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 sti ll in the eclogite-facies, garnet with lower grossular-content and a f irst generation of amphibole are formed under lower pressure condition s, giving rise to the assemblage et-omphacite-phengite-kyanite-quartz- rutile-calcic amphibole (I). Continuing depressurization caused the su ccessive destabilization of omphacite, garnet, kyanite, and phengite. In some places the primary eclogite assemblage was completely transfor med into amphibolite-facies mineral assemblages. Estimated peak pressu res in the Snieznik and Miedzygorze units lie in the coesite stability field (P > 27 kbar at temperatures of about 700-800 degrees C) and ar e not significantly different to the Zlote unit. Although isofacial hi gh-P metamorphism of eclogites and associated Gieraltow gneisses has b een previously suggested, we show for the first time geobarometric evi dence that the country rock gneisses experienced pressures >27 kbar, i ndicated by the mineral assemblage 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 exhuma tion of the Snieznik and Miedzygorze units may be related to ductile p ure shear processes, whereas the cooling and decompression of the Zlot e unit may be related to lithospheric simple shear involving isostatic compensation.