Constraints on the Variscan P-T evolution by EMP Th-U-Pb monazite dating in the polymetamorphic Austroalpine Oetztal-Stubai basement (Eastern Alps)

Polyphase metamorphism with Ordovician, Carboniferous and Cretaceous events has been reported from the Oetztal-Stubai basement in the Austroalpine nappe. Monazite in garnet-metapelites from the Solden, Umhausen and Sellrain regions was analysed by the electron microprobe (EMPA-CHIME) Th-U-Pb age dating method. The ages vary from 330-304 Ma without regional preference. To the E of Solden an additional population of Silurian-age monazites (similar to 420 Ma) has been found. Garnet in the metapelites crystallised during formation of the main foliation S-2. The porphyroblasts display core-to-rim growth zonations with increase of Mg while Mn decreases and Ca increases and then decreases. Intermineral microstructures provided criteria for local equilibria and structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The prograde P-T evolution passed 11-13 kbar at around 600 degrees C and involved decompression to 5 kbar near 700 degrees C. Similar P-T path segments are observed in the regions to the N and SE of the central Oetztal metabasites. According to the monazite ages, the P-T evolution can be assigned to a dominant Variscan (350-305 Ma) metamorphism. Variscan monazites are highly refractory in regions where K-Ar and Rb-Sr mica ages indicate significant Cretaceous metamorphic overprinting. The pre-Variscan metamorphism appears as a localised event in the Oetztal-Stubai basement.