Fluid inclusions of the high-pressure granulites from the Namche Barwa complex of the eastern Himalayan syntaxis, Tibet: Fluid composition and evolution in the continental subduction-zone

The Namche Barwa complex in the eastern Himalayan syntaxis has experienced high-, medium-pressure granulite-facies and amphibolite-facies metamorphism. The high-pressure granulites contain complex fluid inclusion assemblages, including H2O–CO2 ± CH4, carbonic (CO2 ± CH4 ± N2), CO2-solid, brine, medium- to low-salinity aqueous inclusions and low-density gaseous inclusions. Textural evidence suggests that the isolated H2O–CO2 ± CH4 inclusions occurring in quartz blebs enclosed in garnet porphyroblasts were trapped in the peak metamorphic stage whereas isolated and trail-bound CO2 inclusions (along intragranular fractures) in quartz, plagioclase and garnet were mainly trapped during the early decompression stage of the high-pressure granulites although some CO2-rich inclusions have formed by selective leakage of H2O out of H2O–CO2 inclusions. This together with wide variations in homogenization temperatures indicates modifications and density resetting of the fluid inclusions during exhumation. In contrast to the common presence of high-density CO2 fluid inclusions in granulite terrains worldwide the high-pressure granulites in the Namche Barwa complex contain dominantly medium- to low-density H2O–CO2 ± CH4 and CO2 inclusions. This may be explained as that the high-pressure granulites occur in the continental subduction-zone tectonic setting.