The Genesis And Metamorphic Evolution Of Garnet Amphibolite Monzogneiss From The Suichang-Dazhe Region In Southwestern Zhejiang Province

The Suichang-Dazhe area in Southwest Zhejiang is located in the eastern margin of the Cathaysian Block.The main terrane in the area are Paleoproterozoic Badu Group.The Badu Group groups are mainly composed of a set of metapelitic rocks and metagreywack.It's the oldest,higher metamorphic degree and most complicated metamorphic rocks in Cathaysian Block.It has great significance to study the crust formation and tectonic evolution of the whole Cathaysian Block.The most widely exposed rocks of the Badu Group in Suichang-Dazhe area is the mozogneiss containing garnet and hornblende.It has a typical garnet"white eye socket"reaction structure.The previous studies on it are mostly concentrated on chronology and geochemistry,but the study of microlithology and mineral chemistry is still relatively weak.The nature and age of the metamorphic events reflected by the"white eye socket"structure are not clear.In order to understand its genesis and metamorphic evolution,this paper has carried out a comprehensive study of the mineral X ray microanalysis,LA-ICP-MS zircon U-Pb dating and main and trace elements of the rock.A new understanding of the following aspects is obtained:Three generations of mineral assemblages have been recognized: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 shows that the metamorphic conditions of the garnet amphibolite monzogneiss are 600⁷00oC/0.60⁰.65GPa for the M1 stage,800⁸20oC/0.94⁰.96GPa for the metamorphic peak M2stage,and 550⁷00oC/0.56GPa⁰.71GPa for the retrograde M3 stage,respectively.Metamorphic grade reached granulite facies and metamorphism PTt Paths 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 and cantain some mantle-derived components,Formed in continental crust extensional environment during post orogeny.LA-ICP-MS zircon U-Pb dating indicate two-stage age characteristics,the diagenetic age of 1.83-1.85Ga suggests the protolith of monzogneiss formed in the Paleoproterozoic and Metamorphic age of 220-230Ma showing the reworking of Indosinian metamorphism to Paleoproterozoic granite.It is the response to the Subduction of Indo Pacific plate toward to South China plate during Indosinian and also provides new evidence for Indosinian Orogeny in the Southwestern Zhejiang.