Study On The Material Composition And Structural Deformation Of Metamorphic Strata In Dexing-yiyang Area,northeastern Jiangxi

Our research area locates in the northeast of Jiangxi province,eastern part of the Qin-Hang joint belt,which is an giant tectonic melange zone stitching Yangtze and Cathaysian plate within South China continent.So,it's a significant window to study the tectonic evolution of south China block.Around this area,there are many low-grade metamorphic stratum such as Shuang Qiaoshan group,Zhangcun group,Xikou group,and He Shangzhen group,whose stratigraphic age and nature define the properties of the Qin-Hang joint belt.For this reason,we aimed at Zhangcun group and He Shangzhen group in this paper,researched their tectonic framework,rock composition,deformation and metamorphic characteristics through the detailed field investigation and profile measurement.Using techniques such as LA-ICP-MS Zircon U-Pb Dating,Electron Backscatter Diffraction,⁴⁰Ar/³⁹Ar Dating and Geochemical Analysis,we found out their deformation mechanisms,forming ages and forming environments,and discussed the geological evolution in the area.The major conclusions are:Zhangcun group is the essential substance of ophiolite mélange zone in norestern Jian Xi,and is divided into two parts:blocks and matrix.The LA-ICP-MS zircon U-Pb dating of blocks give us the two ages:769±2Ma for andesite and 833±5Ma for gabbro.The geochemical characteristics of andesite show the affinity to island arc.U-Pb age data of detrital zircon grains from the matrix rocks show the peak age focus on 850?920Ma and 882?925Ma,average ages are 896±5Ma and 900±5Ma,in addition,we get the age 483Ma from single zircon.We find there are at least three structure activities in the mélange zone:NW-SE terrene connection and accretion;NE-SW sinistral strike-slip movement and NNE-SSW right-lateral shear and thrust.The muscovite ⁴⁰Ar/³⁹Ar dating yields the plateau age of 468Ma and a total age of473Ma standing for the age of NW-SE thrusting action in the caledonian epoch.It is showed that He Shangzhen group in northeastern Jiang Xi and western Zhejiang are quite different.We regrouped the He Shangzhen group in northeastern Jiang Xi,and divided it into Luo Jiamen formation,Shangshu formation and Hong Chicun formation from bottom to up.The geochemical characteristics of volcanic rock in Shangshu formation showed they formed in the island-arc environment.The ²⁰⁶Pb/²³⁸U weighted average age of rhyolite rocks are 772Ma±2and 829±5Ma.Volcanic rocks in Shangshu formation are the material source for Hong Chicun formation which formed in the forearc basin system.The²⁰⁶Pb/²³⁸U weighted average age of granite in Shangshu formation is about 880Ma,the granite existed in the form of tectonic block in the arc complex belt.We also studied the cretaceous acidic volcanic rocks that isolated from the Shangshu formation this time.We observed the characteristics of paleozoic sedimentary rock in the area too,in order to provide the constraints.Through these research above,we got the conclution that the accretionary orogenic process in northeastern Jiang Xi started since neoproterozoic and continued to move southeast,the mélange belt completely formed in early Paleozoic later.