Isotope geochemistry (Pb-Pb-zircon, Rb-Sr, Sm-Nd, O) in meta-magmatites and pre-Variscan geodynamics in the Austroalpine basement to the south of the Tauern Window
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The Variscan and partly Alpine metamorphic Austroalpine basement to the south of the central Tauern Window is subdivided into four lithological units: (1) the Northern Defereggen/Petzeck Group with the metabasic Rotenkogel, Torkogel, Michelbach and Prijakt Subgroups; (2) the Durreck/Cima Dura Muscoviteschist Group; (3) the Defereggen Group and (4) the Palaeozoic Thurntaler Phyllite Group. Several suites of meta-magmatic rocks can be characterized by whole-rock geochemistry. Protolith ages of defined members in these suites were dated by the Pb-Pb single zircon evaporation method. Eclogitic amphibolites of the Prijakt Subgroup in the Schobergruppe have element abundances typical of N-MORB. Their magmatic LIL and LREE enrichment and (Th/Ta)N of 1.38 - 2.5 can be interpreted as a sign of backarc magmatism. The protolith age is 590 ± 4 Ma (weighted average from 10 zircons). Hornblende-plagioclase gneisses of the Rotenkogel and Prijakt Subgroups have basic to intermediate compositions. They display Ti/Zr, Ti/V and Cr/Y ratios typical of calk-alkaline volcanic arc magmatites. Their protolith ages range between 550 ± 6 and 533 ± 4 Ma. Orthogneisses with compositions ranging from metaluminous dioritic to peraluminous granitic are closely associated with the magmatic arc suite, but occur as well widespread in the Defereggen Group. Their compositions range from metaluminous dioritic to peraluminous granitic. Protoliths of orthogneisses with compositions matching the continental arc granites discriminant field appear as slightly older (471 to 461 Ma) as those of orthogneisses ranging in the continental collision field (457 to 448 Ma). Geochemical signatures and REE patterns of these orthogneisses and the meta-porphyroids in the Thurntaler Phyllite Group are very similar. The meta-porphyroid protolith ages are 473 ± 7 and 469 ± 6 Ma. However, when corresponding single zircon ages are compared, the porphyroid volcanism appears as slightly older as the granitoid plutonism. Both acid rock groups bear many inherited zircons with ages up to 1930 Ma. Amphibolites in the Northern Defereggen/Petzeck and in the Thurntaler Phyllite Groups have Zr-Ti-Y contents ranging from MORB tholeiites to within-plate alkalibasalts. A highly differentiated sample from the alkalibasalt-like suite in the Torkogel Subgroup yielded a 430 ± 2 Ma Pb-Pb zircon age. The Austroalpine basement was part of a north-Gondwanan active continental margin. Active margin development involved a Neoproterozoic extensional magmatism with backarc signature and culminated in the formation of an Early-Cambrian magmatic arc. The Ordovician acid volcanism and plutonism appears as a mature stage of this subduction-controlled evolution. An Early-Silurian alkalibasaltic magmatism then signalizes a subsequent and partly simultaneously starting passive margin evolution, which was probably related to the opening of the Palaeo-Tethys.
Projekt period: from 09.1999 to 06.2002
DFG ,Granting date: 14.06.1999