The Metamorphism And Its Tectonic Implications Of Early Mesozoic Pelitic Granulites From The Badu Complex Of The Southwestern Zhejiang Province,South China

The Badu Complex is the oldest metamorphic rock in Cathaysia Block,and experienced several episodes of metamorphism.Especially,it is obvious that indosinian metamorphic rework in the southwestern Zhejiang Province.However,there is still insufficient understanding of the typical petrographic features,metamorphic evolution,metamorphic times and tectonic significance of pelitic granulites of the southwestern Zhejiang Province.Therefore,we present the first coherent studies from pelitic granulites of the Badu Complex,which is composed of“sillimanite+garnet+cordierite+spinel+biotite”.Through a combined study of petrography,mineral chemistry,geochemistry,metamorphic evolution and geochronology,we get these following several new findings.The protoliths of the pelitic granulites are mainly shale,claystone and graywackewith low maturity and proximal sedimentation.The uppercrust felsic sources with some mafic components make a contribution to their provenance.Tectonic discrimination diagrams show that the protolithsof the pelitic granulites formed in the passive continental margin.Pelitic granulites records four stages of metamorphic mineral assemblages,include the early prograde?M₁?,pressure peak?M₂?,Peak?M₃?and the post-peak decompressional?M₄?stages.The mineral assemblage of early prograde?M₁?is Grt1?core?+St+Ky+Bt+Qz±Rt±Chl;the mineral assemblage of pressure peak?M₂?is Grt1?mantle?+Ky+Rt+Sud+Crn+Bt+Qz;the mineral assemblage of peak?M₃?is Grt2?core-mantle?+Sil±Kfs+Bt+Qz±Pl±Ilm;the post-peak decompressional M_4-1 is consist of Grt1/Grt2?the rim of garnet is contact with cordierite?+Crd+Bt+Sil+Qz+Ilm±Spl±Pl±Kfs,and M_4-2 is Grt₂?the rim of garnet is contact with plagioclase?+Crd+Pl+Bt+Sil±Kfs.Phase equilibria modeling in the system MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂ and traditional thermobarometry were applied to obtain P-T conditions of different metamorphic stages.The P-T conditions is about 785⁸20?and 8.9⁹.9kbar for M₃,780⁸60?and 5.7⁶.2kbar for M_4-1,705⁷20?and4.5⁴.7kbar for M_4-2.A clockwise P-T path is confirmed in Pelitic granulite of the Badu Complex.The P-T path reflected a near-isothermal decompression metamorphism process.LA-ICP-MS Zircon U-Pb dating indicate metamorphic age is 233.5²38.9Ma.The clockwise P-T path reveals that the Badu Complex of Cathaysia Block involved in the subduction/collision followed by exhumation and cooling events.the events are related with the amalgamation of the Indochina Block—South China—North China in the paleo-Tethyan domain.