Variscan vs Permian metamorphism in the Austroalpine units

The central Eastern Alps consist mainly of polymetamorphic crystalline rocks which constitute the major part of the structurally complex Austroalpine nappe edifice. The Austroalpine crystalline basement and the crystalline units of the Southern Alps south of the Periadriatic Lineament were a part of the Apulian crust and formed the southern/western margin of the Alpine Tethys ocean during Mesozoic time. Recently, two important issues concerning the knowledge of the polyphase crystallization and deformation history of this "Altkristallin" have been addressed: 1) It has been shown that the southern/eastern parts of the Austroalpine basement have been subducted to depths of at least 60-70 km. This process was accompanied by intense deformation and an eclogite- to amphibolite-facies metamorphic overprint during eo-Alpine (Cretaceous) convergence-collision (Thöni & Jagoutz, 1993; Miller & Thöni, 1997). 2) It has alo been documented that considerable parts of this crystalline experienced - probably for the first time - intense metamorphism at low pressure conditions during Permian-Triassic lithospheric thinning and initial rifting (Schuster, Scharbert & Abart, 1998). Thus, the aim of the proposed study is to unravel the geotectonic background and the geodynamic processes within the Austroalpine basement in the Late Palaeozoic to Early Mesozoic, i.e. the time span between Variscan decompressional exhumation and cooling and Permian-Triassic lithospheric thinning and related metamorphism (some 350/320 to 250/200 Ma ago). In particular, the P-T conditions and timing of these processes will be analyzed in detail in two N-S sections of the Austroalpine basement using mineral chemical, petrological and multichronological (Sm-Nd, Rb-Sr, Lu-Hf, Ar-Ar) investigations, especially for garnet-bearing assemblages. The two proposed study areas are the Glein-/Stubalpe- Koralpe section (including the Plankogelserie in the S Koralpe and the Saualpe) in the eastern part and the Ötztal- /Ortler-Campo crystalline units in the western part of the central Eastern Alps. The main scientific objective of the proposed project is to provide sufficient reliable PT and age data to unravel the Variscan (collisional) and Permian-Triassic (extensional) tectono-metamorphic processes in space and time, in order to reconstruct the palaeogeography and geodynamics of the Austroalpine basement at a time, when Alpine convergence started (in the Jurassic).

The aim of the project was to constrain the timing and duration of Permian-Triassic pegmatite-formation, and the correlation with metamorphic processes in the meta-sedimentary host rock. New Sm-Nd ages of magmatic garnet in pegmatites range between 2663 Ma and 2764 Ma in the Matsch Unit at the southern margin of the Ötztal- Stubai Complex (South Tyrol, Italy), and between 2542 and 2283 Ma in the Koralpe Complex (Styria, Austria). The latter showed age variations between different samples, ranging from the outcrop to the grain (cm) scale. Major and trace element compositional data were used to interpret these variations: i) long term magmatic crystallisation is indicated by continuous single-phase garnet growth zoning; ii) episodic magma emplacement is ruled out by evidence of continuous crystallisation in a closed system; iii) opening of the Sm-Nd isotopic system in garnet during metamorphic overprint seems not effective, as the magmatic major element composition is preserved. iv) a contribution of a post-magmatic garnet generation may be excluded, since all but one sample lack metamorphic garnet, which differs in composition from magmatic garnet. v) a contribution of submicroscopic inclusions not in isotopic equilibrium with garnet has to be taken into account, as (re)crystallised apatite, xenotime, kyanite inclusions were observed within deformed garnet domains. In summary, compositional data favour a long- term magmatic crystallisation history (covering at least 21 Ma), but re-equilibrated inclusions within garnet used for dating should be taken into account. The conditions of magmatic crystallisation were roughly constrained for the Koralpe Complex to temperatures between 650 and 750 C and pressures below 0.4 GPa (or less than 15 km crustal depth). Metamorphic crystallisation and deformation in the pegmatite host rocks were assigned to processes i) prior to, ii) related with or iii) postdating the magmatic event. For the Koralpe Complex, garnet cores from the Plankogel Micaschist Unit yielded Sm-Nd ages of 2714 Ma, indicating a Permian metamorphic crystallisation prior to overall pegmatite-emplacement. In contrast, major metamorphic and deformational processes in the Matsch Unit were assigned to the Variscan tectono-metamorphic event, based on microstructural data and garnet Sm-Nd ages from metapelites at 31110 Ma or 3254 Ma. Variscan maximum burial was followed by intense mineral recrystallisation and deformation, producing the major foliation, which was then cut by Permian pegmatites. Subsequent mineral reactions indicate a T-increase after Variscan cooling and, thus, represent the first evidence of a Permian tectono-metamorphic event in the Matsch unit. Still, Cretaceous deformation turned out to be more prominent than expected, affecting not only the boundaries but also internal parts of the Matsch Unit. West- directed shear deformation took place at lower amphibolite to lower greenschist-facies conditions and is probably responsible for the current tectonic arrangement of the different units in the S Ötztal-Stubai-Complex.