Abstract

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 Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O (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 re-equilibration 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 (<0.02) typical of metamorphic origin. In these zircons, nearly flat HREE patterns, (Lu/Gd)N = 1–4, and only small negative Eu anomalies indicate formation in the presence of garnet and absence of plagioclase, which is corroborated by occurrence of Mg- and Ca-rich garnet inclusions. Therefore, these zircons are interpreted to record the Cretaceous HP/UHP metamorphism. The 92.0 ± 0.5 Ma age obtained in this study agrees with that (93–91 Ma) determined earlier in the Pohorje eclogites from U/Pb zircon, Sm-Nd and Lu-Hf garnet-whole-rock dating. This implies that the eclogites and their country rocks were subducted and exhumed together as a coherent piece of continental crust. There is no evidence for a mélange-like assemblage of rocks, which followed different P–T–t paths, or several subduction and exhumation cycles as proposed for some other UHP metamorphic terranes.

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