Metamorphism and Its Tectonic Implications of Early Mesozoic Pelitic Granulites from Badu Complex of Southwestern Zhejiang Province, South China

Dong Yunfeng; Zheng Changqing; Zhou Xiwen; Xu Jiulei; Zhou Xiao; Wang Zhaoyuan; Song Yang; Zhong Dingding

Abstract：Suichang-Dazhe pelitic granulites,showing distinct decompressional textures,crop out in Badu complex of the southwestern Zhejiang Province,the Cathaysia block.However,its metamorphic evolution characteristics,metamorphism ages and tectonic implications remain unclear.Results of our study of its petrology,mineral chemistry and LA-ICP-MS zircon U-Pb ages indicate that the rocks contain four distinct metamorphic assemblages,namely the early prograde (M1),pressure peak (M2),peak (M3) and the post-peak (M4) stages.The early prograde (M1) assemblage consists of garnet (core)+ biotite+ quartz.The pressure peak (M2) assemblage consists of garnet (mantle) + sudoite + rutile + kyanite + corundum + biotite + quartz ±staurolite,the mineral assemblage may indicate that the rock has undergone the process of ultrahigh pressure metamorphism.The peak (M3) assemblage consists of garnet (rim-mantle) + biotite+ sillimanite+ quartz± K-feldspar± plagioclase± ilmenite.Post-peak near-isothermal decompressional (M4) stage following the peak stage could be subdivided into two stages M4-1 and M4-2.The M4-1 stage assemblage consists of garnet (rim) + cordierite+ biotite+ sillimanite+ quartz+ ilmenite ± spinel± plagioclase± K-feldspar,and the M4-2 stage is represented by garnet (rim) + cordierite+ sillimanite+ plagioclase+ biotite+ quartz±K-feldspar.Quantitative phase equilibria modeling in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2 O-TiO2,in combination with traditional thermobarometry,was applied to obtain P-T conditions of 780-810 ℃ and 8.0-9.2 kbar for M3,780-860 ℃ and 5.7-6.0 kbar for M4-1,about 700 ℃ and about 4.4 kbar for M4-2.The combination of the mineral assemblages,mineral compositions,and metamorphic reaction history in the Suichang-Dazhe pelitic granulite defines a clockwise P-T path that involves near-isothermal decompression that followed the peak granulite-facies metamorphism.Analyses of LA-ICP-MS zircon U-Pb dating indicate that metamorphic zircons recorded metamorphic age of the Suichang-Dazhe pelitic granulite is 233.5-238.9 Ma.The clockwise P-T path reveals that the southwestern Zhejiang Province terrane involved in the subduction or collision followed by exhumation and cooling events and experienced the Early Mesozoic (233.5-238.9 Ma) granulite-facies metamorphism with the amalgamation of the Indochina-South China-North China block in the paleo-Tethyan domain.