Pervasive rehydration of granulites during exhumation -an example from the Pulur complex, Eastern Pontides, Turkey

摘要

High-grade gneisses from the Pulur complex in NE Turkey bear evidence for biotite-dehydration melting at > = 820 deg C and 0.7-0.8 GPa, melt segregation and near-isothermal decompression to 0.4-0.5 GPa. During further exhumation, the rocks underwent secondary pervasive rehydration at temperatures between approx 400 and 230 deg C and fluid pressures between approx 0.3 and 0.1 GPa. Metamorphic peak conditions are dated at 331-327 Ma, while hydrothermal retrogression occurred significantly later at 315-310 Ma under static conditions. During the rehydration event, primary high-grade mineral assemblages including garnet, cordierite, sillimanite, spinel, biotite, plagioclase and ilmenite were extensively replaced by muscovite, paragonite, margarite, corundum, diaspore, chlorite, kaolinite, pumpellyite, prehnite, epidote, titanite, anatase, pyrite and chalcopyrite. Secondary mineral assemblages indicate that the infiltrating fluids were characterized by low fO_2, very low X_(CO_2) (<0.002), variable activities of Ca~(2+), K~+, Na~+ and H~+ and relatively high activities of H_2S and CH_4. Quartz veins that might have acted as pathways for the fluids are rare. Ubiquitous veinlets consisting of (i) albite, (ii) chlorite + calcite + quartz or (iii) K-feldspar + calcite + quartz were formed after the pervasive rehydraton event by precipitation from aqueous solutions that were somewhat richer in CO_2.