40 Ar/ 39 Ar ages of detrital white mica from Upper Austroalpine units in the Eastern Alps, Austria: Evidence for Cadomian and contrasting Variscan sources

Abstract

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 40Ar/39Ar 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 one is 562.2±0.7 Ma, whereas the other yielded a 40Ar/39Ar plateau age of 607.3±0.3 Ma. These results indicate a source area affected by Cadomian tectonothermal activity. Three detrital muscovite concentrates from post-Variscan, Late Carboniferous and Permian cover sequences exposed within three different Alpine nappes yielded 40Ar/39Ar plateau ages of 359.6 ± 1.1 Ma, 310.5±1.2 Ma, and 303.3±0.2 Ma. The contrasting detrital white mica ages are interpreted to reflect different source areas. Detrital muscovite from a post-Variscan Carboniferous molasse-type sequence and from a Permian Verrucano-type sequence record ages which indicate “late” Variscan (e.g. 330–300 Ma) metamorphic sources. By contrast, detrital white mica from another Permian Verrucano-type sequence suggests a source area affected by “early” Variscan (e.g. 400–360 Ma) metamorphism. These results help clarify palinspastic relationships and tectonic correlations between pre-Late Carboniferous metamorphic basement sequences and Carboniferous to Permian cover sequences.