U-PB AND PB-PB ZIRCON DATING OF THE OLDER ORTHOGNEISS SUITE IN THE SILVRETTA NAPPE, EASTERN ALPS - CADOMIAN MAGMATISM IN THE UPPER AUSTRO-ALPINE REALM

Two orthogneiss suites dominate the Silvretta nappe. Primary crystalli zation of the larger suite ('younger orthogneisses') is assumed to be Ordovician in age. The second, adjacent magmatic suite consists of old er, alkaline to calc-alkaline, ultrabasic, basic to intermediate and g ranitic rocks known as 'older orthogneisses'. U-PB data of multigrain zircon fractions, as well as single zircon stepwise evaporation Pb-207 /Pb-206 results suggest a latest Proterozoic to early Cambrian intrusi on age for the protoliths of the 'older orthogneisses' as both dating methods yield early Cambrian crystallization ages of 526 +/- 7 and 519 +/- 7 Ma for an alkaline granite gneiss; similar results were obtaine d for two neighbouring calc-alkaline orthogneisses (Pb-207/Pb-206 ages of 533 +/- 4 and 568 +/- 6 Ma, respectively). The crystal habitus cor responds to P5, S19 and S9 zircons of magmatic origin. Whole-rock init ial Sr isotope ratios indicate a primitive source. The igneous protoli ths of these 'older orthogneisses' represent a fragment of a Cadomian (Pan-African) crust found in places within the basement of the Europea n Hercynides.