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Mesozoic Metamorphic evolution of the Tauern Window

The oldest rocks in the Tauern Window are found in a volcano-sedimentary sequence comprising ophiolites, island arc volcanics and associated sediments of Late Proterozoic to Paleozic age (Habach Formation). A part of this sequence underwent pre-Mesozoic metamorphism, partly migmatisation and was intruded by Variscan granitoids. The Postvariscan sequences start with Permotriassic quarzites, middle Triassic limestones and dolomites and late Triassic sandstones and shales (Keuper). The Triassic rocks are overlain by shales, marls and shaly limestones of Jurassic to early Cretaceous age (Bündnerschiefer Formation). Locally, sandstones, breccias and arcoses occur. Associated with the sediments are ophiolites and other basic intrusions and volcanics. The youngest sediments proven so far are of early Cretaceous age but by comparison with lithologically similar sediments in the Lower Engadin Window and the Penninic realm in the Western Alps the occurrence of younger sediments seems at least possible. Tectonically two nappes are generally delineated (Frisch, 1976): the lower Venediger nappe comprising most of the pre-Mesozoic rocks and relatively little Mesozoic sediments and volcanics and the higher Glockner nappe including most of the Triassic rocks, the Bündnerschiefer and the ophiolites. Both nappes were later folded, forming a huge anticline with an axis following approximately the main ridge of the Alps. Apart from the pre-Mesozoic metamorphism three episodes of metamorphic events were recognized: an eclogite event, a blueschist metamorphism, and the final greenschist to amphibolite facies metamorphism called "Tauernkristallisation" by SANDER (1912). The eclogitisation affects only a relatively small strip mainly at the southern escarpment of the Tauern Window, the blueschist metamorphism is more widely distributed but restricted to the ophiolites, their immediate cover and the areas tectonically below. The Tertiary greenschist to amphibolite facies metamorphism can be seen in all rocks of the Tauern Window.