Ultrahigh-temperature metamorphism revealed by felsic granulite xenoliths in southern Tibet

Abstract

The Himalayan-Tibetan orogen experienced extensive thermal disturbance during the Indian-Asian collision, yet evidence for ultrahigh-temperature metamorphism (UHTM, >900 °C) remains scant. We report a UHTM event in the Lhasa Block, identified from felsic granulite xenoliths in Sangsang near the Tangra Yum Co Rift (TYR) of southern Tibet. These xenoliths exhibit a mineral assemblage of garnet + rutile + quartz + K-feldspar + plagioclase ± kyanite. Elemental mapping indicates garnet underwent high-temperature chemical homogenization. Employing Raman elastic geothermobarometry, Ti-in-quartz, and Zr-in-rutile geothermometers, we deduced pressure-temperature (P-T) conditions of 12−16 kbar and 870−975 °C, suggesting conditions were in the ultrahigh-temperature (UHT) range. The predicted P-T range is the most reliable estimation for Sangsang xenoliths to date, although the actual P-T conditions were likely higher due to viscous reset and chemical diffusion. An in situ garnet Lu-Hf date (ca. 26 ± 8 Ma) suggests that the UHT event occurred during the early Miocene, and in situ rutile U-Pb dates (ca. 12−11 Ma) indicate that the UHT conditions persisted until at least the timing of xenolith exhumation. Combined with the geological records in the Himalayan-Tibetan orogen, we suggest that the mantle upwelling contributed most to heating the Tibetan crust in the TYR and ultimately induced UHTM. Our study serves as an example of the UHTM formation mechanism within the active orogenic system.