Adriatic Plate (eastern Alps) basement to the south of the Tauern Window as a part of the Geodynamic evolution of an Alpine terranethe Austroalpine Geological Society, London, Special Publications

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-MORBtype 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 withinplate 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 Shear zone activity ceased at 15 Ma and was superseded by brittle strike-slip movements along NW and SE trending faults. The Alpine orogen resulted from the closure of the Penninic ocean and a subsequent multiphase collision of the southern Mediterranean microplates with the northern European Plate during Cretaceous to Oligocene times. During this collision, parts of the Adriatic microplate were thrusted in giant nappes upon the Penninic units exposed in the Tauern Window (Schmid et al. 2004). The Austroalpine basement complex in the Eastern Alps (Fig. 1) represents a major part of the Adriatic crust. Its erosional exposure to the south of the Tauern Window allows an insight into the complex Alpine (Cretaceous to Tertiary) and preMesozoic magmatic and metamorphic evolution of the Adriatic–Apulian Plate. The remnants of a pre-Mesozoic evolution turned out to be especially well preserved in this part of the Austroalpine basement, regardless of a more or less intense Alpine overprint. The Austroalpine basement represents one of the Intra-Alpine or Proto-Alpine terranes in the southern part of the European Variscan belt (Schatz et al. 2002). This is a huge collisional orogen assembled by crustal segments that were parts of the former microcontinents Avalonia, Cadomia, Armorica and the Intra-Alpine terranes. Since the Neoproterozoic, these microcontinents and terranes split off from the North-Gondwanan margin at various times, were moved in northern directions and successively collided with the southern margin of Laurussia (Stampfli 1996; von Raumer 1998; Stampfli & Borel 2002; Schatz et al. 2002; Stampfli et al. 2002; von Raumer et al. 2002, 2003, 2006). Rocks of the Austroalpine basement are increasingly important for the detailed reconstruction of the pre-Variscan evolution (Frisch et al. 1984; Frisch & Neubauer 1989; von Raumer & Neubauer 1993; Neubauer 2002a), as From: SIEGESMUND, S., FUGENSCHUH, B. & FROITZHEIM, N. (eds) Tectonic Aspects of the Alpine-DinarideCarpathian System. Geological Society, London, Special Publications, 298, 5–44. DOI: 10.1144/SP298.2 0305-8719/08/$15.00 # The Geological Society of London 2008. geochemical and isotopic characteristics of pre-Carboniferous magmatic rocks and sediments can be recognized despite a considerable Variscan and Alpine overprint. The Austroalpine basement south of the Tauern Window involves the presumably post-Early Ordovician Thurntaler Phyllite Group, and lithologies appearing in several preEarly Ordovician basement units (Schulz et al. 1993, 2001, 2004). They can be related to the Noric Composite terrane and the Celtic terrane, as established by Frisch & Neubauer (1989) in their listing of the Austroalpine pre-Alpine terranes. Here we present insights to the complex geodynamics of a part of the Adriatic–Apulian Plate. The pre-Variscan evolution is illuminated by whole-rock geochemistry and zircon dating of meta-magmatic rocks and then interpreted within a frame of peri-Gondwanan plate tectonics. A time schedule of the Variscan collisional event and its Alpine overprint is detailed by structural Fig. 1. Lithological units in the Austroalpine basement to the south of the central Tauern Window, Eastern Alps. Sampling locations of zircon dating and cross-sections in Figure 2 are marked. A, Antholz/Anterselva muscovite orthogneiss; BQ, Brixen Quartzphyllite; C, Casies/Gsies biotite orthogneiss; CA, Carnic Alps; CNSg, Croda Nera Subgroup (metabasites); CT, Campo Tures orthogneiss; DAV, Defereggen-Antholz-Vals line; DG, Defereggen Group (monotonous metapsammopelites); DMG, Durreck Muscoviteschist Group; EW, Engadine Window; H, Hochgrabe biotite orthogneiss; IQ, Innsbrucker Quartzphyllite; K, Kristeinertal biotite orthogneiss; KC, Koralpen Crystalline; KV, Kalkstein-Vallarga line; MSg, Michelbach Subgroup (amphibolites); MZ, Matreier Zone (Penninic); NDF, Northern Drauzug fault; NDPG, Northern-Defereggen-Petzeck Group; NCA, Northern Calcareous Alps; OB, Oetztal-Stubai Basement; P, Penninic Upper Schieferhuelle; PG, Palaeozoic of Graz; PGN, Palaeozoic of Gurktal Nappes; PGZ, Palaeozoic of Greywacke Zone; PL, Periadriatic Lineament, Pustertal line; PSg, Prijakt Subgroup (eclogitic amphibolites and hornblende-gneisses); R, Rieserferner tonalite; RSg, Rotenkogel Subgroup (hornblende-gneisses, orthogneisses); SA, Southern Alps; SAM, southern limit of Alpine metamorphism (Hoinkes et al. 1999); SC, Saualpen Crystalline; SG, Schobergruppe basement; T, Permo-Trias and Trias; TPG, Thurntaler Phyllite Group; TSg, Torkogel Subgroup (amphibolites); Z, Zinsnock tonalite. B. SCHULZ ET AL. 6