–30% partial melting from already depleted mantle in a suprasubduction zone geotectonic setting. Orthopyroxenites crystallised from high-(Si, Mg) melts with a significant crustal component, probably during a Cambrian subduction event. In Mesozoic units of the Penninic windows (Lower Engadine, Tauern and Rechnitz windows), serpentinised ultramafic rocks of harzburgitic composition (#Mg=90–92; 1.1–2.0 wt.% Al 2O 3) are associated with metagabbro, metabasalt, radiolarite and ophicarbonate. Furthermore, serpentinised lherzolites (#Mg=88–90; 2.9–4.6 wt.% Al 2O 3) are exposed in the Matrei Zone (a tectonic mélange zone between Penninic and Austroalpine nappes), in the Lower Austroalpine Reckner Complex and in a body of unclear tectonic affiliation at the southern margin of the Lower Engadine Window at Nauders. The harzburgites are residual mantle left behind after 10–15% partial melting of a fertile source, and form part of the Mesozoic (Jurassic) incomplete ophiolite sequences of the Ligurian–Piemontais ocean. The lherzolites, having experienced less than 10% partial melting, are either attributable to an early (Permian) rifting episode during break-up of Pangaea, or represent subcontinental mantle from the Adria plate tectonically incorporated into the Neo-Tethys.