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Reconstructing Pre-Alpine Europe: The Signifi cance of the Palaeozoic Granitic Rocks

Variably deformed granitoid rocks constitute a large part of the remnant Palaeozoic crust preserved in the Alpine orogenic system. These granitoids carry important information regarding the geological history and structure of the pre-Alpine Europe (VON RAUMER, 1998, VON RAUMER et al., 2003). Based on formation ages and geochemical data, and by comparison with the situation in the (extra-Alpine) Variscides, distinct granite forming events and magmatic provinces can be defined, which in turn represent distinct geological/tectonic elements within the pre-Alpine crustal architecture: I-type granitoids with ages of ca. 600-520 Ma represent remnants of the arc-type crust of the Avalonian-Cadomian Pacific-type orogenic belt (e.g. the older orthogneisses in the Silvretta crystalline complex - SCHALTEGGER et al., 1997). Widespread granitic magmatism (dominantly S-type, but also I-type) characterises the Ordovician - Silurian time span and is generally interpreted in terms of (post-collisional, plume-induced or back-arc) crustal extension related to the Early Palaeozoic fragmentation of the Avalonian-Cadomian orogen at the northern Gondwana margin (e.g. granitoid rocks of the Ötztal crystalline basement - SCHINDLMAYR, 1999). Within the Variscan orogenic cycle several distinct granite-forming events have occurred (FINGER et al., 1997): Devonian to Lower Carboniferous I-type granitoids (mainly quartz-diorites, tonalites, granodiorites) with primitive chemical and isotope signatures formed along the early-Variscan active margins (e.g. the Cetic granitoid massif - FRASL & FINGER, 1988). Chains of "Durbachite-plutons" (i.e. highly potassic, mostly coarse-grained melagranites with mafic enclaves) formed at ca. 330-340 Ma, probably as a result of slab-break-off processes, through mixing and mingling of crustal and enriched mantle magmas. Lower Carboniferous S-type leucogranites and migmatites are indicative of fluid-present melting events along major Variscan thrust zones. High-T melting of lower crust in post-collisional high-heat-flow zones generated large batholitic complexes consisting of S- and (mostly high-K2O) I-type granitoids between ca. 320 and 350 Ma, e.g. in the western Carpathians (PETRIK et al., 1994, FINGER et al., 2003). A distinct pulse of late-Carboniferous I-type plutonism (290-310 Ma) marks a zone of (back-arc ?) extension north of the Palaeotethys subduction system (STAMPFLI et al., 2001) and is mainly seen in the Penninic unit and the Southern Alps (e.g. EICHHORN et al., 2000, CESARE et al., 2002). Finally, I-, S- and A-type granitic plutons intruded along Permian rift zones (FINGER et al., 2003). Present research has shown that single basement fragments in the Alpine chain are often well characterised by their particular granitoid inventory, and that on this basis efficient correlations can be obtained and pre-Alpine tectonic zones reconstructed (e.g. SCHERMAIER et al., 1997, VON RAUMER, 1998, VON RAUMER et al., 2003). With the increasing availability of modern geochronological methods, the information-potential of the granitic rocks can be better and better exploited. The study of Palaeozoic granitic lithologies also plays a key role in the current research project "Correlation of Variscan crust in Austria and Croatia", which is a cooperation project between the mineralogical institutes of the universities of Zagreb and Salzburg. One task of this project is to compare the granite inventory of the Tisia block with that of the southern Bohemian Massif. CESARE, B., RUBATTO, D., HERMANN, J., BARZI, L. (2002): Evidence for Late Carboniferous subduction-type magmatism in mafic-ultramafic cumulates of the SW Tauern window (Eastern Alps).- Contrib. Mineral. Petrol. 42, 449-464. EICHHORN, R., LOTH, G., HÖLL, R., FINGER, F., SCHERMAIER, A., KENNEDY, A. (2000): Multistage Variscan magmatism in the central Tauern Window (Austria) unveiled by U/Pb SHRIMP zircon data.- Contrib. Mineral. Petrol. 139, 418-435. FINGER, F., ROBERTS, M.P., HAUNSCHMID, B., SCHERMAIER, A., STEYRER, H.P. (1997): Variscan granites of central Europe: their typology, potential sources and tectonothermal relations.- Mineralogy and Petrology 61, 67-96. FINGER, F., BROSKA, I., HAUNSCHMID, B., HRASKO, L., KOHÚT, M., KRENN, E., PETRÍK, I., RIEGLER, G., UHER, P. (2003): Electron-microprobe dating of accessory monazites from Western Carpathian basement granitoids: Plutonic evidence for an important Permian rifting event subsequent to Variscan crustal anatexis.- Int. J. of Earth Sci. (Geol Rundschau) 92, 86-98. FRASL, G., FINGER, F. (1988): The Cetic Massif below the Eastern Alps - characterized by its granitoids. SMPM 68, 433-439. PETRÍK, I., BROSKA, I., UHER, P. (1994): Evolution of the Western Carpathian granite magmatism: age, source rocks, geotectonic setting and relation to the Variscan structure.- Geol. Carpath. 45, 365-371. SCHALTEGGER, U., NÄGLER, T.F., CORFU, F., MAGGETTI, M., GALETTI, G., STOSCH, H.G. (1997): A Cambrian island arc in the Silvretta nappe: constraints from geochemistry and geochronology.- SMPM 77, 337-350. SCHERMAIER, A., HAUNSCHMID, B., FINGER, F. (1997): Distribution of Variscan I- and S-type granites in the Eastern Alps: a possible clue to unravel pre-Alpine basement structures.- Tectonophysics 272, 315-333. SCHINDLMAYR, A. (1999): Granitoids and Plutonic Evolution of the Ötztal-Stubai Massif - A Key for Understanding the Early Palaeozoic History of the Austroalpine Crystalline Basement in the Western Eastern Alps.- PhD thesis, University of Salzburg, 288 p. STAMPFLI, G.M., VON RAUMER, J.F., BOREL, G.D., BUSSY, F. (2001): Variscan and pre-Variscan evolution.- In: Geology of the Western Swiss Alps - a guide book, Stampfli G.M. (ed) Mémoires de Géologie (Lausanne) 36, 28-41. VON RAUMER, J.F. (1998): The Palaeozoic evolution in the Alps: from Gondwana to Pangea.- Geol. Rundsch. 87 (3), 407-435. VON RAUMER, J.F., STAMPFLI, G.A., BUSSY, F. (2003): Gondwana-derived microcontinents - the constituents of the Variscan and Alpine collisional orogens.- Tectonophysics 365 (1-4), 7-22.

Granitoid rocks ; Palaeozoic ; Variscides ; Pre-Alpine European crust

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