Tectono-metamorphic Evolution Of The Barrovian Metamorphic Zone In Mid-Jinshajiang Suture Zone, Eastern Tibetan Plateau

The Jinshajiang Metamorphic zone located in mid-Jinshajiang Suture Zone is a key area to elucidate deformation-metamorphic process of the suture and an ideal place to analyze tectonic evolution of the Tethys. Combining with geologic survey, petrological observation and mineral chemistry analysis, the author intends to elucidate regional deformation-metamorphic evolution and constrain metamorphic pressure and temperature conditions by conventional geothermobarometers.The author recognized three deformational episodes by detailed structural analysis. The first event (D1-M1) is recorded by main schistosity (S1) in metamorphic rocks and prograde chlorite- to sillimanite- grade Barrovian metamorphism during subduction of Jinshajiang Ocean Plate and subsequent crustal thickening and shortening. The second deformation (D2) is recorded as S2 schistosity and ductile shear zone. The retrograde metamorphic stage (M2) is deciphered to be coeval with the D2 deformational event. The D2-M2 event represents regional extension after previous crustal thicking. The third deformation (D3) is characterized by brittle fault, S3 foliation, and related fault breccias and gauge. The last movement of fault is ~200 - ~135 ka, indicated by ESR ages.According to petrological observation, the author recognized two stages of metamorphism. The peak metamorphic assemblages (M1) are formed by progade Barrovian metamorphism. The M2 metamorphism is characterized by a series of retrograde metamorphic minerals, such as chlorite and sericite, in every metamorphic zone. Metamorphic P-T conditions were calculated by garnet- biotite- plagioclase- quartz (GBPQ) geothermobarometer and hornblende- plagioclase- quartz (HPQ) geothermobarometer and chlorite compositional geothermometer. Peak metamorphic (M1) P-T conditions of the sillimanite zone should be higher than 629℃and 5.832 kbar, obtained by GBPQ geothermobarometer. That of staurolite-kyanite zone should be higher than 641℃, 5.077 kbar by GBPQ geothermobarometer. While peak metamorphic conditions by HPQ geothermobarometer is ~702-~744℃and ~6.370 kbar;The metamorphic temperature of retrograde metamorphism (M2) calculated by chlorite composition geothermometer is ~295.777-~319.861℃。Clockwise P-T segments were inferred for the staurolite-kyanite and sillimanite zones.According to these analysis, the author proposed that, during the Middle Triassic, the Jinshajiang Ocean Plate suducts under the Yidun Arc to the east and builds the Tethys Orogeny which formed the Barrovian Metamorphic zones; During the late Indochina period, the area experienced regional extension which formed the ductile shear zone and S2 shistosity; The collision of India subcontinent with Euroasian formed the uplifting of the Tibetan Plateau and a series of brittle faults, which activates until Pleistocene.