Ultrahigh-pressure metamorphism in the Snieznik Mountains (Sudetes, Poland): P-T constraints and geological implications

The petrology and P-T conditions of eclogites and associated gneisses from the Snieznik Mountains (Sudetes, SW-Poland) were investigated to establish an exhumation path for these ultrahigh-pressure rocks. Peak-pressure conditions are represented by the assemblage garnet-omphacite-phengite-kyanite-coesite-rutile. Post-dating this assemblage, but still in the eclogite-facies, garnet with lower grossular-content and a first generation of amphibole are formed under lower pressure conditions, giving rise to the assemblage garnet-omphacite-phengite-kyanite-quartz-rutile-calcic amphibole (I). Continuing depressurization caused the successive destabilization of omphacite, garnet, kyanite, and phengite. In some places the primary eclogite assemblage was completely transformed into amphibolite-facies mineral assemblages. Estimated peak pressures in the Snieznik and Miedzygorze units lie in the coesite stability field (P > 27 kbar at temperatures of about 700-800 degrees C) and are not significantly different to the Zlote unit. Although isofacial high-P metamorphism of eclogites and associated Gieraltow gneisses has been previously suggested, we show for the first time geobarometric evidence that the country rock gneisses experienced pressures >27 kbar, indicated by the mineral assemblage garnet(grossular(30-50))-zoisite-titanite(rutile)-quartz. The post-eclogite stage P-T path of the three tectonic units is different. The isothermal, rapid decompression during exhumation of the Snieznik and Miedzygorze units may be related to ductile pure shear processes, whereas the cooling and decompression of the Zlote unit may be related to lithospheric simple shear involving isostatic compensation.