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Geological Society, London, Special Publications; 2008; v. 298; p. 5-44;
DOI: 10.1144/SP298.2
© 2008 Geological Society of London

Articles

Geodynamic evolution of an Alpine terrane—the Austroalpine basement to the south of the Tauern Window as a part of the Adriatic Plate (eastern Alps)

Bernhard Schulz1, André Steenken2 & Siegfried Siegesmund3

1 Institut für Mineralogie der TU Bergakademie, Brennhausgasse 14, D-09596 Freiberg/Sachsen, Germany (e-mail: Bernhard.Schulz{at}mineral.tu-freiberg.de)
2 Institudo de Geosciêncas Universidade de Saõ Paulo, Rua do Lago 562, Cidade Universitária, 05508–080 Saõ Paulo, Brazil
3 Geowissenschaftliches Zentrum Göttingen (GZG), Goldschmidtstraße 3, D–37077 Göttingen, Germany

The Austroalpine basement underwent a multistage Precambrian to Tertiary evolution. Meta-magmatic rocks occur in pre-Early Ordovician and post-Early Ordovician units. Protolith zircon ages and whole-rock trace element data define two magmatic evolution lines. An older trend with Th/Yb typical of subduction-related metamorphism, started by 590 Ma N-MORB-type and 550–530 Ma volcanic arc basalt-type basic suites which mainly involved depleted mantle sources, and was continued by mainly crustal-source 470–450 Ma acid magmatic suites. A presumably younger evolution by tholeiitic MORB-type and 430 Ma alkaline within-plate basalt-type suites is characterized by an intraplate mantle metasomatism and multicomponent sources. These magmatic trends can be related to a Neoproterozoic to Ordovician active margin and a subsequent Palaeo-Tethys passive margin along the north-Gondwanan periphery. During Variscan collision, the Austroalpine basement underwent multiphase deformation and metamorphism. Early deformation involved non-coaxial shearing with formation of sheath folds and calcsilicategneiss bodies in some regions. Syndeformational clockwise P–T paths in lower basement parts passed high-pressure and high-temperature amphibolite-facies stages and are interpreted by a Devonian to Carboniferous crustal stacking. A post-collisional Permian thermal event is documented by pegmatite intrusions, LP-HT assemblages and monazite ages. Ductile overprinting under greenschist-facies conditions during the Cretaceous is indicated by foliated pegmatites and monazite ages in samples with retrogressed garnet. The emplacement of the Oligocene Rieserferner pluton was controlled by sinistral shear zone deformation along the Defereggen–Antholz–Vals line. Shear zone activity ceased at 15 Ma and was superseded by brittle strike-slip movements along NW and SE trending faults.