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

Abstract

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 H 2O–CO 2 ± CH 4, carbonic (CO 2 ± CH 4 ± N 2), CO 2-solid, brine, medium- to low-salinity aqueous inclusions and low-density gaseous inclusions. Textural evidence suggests that the isolated H 2O–CO 2 ± CH 4 inclusions occurring in quartz blebs enclosed in garnet porphyroblasts were trapped in the peak metamorphic stage whereas isolated and trail-bound CO 2 inclusions (along intragranular fractures) in quartz, plagioclase and garnet were mainly trapped during the early decompression stage of the high-pressure granulites although some CO 2-rich inclusions have formed by selective leakage of H 2O out of H 2O–CO 2 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 CO 2 fluid inclusions in granulite terrains worldwide the high-pressure granulites in the Namche Barwa complex contain dominantly medium- to low-density H 2O–CO 2 ± CH 4 and CO 2 inclusions. This may be explained as that the high-pressure granulites occur in the continental subduction-zone tectonic setting.