Cadomian 40 Ar/ 39 Ar apparent age spectra of detrital muscovites from the Eastern Alps

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Abstract
Concentrates of detrital muscovite were prepared from the Bischofalm Quartzite (late Ordovician) exposed in the Southalpine Carnic Alps, Austria. Two size fractions (150–175 pm and 85-100 μm) from two samples display similarly discordant 40 Ar/ 39 Ar age spectra in which apparent ages systematically increase from c. 300 Ma to c. 600 Ma in low-temperature portions of each analysis. Intermediate- and high-temperature increments are characterized by apparent ages of c . 620–645 Ma. Characteristics of the age spectra suggest minor late Palaeozoic, Variscan or combined Variscan/Alpine rejuvenations of argon isotopic systems within detrital grains which originally crystallized within a late Precambrian source terrain. The detrital muscovite ages provide evidence for a Cadomian-age source region for some basement sequences within the Austroalpine/Southalpine sectors of the Eastern Alps. The occurrence of Cadomian-age detritus in the Eastern Alps suggests links with Armorica and other, similarly aged tectonic elements within Alpine chains initiating along northern margins of Gondwana.