Abstract

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 ⁴⁰Ar/³⁹Ar systems. Within the underlying polymetamorphic basement, ⁴⁰Ar/³⁹Ar 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 ⁴⁰Ar/³⁹Ar ages from basement mylonites yield Variscan ages. To date this deformation directly, we isolated different mineral size fractions (63–30 and 30–10 μm) 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 μim) 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°C). The data suggest that mineral ages that date low-grade mylonitization (e.g., white mica ⁴⁰Ar/³⁹Ar) should be considered with caution.