Study On Indosinian Metamorphism-Deformation Sequence Of Badu Group In Suichang-Dazhe Region Of Southwestern Zhejiang Province

Badu Group in Suichang-Dazhe area of Southwestern Zhejiang Province is the oldest metamorphic basement known to the Cathaysian block.Under the influence of Indosinian metamorphic events,rocks generally undergo metamorphism and deformation.Previous studies on Metamorphism and tectonic deformation of Badu Group have been carried out systematically,but little has been done on the relationship between tectonic deformation and metamorphism.However,This study is of great significance for exploring the tectonic environment of Indosinian metamorphic event.Therefore,through detailed field geological survey and field profile survey,combined with the outcrop characteristics of Badu Group rocks in the study area,on the basis of dividing the structural deformation sequence,we collect representative rock samples of the main lithology for petrographic analysis and identify the symbiotic metamorphic mineral assemblages in each deformation stage,and then conclude that the Badu Group in the study area can be divided into three stages of metamorphic deformation sequence in Indosinian period:D₁ deformation period:residual tight folds recorded by NW-trending S₁ gneisses.Symbiotic mineral assemblages are garnet metamorphic phenocrysts and inclusion minerals with directional distribution,garnet+biotite+quartz?argillaceous?and garnet+amphibole+plagioclase+quartz?feldspar?;D₂deformation period:regional wide-gentle folds and NE-trending gently inclined permeable S₂ gneisses;symbiotic mineral assemblages are garnet metamorphic phenocrysts and matrix minerals.sillimanite+garnet+biotite+quartz+plagioclase+K-feldspar?argillaceous?and garnet+K-feldspar+plagioclase+biotite+quartz?feldspar?;D₃ deformation stage:NE-trending steeply inclined ductile-brittle fractures are mostly filled by granite porphyry veins and Inhomogeneous distribution of ductile shear zones,symbiotic mineral assemblages are garnet metamorphic phenocrysts and their retrograde edge minerals,garnet+sillimanite+cordierite+plagioclase+biotite+quartz+K-feldspar?argillaceous?and amphibole+plagioclase+biotite+ilmenite?feldspar?.Combining with previous research results,it is shown that the peak metamorphism degree of Indosinian metamorphic event of Badu Group has reached granulite facies.Showing clockwise near isothermal decompression?ITD?type P-T evolution track,The metamorphism and deformation sequence reflects the evolution process from subduction collision to rapid reentry cooling.Zircon U-Pb dating results of granite porphyry restrict the completion time of the evolution process to 229.7Ma,which may be the response of Southwestern Zhejiang to subduction-collision process between the Indosinian-South China-North China plate in the Paleotethyan Ocean during Indosinian.