Deformation-induced resetting of Rb/Sr and (super 40) Ar/ (super 39) Ar mineral systems in a low-grade, polymetamorphic terrane (Eastern Alps, Austria)

A paradox in dating metamorphic events in low-grade, polymetamorphic terranes is exemplified by the eastern Lower Austroalpine nappes of the Eastern Alps. Here, the last metamorphic event, best recorded in post-Variscan cover rocks, is dated as Late Cretaceous in age (c. 80-85 Ma) by white mica Rb/Sr and (super 40) Ar/ (super 39) Ar systems. Within the underlying polymetamorphic basement, (super 40) Ar/ (super 39) Ar and Rb/Sr ages of phengitic white mica record only Early and/or Late Variscan ages (375-270 Ma), indicating that the Alpine greenschist facies metamorphic overprint virtually caused no rejuvenation of Variscan mineral ages. Based on these results, the timing of a penetrative, ductile, top-to-WNW simple shear deformation recorded within both basement and cover rocks was contradictory. Deformation within the post-Variscan cover rocks had to be Alpine in age, whereas phengite (super 40) Ar/ (super 39) Ar ages from basement mylonites yield Variscan ages. To date this deformation directly, we isolated different mineral size fractions (63-30 and 30-10 mu m) from highly strained shearbands within the Wechsel basement nappe. A resulting Rb/Sr errorchron pointed to an Upper Cretaceous age (c. 85 Ma) for the deformation, consistent with the timing of Alpine metamorphism in the cover rocks. Coarser-grained white mica (150-300 mu m) from similar basement mylonites do not reflect any Alpine overprint of either K/Ar and/or Rb/Sr systems. It follows that dynamic re-and/or neocrystallization induced by ductile deformation within the shearbands was the dominant process by which the Rb/Sr system locally virtually re-equilibrated. This is valid even for overprinting metamorphic conditions below the temperatures required for Ar diffusional loss in phengitic white mica (c. <350 degrees C). The data suggest that mineral ages that date low-grade mylonitization (e.g., white mica (super 40) Ar/ (super 39) Ar) should be considered with caution.