Provenance shift at the northern margin of Gondwana during the Ordovician and Silurian recorded by detrital U-Pb zircon dating from the Eastern Alps

The paleogeographic position of the Alpine terrane with respect to (peri-) Gondwana during the Paleozoic is still a matter of debate. In this study we use a multi-proxy approach to analyze the provenance of siliciclastic sedimentary rocks from the Carnic Alps and the Northern Greywacke Zone, which include detrital U-Pb zircon dating, petrology and bulk geochemistry. The biostratigraphically well constrained deposits from the Carnic Alps have been used as a reference profile, and these results were compared to samples from the monotonous sediment record of the Northern Greywacke Zone. The stratigraphic interval ranges from the Middle Ordovician to the Silurian. The petrological and geochemical data indicate a high compositional maturity, which is supported by ZTR-dominated heavy mineral spectra. The results of U-Pb zircon dating (in total 957 concordant ages) show signatures typical for sedimentary rocks from the margin of Northern Gondwana and can be assigned to the East African-Arabian province. Most of the zircons show Pan-African ages, but presence of Meso- to Palaeoproterozoic and few Archean ages point to contributions from central Gondwana. Cambro-Ordovician zircon ages are linked to local volcanism at the northern margin of Gondwana. A stratigraphic trend both in the Carnic Alps and the Greywacke Zone is characterized by an increase of zircons with Tonian ages (900-700 Ma) and a decrease of zircons with Pan-African ages (700-550 Ma). We interpret the shift in detrital zircon ages by enhanced sediment input from the center of Gondwana mainland and reduced input by detritus from Cadomian crust of Northern Gondwana.