CADOMIAN AR-40 AR-39 APPARENT AGE SPECTRA OF DETRITAL MUSCOVITES FROMTHE EASTERN ALPS

Concentrates of detrital muscovite were prepared from the Bischofalm Q uartzite (late Ordovician) exposed in the Southalpine Carnic Alps, Aus tria. Two size fractions (150-175 mum and 85-100 mum) from two samples display similarly discordant Ar-40/Ar-39 age spectra in which apparen t ages systematically increase from c. 300 Ma to c. 600 Ma in low-temp erature portions of each analysis. Intermediate- and high-temperature increments are characterized by apparent ages of c. 620-645 Ma. Charac teristics of the age spectra suggest minor late Palaeozoic, Variscan o r combined Variscan/Alpine rejuvenations of argon isotopic systems wit hin detrital grains which originally crystallized within a late Precam brian source terrain. The detrital muscovite ages provide evidence for a Cadomian-age source region for some basement sequences within the A ustroalpine/Southalpine sectors of the Eastern Alps. The occurrence of Cadomian-age detritus in the Eastern Alps suggests links with Armoric a and other, similarly aged tectonic elements within Alpine chains ini tiating along northern margins of Gondwana.