High-pressure metamorphism in the SW tauern window, Austria: P-T paths from hornblende-kyanite-staurolite schists

The hornblende garbenschist horizon of the Lower Schieferhulle series (LSH) in the SW Tauern Window, Austria, contains the assemblage hornblende + kyanite + staurolite + garnet + biotite + epidote + plagioclase + ankerite + quartz + rutile + ilmenite, with either chlorite or paragonite present in all samples. These assemblages are divariant in the system SiO₂-Al₂O₃-TiO₂-Fe₂O₃-MgO-FeO-MnO-CaO-Na₂O-K₂O-H₂O-CO₂. Garnet-biotite geothermometry yields temperatures of final equilibration of ̃550 °C, and garnet-plagioclase-kyanite-quartz geobarometry indicates pressures of 6-8 kb for the matrix assemblage and 9-10 kb for plagioclase inclusions in garnet. Quantitative modelling of zoned garnet, hornblende, and plagioclase indicates growth and equilibration along a decompression path from ̃530 °C, 10 kb to ̃550 °C, 7 kb. Fluid inclusion data constrain the uplift path to have passed through a point at ̃ 375 °C, 1.5 kb.These data permit the construction of a relatively complete P-T loop for metamorphism associated with the Alpine orogenic cycle in the LSH of the SW Tauern Window. The maximum pressure conditions (̃10 kb at 530 °C) recorded along this loop are considerably higher than previous estimates of 5-7 kb for the region. Simple overthrust models developed for the Tauern Window cannot account for pressures of this magnitude; a more likely scenario involves partial subduction of the rocks to a depth of ̃35 km, followed by prolonged heating in response to decay of the subduction isotherms. Initial uplift appears to have been rapid and occurred along a nearly isothermal path. Significant cooling did not occur until the rocks were within ̃5 km of the surface. Detailed tectonic models for the evolution of the Tauern Window must be able to account for the quantitative features of the P-T loop.