UHP KYANITE-BEARING ECLOGITE FROM MIEDZYGORZE IN THE WEST SUDETES (BOHEMIAN MASSIF) REVISITED

Ultrahigh-pressure (UHP) metamorphism in the west Sudetes was suggested by several studies over last 30 years. However, some recent studies questioned the UHP origin of the west Sudetes eclogites and associated rocks. We revisited the classical locality of the kyanite-bearing eclogites in that region and applied the currently available geothermobarometric and modeling techniques do estimate the pressure-temperature (P-T) conditions under which these eclogites were formed.

The Miedzygorze eclogite is banded with alternating garnet and omphacite dominated layers with subordinate kyanite, phengite and quartz. Accessories are represented by rutile, apatite, zircon and Fe-oxides. Garnet occurs as anhedral to subhedral porphyroblasts. It contains rare inclusions of kyanite, rutile and quartz. Seldom early zoisite is also found overgrown by garnet. Garnet zoning is expressed by small changes in concentrations of grossular, pyrope and almandine, whereas spessartine content is minor and does not show any zoning. The cores contain 37-38 mol% of pyrope and 26 mol% of grossular, whereas the rims up to 39 mol% of pyrope and 24 mol% of grossular, respectively. Garnet often reveals a composite internal character with multiple cores (and rims). It is considered to be an effect of coalescence of several smaller grains. Omphacite is characterized by jadeite up to 37 mol%, Ca-Tschermak up to 3.2 mol% and Ca-Eskola up to 2.6 mol%. Rod-shaped inclusions of SiO₂oriented parallel to the c-axis are present in the omphacite. The latter partly decomposes to the diopside-plagioclase symplectite, whereas phengite (up to 3.33 Si apfu) is marginally replaced by the biotite-plagioclase symplectite. Late amphibole is rarely present in the matrix.

The peak pressure assemblage is represented by Grt-Omp-Ky-Ph-Q-Ru. Hence the P-T conditions could have been derived using the Grt-Omp-Ky-Ph thermobarometry. It was coupled with the P-T pseudosection modeling in the NCKFMMnASHT system.

The obtained P-T conditions of 2.8-3.05 GPa and 770-830°C fall above the quartz-coesite boundary, thus confirming the UHP conditions during formation of the west Sudetes eclogites. The retrograde assemblage does not comprise typical high temperature minerals. Hence simultaneous decompression and cooling should have followed the peak pressure stage.