Metamorphism And Exhumation Mechanism Of LT/HP Rocks In Lanling Area,Qiangtang Terrane Of North Tibetan Plateau

Subduction and exhumation of low-temperature/high-pressure(LT/HP)metamorphic rocks are affected by both tectonic force and buoyancy induced by density contrast between LT/HP rocks and their surroundings.The LT/HP rocks exposed in the Qiangtang Terrane,central Tibetan Plateau,are well studied on its structural characteristics,but controversy still exists about how these LT/HP rocks were subducted and exhumed during the closure of Paleo-Tethys.In this study,we analyzed the newly found lawsonitebearing eclogite,epidote eclogite,lawsonite-bearing blueschist,chloritoid-bearing blueschist and chloritoid-lawsonite-bearing phengite schist outcrops from Lanling area in the central Qiangtang Terrane.Detailed petrologic observations and geochemical analyses illustrate that porphyroblastic garnet in all samples show clearly zonation characterized by an inclusion-rich core(e.g.lawsonite,omphacite and glaucophane)overgrown by an inclusion-poor rim.In addition,profile of increasing pyrope and decreasing spessartine contents from garnet core to rim is indicative of garnet growth process typically under heating.Phase equilibria modeling reveals that garnet core grew at the pressure peak stage at 420-490 °C and 17.5-24.5 kbar,which corresponds to lawsonite eclogite/lawsonite blueschist facies metamorphism featured by the mineral assemblage of glaucophane + lawsonite ± omphacite ± chloritoid ± chlorite ± talc ± actinolite ± phengite ± quartz.In contrast,garnet rim was formed at the temperature peak stage at 520-550 °C and 20-22.5 kbar in the lawsonite eclogite facies condition with the mineral assemblage of omphacite + glaucophane + lawsonite + quartz ± phengite.All our studied samples indicate a clockwise metamorphic P-T evolution path involving pronounced heating decompression(except for chloritoid-lawsonite-bearing phengite schist)and subsequent isothermal decompression.Combine with previous geochronology datas,we conclude that the subduction rate is 4-9 mm/yr while the rate of isothermal decompression stage is ~10 mm/yr.By comparing the calculated density of LT/HP rocks at different metamorphic stages or exhumation stages,we found that density variation probably controls the exhumation behavior of LT/HP rocks in the Qiangtang Terrane.Moreover,a cold subduction system is suggested to develop in the Paleo-Tethys realm.