Evolution of H2O content in a polymetamorphic terrane: the Plattengneiss Shear Zone (Koralpe, Austria) |
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| Authors: | V TENCZER R POWELL K STÜWE |
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| Institution: | Institut für Erdwissenschaften, Universität Graz, Heinrichstr. 26, A-8010 Graz, Austria ();
School of Earth Sciences, University of Melbourne, Melbourne, Vic. 3010, Australia |
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| Abstract: | The Koralpe of the Eastern European Alps experienced high-temperature/low-pressure metamorphism (～650 °C and 6.5 kbar) during the Permian and eclogite facies metamorphism (～700 °C and 14 kbar) during the Eo-Alpine (Cretaceous) metamorphic event. In the metapelitic Plattengneiss shear zone that constitutes much of the Koralpe, the second metamorphism caused only partial re-equilibration of the assemblages formed during the first metamorphism. It is shown here that the Eo-Alpine mineral assemblage, garnet + biotite + muscovite + plagioclase + quartz (with or without kyanite), formed under low water activity conditions that are consistent with the level of dehydration that occurred during the Permian event. This implies that the rocks were essentially closed-system from the peak of the Permian metamorphism through the Eo-Alpine event. The evolution of water content of the rocks is traced through time: that prograde dewatering during the Permian metamorphic event terminated at the metamorphic peak with a water content around 3–4 mol.%. The water content remained then constant and led to water-undersaturation during the subsequent Eo-Alpine metamorphism. From the water content and activity evolution a post-peak isothermal decompression path close to the solidus is inferred for the Eo-Alpine event. |
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| Keywords: | polymetamorphism Plattengneiss shear zone Koralpe region water content water activity phase diagrams |
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