AR-40 AR-39 AGES OF DETRITAL WHITE MICE FROM UPPER AUSTROALPINE UNITSIN THE EASTERN ALPS, AUSTRIA - EVIDENCE FOR CADOMIAN AND CONTRASTING VARISCAN SOURCES/

Five detrital white mica concentrates from very low-grade, metaclastic sequences within pre-Variscan basement and post-Variscan cover units of the Upper Austroalpine Nappe Complex (Eastern Alps) have been dated with Ar-40/Ar-39 incremental heating techniques to constrain the age of tectonothermal events in their respective source areas. Two samples from early Palaeozoic sandstone exposed within the same Alpine nappe record slightly discordant age spectra. The maximum age recorded in on e is 562.2 +/- 0.7 Ma, whereas the other yielded a Ar-40/Ar-39 plateau age of 607.3 +/- 0.3 Ma. These results indicate a source area affecte d by Cadomian tectonothermal activity. Three detrital muscovite concen trates from post-Variscan, Late Carboniferous and Permian cover sequen ces exposed within three different Alpine nappes yielded Ar-40/Ar-39 p lateau ages of 359.6 +/- 1.1 Ma, 310.5 +/- 1.2 Ma, and 303.3 +/- 0.2 M a. The contrasting detrital white mica ages are interpreted to reflect different source areas. Detrital muscovite from a post-Variscan Carbo niferous molasse-type sequence and from a Permian Verrucano-type seque nce record ages which indicate ''late'' Variscan (e.g. 330-300 Ma) met amorphic sources. By contrast, detrital white mica from another Permia n Verrucano-type sequence suggests a source area affected by ''early'' Variscan (e.g. 400-360 Ma) metamorphism. These results help clarify p alinspastic relationships and tectonic correlations between pre-late C arboniferous metamorphic basement sequences and Carboniferous to Permi an cover sequences.