Eclogite-hosting metapelites from the Pohorje Mountains (Eastern Alps): P-T evolution, zircon geochronology and tectonic implications

Phase-equilibrium modelling, geothermobarometry, ion-microprobe dating and mineral chemistry of zircon have been used to constrain the P-T-t evolution of metapelitic kyanite-bearing gneisses from the ultrahigh-pressure (UHP) metamorphic terrane of the Pohorje Mountains in the Eastern Alps. These eclogite-hosting rocks are part of the continental basement of the Austroalpine nappes. Based on calculated phase diagrams in the system NaO-CaO- KO-FeO-MgO-MnO-AlO-SiO-H O (NCKFMMnASH) and conventional geothermobarometry, the garnet-phengite-kyanite-quartz assemblages of gneisses record metamorphic conditions of 2.2-2.7 GPa at 700-800 °C. These are considered as minima because of the potential for a diffusion-related modification and reequilibration of the garnet and phengite during early stages of decompression. It is therefore most likely that the gneisses experienced the same peak UHP metamorphism at ∼3 GPa as associated kyanite eclogites. Decompression and cooling to ∼0.5 GPa and 550°C led to the consumption of garnet and phengite, and the development of matrix consisting of biotite, plagioclase, K-feldspar ± sillimanite and staurolite. Textures and phase diagrams suggest a low extent of partial melting during decompression. Cathodoluminescence images as well as zircon chemistry reveal cores encompassed by two types of metamorphic zircon rims. Ion probe U-Pb dating of three zircon cores yielded Permian (286 ± 10, 258 ± 7 Ma) and Triassic (238 ± 7 Ma) concordia ages. The zircon rims are Cretaceous with a mean concordia age of 92.0 ± 0.5 Ma and some cores gave a similar age. The Cretaceous zircons all exhibit very low Th/U ratio (