Metamorphism And Tectonic Implications Of The Bueschists In The Central Yarlung Zangbo Suture Zone,Southern Tibet

Blueschists are mostly metamorphosed basaltic rocks that form at high-pressure(HP)and low-temperature(LT)conditions and characterize sodic amphibole as the major constituent mineral.Owing to blueschists typically form under cold geothermal gradients which are only found today in subduction zones,the distribution of blueschists belt can be regarded as a diagnostic petrological signature of subduction zones.Therefore,better constraining the metamorphic evolution of blueschists is vital to our comprehension of the plate tectonics as they preserve detailed information on the subduction,accretion and collision history of modern or fossil convergent plate margins.Blueschists in central southern Tibet are distributed as discontinuous blocks in sedimentary-matrix melange within the central section of Yarlung Zangbo Suture Zone(YZSZ).They have been subjected to the subduction of the Neo-Tethyan ocean below the Asian plate and provide important clues for better understanding of the evolution of the India-Asia convergence zone.In this paper,the systematical petrographic and mineral chemical studies and pseudosection modelling were carried out to better constrain the metamorphic conditions and evolution of blueschists in the central YZSZ.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 light blue and occur as platy to prismatic crystals,which are identified as magnesioriebeckite in chemical compositions.Pseudosection calculations and intersection of isopleths indicate peak metamorphic conditions of 7.8kbar at 355?.Metamorphic mineral assemblages from Xiongma blueschist contain sodic amphibole,epidote,chlorite,omphacite and sphene,implying P-T conditions of>320?and>7.1 kbar.The blueschists from Kadui area contain metamorphic assemblages of sodic amphibole,epidote,chlorite,omphacite with accessory sphene.Electron microprobe analyses demonstrate that the amphibole in Kadui blueschist shows zoned from actinolite core to ferrowinchite/riebeckite rim and actinolite outmost rim composition indicating that the sodic amphibole was formed during a prograde metamorphic event.The metamorphic conditions of Kadui blueschists are estimated to be 360? 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?/km.The metamorphic characteristics of blueschists in the central YZSZ show large contrasts to the deeply subducted rocks that experienced HP-LT blueschist-faceis metamorphism but display similarities to the metamorphic rocks which only experienced low-grade metamorphism at shallow depths in subduction channel.Our new findings indicate that the blueschist in the central part of YZSZ experienced low-grade blueschist-facies metamorphism as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India-Asia collision stage.