Grain-size effects on the closure temperature of white mica in a crustal-scale extensional shear zone - Implications of in-situ ⁴⁰Ar/³⁹Ar laser-ablation of white mica for dating shearing and cooling (Tauern Window, Eastern Alps)

In-situ ⁴⁰Ar/³⁹Ar laser ablation dating of white-mica grains was performed on samples from the footwall of a crustal-scale extensional fault (Katschberg Normal Fault; KNF) that accommodated eastward orogen-parallel displacement of Alpine orogenic crust in the eastern part of the Tauern Window. This dating yields predominantly cooling ages ranging from 31 to 13 Myr, with most ages clustering between 21 and 17 Myr. Folded white micas that predate the main Katschberg foliation yield, within error, the same ages as white-mica grains that overgrow this foliation. However, the absolute ages of both generations are older at the base (20 Myr) where their grain size is larger (300-500 μm), than at the top and adjacent to the hangingwall (17 Myr) of this shear zone where grain size is smaller (<100-300 μm). This fining-upward trend of white-mica grain size within the KNF is associated with a reduction of the closure temperature from the base (~445 °C) to the top (<400 °C) and explains the counter-intuitive trend of downward-increasing age of cooling in the footwall. The new data show that rapid cooling within the KNF of the eastern Tauern Window started sometime before 21 Myr according to the ⁴⁰Ar/³⁹Ar white-mica cooling ages and between 25-21 Myr according to the new Rb/Sr white-mica ages, i.e., shortly after the attainment of the thermal peak in the Tauern Window at ~25 Myr ago. These new data, combined with literature data, support earlier cooling in the eastern part of then Tauern Window than in the western part by some 3-5 Myr.