Low-grade Blueschist-facies Metamorphism in the Central Yarlung Suture Zone, Southern Tibet

Blueschists are mostly metamorphosed basaltic rocks that form at high-pressure and low-temperature conditions and characterize sodic amphibole as the major constituent mineral. Blueschists in southern Tibet are distributed as discontinuous blocks in sedimentary-matrix mélange within the Yarlung Suture Zone (YSZ). In this paper, petrographic and mineral chemical studies and pseudosection modelling were carried out to constrain the metamorphic conditions and evolution of blueschists in the central YSZ. Blueschists from Sangsang area are characterized by presence of sodic amphibole, epidote, chlorite, muscovite, omphacite and sphene in the peak metamorphic assemblage. Sodic amphiboles in Sangsang blueschist are identified as magnesioriebeckite in chemical compositions. Pseudosection calculations and intersection of isopleths indicate peak metamorphic conditions of 7.8kbar at 355°C. Blueschists from Kadui area contain metamorphic assemblages of sodic amphibole, epidote, chlorite, omphacite with accessory sphene. The amphibole in Kadui blueschist shows zoned from actinolite core to ferrowinchite/riebeckite rim and actinolite outmost rim. The metamorphic conditions of Kadui blueschists are estimated to be 360°C and 7.5kbar based on available phase equilibria and sodic amphibole compositions. These P–T conditions are consistent with metamorphism in the low-grade blueschist facies, burial depths of 25–27 km and a thermal gradient of 13–14°C/km. Our new findings indicate that the blueschist in the central part of YSZ experienced low-grade blueschist-facies metamorphism at shallow depths in subduction channel as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India–Asia collision stage.