U-Pb ages of apatite in the western Tauern Window (Eastern Alps): Tracing the onset of collision-related exhumation in the European plate

We performed single spot U-Pb analyses of zircons and apatites from eighteen samples distributed along three sections crossing the western Tauern Window (Eastern Alps). Zircon analyses yield concordant ages of ∼295Ma dating the Variscan emplacement of the Zentralgneiss batholith. Common Pb-corrected apatite U-Pb ages show two age groups; the first is characterized by apparent 206Pb/238U ages scattering between ∼300 and 40Ma and a second having a relatively uniform cluster of age values around 30 Ma. We applied an age extractor algorithm (TuffZirc ISOPLOT) to all apatite analyses of each sample to calculate median ages from the younger age cluster and asymmetric 2σ errors as their uncertainties, which are mostly in the range of ∼10%. These median ages are interpreted to date cooling below ∼450-500 °C during Alpine orogeny after the thermal climax of Barrovian metamorphism in the western Tauern Window. Cooling ages give a consistent pattern over the study area of ∼31-29Ma. Along each section younger cooling ages were obtained in the center compared to its margins. The eastern section shows older cooling ages of ∼36-34Ma whereas the western section younger cooling ages of ∼26-25Ma. The reliability of both age trends, (1) younging towards the centers of the sections and (2) younging towards the west across all three sections were verified by six independent two-sample Student's t-tests. Spatially the cooling age pattern resembles those observed from geochronometers having lower closure temperatures (∼375-110 °C), that are largely related to doming of the Tauern Window. Cooling rates of five samples, where additional fission track ages are available, yielded uniform results and were averaged to 15.3±1.9K/Myr (2σ). Linear extrapolation using the specific cooling rates and the estimated maximum temperature (Tmax) indicates a time range of ∼37-34Ma, interpreted to date the thermal climax of Barrovian metamorphism in the western Tauern Window.