Genesis and Metamorphic Evolution of Garnet Amphibolite Monzogneiss from Suichang-Dazhe Region in Southwestern Zhejiang Province

Zhou Xiao; Zheng Changqing; Zhou Xiwen; Dong Yunfeng; Wang Zhaoyuan; Song Yang; Han Xiaomeng

Abstract：There is a set of garnet amphibolite monzogneiss which developed typical garnet "white eye socket" structure from the Badu Group in Suichang-Dazhe region,southwestern Zhejiang Province.However,its genesis and metamorphic evolution remain unclear.In this study,mineral X-ray microprobe microanalysis,LA-ICP-MS zircon U-Pb dating and whole-rock major and trace element analyses were carried out to address these questions.Three stages of mineral assemblages have been identified:the prograde metamorphic assemblages (M1) are mineral inclusions garnet + amphibole + plagioclase + quartz (Grt1 +Amp1 +Pl1 +Q) preserved in the garnet porphyroblasts,the peak metamorphic assemblages (M2) are represented by the "garnet-mantles" and the matrix minerals garnet+ K-feldspar + plagioclase + biotite+ quartz (Grt2 + Kf2 + Pl2+Bt2 +Q),whereas the retrograde assemblages (M3) are composed of amphibole + plagioclase+ biotite+ ilmenite (Amp3 + Pl3 + Bt3 + Ilm)symplectites rimming the garnet porphyroblasts.Thermobarometric computation and phase equilibrium modeling show that the metamorphic conditions of the garnet amphibolite monzogneiss are 600 700 ℃/0.60-0.65 GPa for the M1 stage,800-820 ℃/0.94-0.96 GPa for the metamorphic peak M2 stage,and 550-700 ℃/0.56-0.71 GPa for the retrograde M3 stage,respectively.Metamorphic grade reached granulite facies and metamorphism PTt paths are characterized by clockwise pattern involving the nearly isothermal decompression (ITD) segments,indicating crustal thickening and rapid exhumation process.Petrogeochemistry shows that the protolith of monzogneiss is metaluminous A-type granite with some mantle-derived compo nents which formed in continental crust extensional environment during post orogeny.LA-ICP-MS zircon U-Pb dating indicates two-stage age characteristics,in which the diagenetic age of 1.83-1.85 Ga suggests the protolith of monzogneiss formed in the Paleoproterozoic and the metamorphic age of 220-230 Ma shows the reworking of Indosinian metamorphism to Paleoproterozoic granite.It is the response to the subduction of Indo Pacific plate into South China plate during Indosinian,which provides new evidence for Indosinian orogeny in the southwestern Zhejiang.