Granulite-facies Metamorphism And Tectonic Setting Of The Dadongcha Formation In Ji'an Group,Tonghua Area

The study on the metamorphism and cause of the metamorphic rock series in Ji'an Group is of great significance for the tectonic setting of the Jiao-Liao-Ji Belt.In this study,the metapelites of the Dadongcha formation in Ji'an Group have been studied systematically in petrography,petrogeochemistry,mineral chemistry and geochronology.The analyzed metapelites include garnet-biotite schist,garnet-biotite-plagioclase gneiss,garnet-sillimanite-cordierite-biotite gneiss,and hypersthene-spinel-cordierite-biotite gneiss.In some rocks,we can see that garnets contain mineral inclusions such as biotite,feldspar and quartz,a large number of minerals such as sillimanite aggregates and biotite in the matrix,the typical cordierite+sillimanite+quartz reactive edge structures around the garnet.The geochemical data of the metapelites indicate that,the chondrite-normalized rare earth element(REE)pattern of analyzed samples displays characteristics of medium fractionation in light and heavy REEs,and strong enrichment in light REEs.The primitive mantle-normalized trace element pattern shows enrichment of large ion lithophile elements(LILEs),and the content of high field strength elements(HFSEs)such as Zr and Hf is relatively low;furthermore,Nb,Ta,P,and Ti are distinctly depleted.Combined with the protolith discrimination diagram and tectonic setting discrimination diagram,the protoliths of the metapelites in Dadongcha formation are clay rocks,which were formed in the active continental margin or island arc environment.According to the petrographic and mineral chemical characteristics,the granulite facies metamorphism of the metapelites in Dadongcha formation can be divided into three stages,the mineral assemblage of peak stage(M₂)is Grt+Sil+Bt+Pl+Kfs+Pth+Qz,the mineral assemblage of post-peak stage(M₃)is Grt+Bt+Sil+Crd+Qz+Pl±Kfs.The results of the traditional geothermometer and phase equilibrium modeling show that,the P–T conditions of peak stage are as follows:?0.6-0.7 GPa/?800-827?,the P–T conditions of post-peak stage are as follows:?0.5-0.6 GPa/?620-760?,which shows a clockwise P–T path as a whole,which is related to a collision.The results of zircon isotopic chronology show that the protoliths of the metapelites in Dadongcha formation was formed later than 2.02Ga,and granulite facies metamorphism occurred in 1.8-1.95Ga,accompanied by?1.84 Ga anatexis.To sum up,the metapelites of Dadongcha formation were formed in an active continental margin environment where the evolved island arcs developed around1.95–2.02 Ga,then were involved in the collision between the Longgang and Langrim blocks in the eastern part of the NCC,and returned to the surface after undergoing regional metamorphism with anatexis around 1.8–1.95 Ga.