Unravelling the pre-Variscan evolution of the Habach terrane (Tauern Window, Austria) by U-PbSHRIMP zircon data

The U-Pb SHRIMP age determinations of zircons from the Habach terrane (Taue rn Window, Austria) reveal a complex evolution of this basement unit, which is exposed in the Penninic domain of the Alpine orogen. The oldest compone nts are found in zircons of a metamorphosed granitoid clast, of a migmatiti c leucosome, and of a meta-rhyolitic (Variscan) tuff which bear cores of Ar chean age. The U-Pb ages of discordant zircon cores of the same rocks range between 540 and 520 Ma. It is assumed that the latter zircons were origina lly also of Archean origin and suffered severe lead loss, whilst being inco rporated into Early-Cambrian volcanic arc magmas. The provenance region of the Archean (2.64-2.06 Ga) zircons is assumed to be a terrane of Gondwana a ffinity: i.e., the West African craton (Hoggar Shield, Reguibat Shield). Th e Caledonian metamorphism left a pervasive structural imprint in amphibolit e facies on rocks of the Habach terrane; it is postdated by discordant zirc ons of a migmatitic leucosome at < 440 Ma (presumably ca. 420 Ma). Alpine a nd Variscan upper greenschist- to amphibolite-facies conditions caused part ial lead loss in zircons of a muscovite gneiss ('white schist') only, where extensive fluid flow and brittle deformation due to its position near a na ppe-sole thrust enhanced the grains' susceptibility to isotopic disturbance . The Habach terrane - an active continental margin with ensialic back-arc development - showed subduction-induced magmatic activity approx. between 5 50 and 507 Ma. Back-arc diorites and arc basalts were intruded by ultramafi c sills and subsequently by small patches of mantle-dominated unaltered and (in the vicinity of a major tungsten deposit) altered granitoids. Fore-arc (shales) and back-arc (greywackes, cherts) basin sediments as well as arc and back-arc magmatites were not only nappe-stacked by the Caledonian compr essional regime closing the presumably narrow oceanic back-arc basin and sq ueezing mafic to ultramafic cumulates out of high-level magma chambers (496 -482 Ma). It also induced uplift and erosion of deeply rooted crystalline c omplexes and triggered the development of a successor basin filled with pre dominantly clastic greywacke-arkosic sediments. The study demonstrates that the basement rocks exposed in the Habach terrane might be the 'missing lin k' between similar units of the more westerly positioned External domain (i .e., Aar, Aiguilles Rouges, Mont Blanc) and the Austroalpine domain to the east (Oetztal, Silvretta).