Pre-Alpine history of the pennine zone in the Tauern Window,Austria: U-Pb and Rb-Sr geochronology

Zircon ages from major lithologies of the Zentralgneis suggest that much of the Variscan magmatism in the Tauern Window is older than previously suggested. In the southeast Tauern Window a tonalite has been dated at 314±7 m.y. and a granodioritic biotite augen gneiss at 313±10 m.y. Two granodiorites from the Granatspitzkern yielded zircon data consistent with a similar age. These zircon data require re-interpretation of some previously published Rb-Sr whole rock ages and raise the possibility that Alpine metamorphism caused more widespread disturbance of Rb-Sr whole rocks than commonly supposed. Rb-Sr data on fabric-forming white micas from two banded gneisses give ages close to 220 m.y., indicating the foliation in these rocks is pre-Alpine and has not been greatly affected by Alpine recrystallisation.