藏东同卡微陆块泥质高压麻粒岩的变质作用及地质意义OA

The Tongka microcontinent block,situated in the eastern segment of the Bangong-Nujiang suture zone in the central Tibetan Plateau,preserves key evidence of the Early Jurassic tectonic evolution of the Tethyan Ocean through its metamorphic and magmatic records.Pelitic high-pressure granulites occur as interlayers within dominant felsic gneisses and are accompanied by leucosomes of varying scales.These granulites are primarily composed of garnet,kyanite,biotite,K-feldspar,quartz,with minor accounts of plagioclase,muscovite,sillimanite,and ru-tile.Integrated analyses—petrography,mineral chemistry,geochronology,and phase equilibrium modeling—reveal a clockwise metamorphic p-T-t path that can be divided into three distinct stages:① Near-isobaric heating prograde metamorphic stage:this stage is characterized by prograde conditions of 0.70～0.88 GPa and 655～680℃,constrained by the compositions of garnet cores and associated biotite inclusions;② Peak metamorphic stage:the peak mineral assemblage—garnet+kyanite+biotite+K-feldspar+quartz+rutile+melt±plagioclase—formed under condi-tions of 0.98～1.06 GPa and 770～790℃,as indicated by phase stability fields and the compositions of garnet mantles and matrix biotite.Zircon U-Pb dating yields a metamorphic age of 175 Ma,representing the timing of high-pressure granulite-facies metamorphism;③ Cooling and decompression retrograde stage:The retrograde as-semblage of biotite+plagioclase+sillimanite+muscovite reflects post-peak conditions of 0.32～0.68 GPa and 600～725℃.This clockwise p-T-t path reflects crustal thickening associated with the collision between the Tongka mi-crocontinent block and the Qiangtang terrane.The near-isobaric heating stage likely records prolonged thermal input from upwelling asthenosphere into the thickened lower crust of the microcontinent.