Neoproterozoic Evolution Of The Western Jiangnan Orogen

The Jiangnan Orogen is the joint zone and an important tectonic hinge between two ancient plates:the Yangtze Block and the Cathaysia Block.It is of great significance to investigate the evolution of the Jiangnan Orogen during the Neoproterozoic era for understanding the tectonic history of Southern China.However,currently available evidence regarding the diagenetic age and diagenetic tectonic setting of diabase in the western Jiangnan Orogen have been inconsistent.Xuefengshan area is an important part of the western Jiangnan Orogen,and the widely distributed Neoproterozoic Banxi Group strata,as a volcano-sedimentary sequence developed in the Nanhua Rift System,recorded important magmatic and tectonic events related to the convergence and breakup of the Rodinia supercontinent.Therefore,the Neoproterozoic tectonic pattern and evolution in the Xuefengshan area is the key to revealing the evolution and accretion process of the Jiangnan Orogen and South China Continent.In order to save this problem and provide more insights into the Neoproterozoic tectonic pattern and evolution,this study focused on the geochemistry and detrital zircon chronology of clastic rocks from the Jiajiantian Formation,Gaojian Group,and Wuqiangxi Formation,Duoyitang Formation,Banxi Group in the Xuefengshan area,to the Northwest of Hunan province.Baddeleyite geochronology,as well as the geochemical characteristics of minerals（biotite,pyroxene,and ilmenite）of the Huaihua diabase intruding into Gaojian Group,were also explored in this study.Overall,the tectonic evolution pattern of the Xuefengshan area during the Neoproterozoic era was thoroughly discussed.The source areas of greywacke from Gajian Group and lithic sandstones from Banxi Group were all located in the east of the Yangtze Block,and their detrital materials were mainly upper crust felsic rocks.Among them,Greywackes from Gaojian Group contained many ancient sedimentary materials,and the rocks were formed in passive continental margin environment.Lithic sandstones from Banxi Group contained more island arc volcanic materials and has the characteristics of forming in continental island arc environment in trace element composition.There are three U-Pb age peaks of～805 Ma,1990 Ma,and 2470 Ma in the detrital zircons of the greywacke.The detrital zircons in～805 Ma primarily record the Neoproterozoic rift magmatism of the western Jiangnan Orogen.There are also three U-Pb age peaks of 845～830 Ma,2000 Ma and 2500 Ma in the detrital zircons of the Lithic sandstone.The detrital zircons in 845～830Ma primarily record the Neoproterozoic Island arc magmatism in the western Jiangnan Orogen.The Neoproterozoic zircons in these abovementioned two types of clastic rocks are affinitive to the continental arc in terms of their constructive properties.Among them,the Y content of detrital zircon in the graywacke（～805 Ma）is high,characterized by the origin of intraplate magma.The content of Y in Neoproterozoic detrital zircons in clastic rocks increases with time,similar to the change of tectonic environment from MOR to WPB.The Paleoproterozoic zircons have a different geochemical composition from the known old basement in the Yangtze Block,indicating that there may be concealed basement of the Columbia era under the western Jiangnan Orogen.The chronology and Hf isotopic geochemistry of detrital zircons indicate that there were three important crustal growth events in Xuefengshan area.The Neoproterozoic（～0.8 Ga）crust growth was characterized by the addition of a small amount of mantle material or/and new crust during the Neoarchean crust remelting.The crust growth in Paleoproterozoic（～2.0 Ga）was dominated by Meso-Paleoarchean crust remelting.The Neoarchean（～2.5 Ga）crustal growth included Meso-Paleoarchean crustal remelting and new crustal formation.The Huaihua diabase is a kind of alkaline basalt with low potassium characteristics and mixed with the crust.The trace elements of the Huaihua diabase were similar to the Ocean Island basalt（OIB）,which was enriched in LREE and LISE,depleted of HREE and slightly depleted HFSE.The SR-Nd isotopic compositions as follows:⁸⁷Sr/⁸⁶Sr_（i）=0.708416～0.715073,¹⁴³Nd/¹⁴⁴Nd_（i）=0.512265～0.512417,ε_Nd（t）=-0.74～2.33,indicated that the magma is not contaminated by the crust.Baddeleyite U-Pb dating shows that the Huaihua diabase was formed in the middle Neoproterozoic with an emplacement age of 788±5.4 Ma.The mafic rocks at 900～800 Ma in the study area were all formed in subduction-related tectonic environment,and the magma originated from mantle wedge.The Huaihua diabase originated from 1-10%partial melting of the pyroxenite asthenosphere mantle dominated by garnet,and its mantle source area was metasomatized by subduction plate melt.Represented by the Huaihua diabase,a series of Bimodal alkaline rocks at 800～750 Ma in the study area were formed in the thickened lithospheric mantle of the late orogenic belt,which served as the tectonic setting for plate fracture or delamination of the intra-continental rift,and could serve as an important symbol of large-scale asthenosphere mantle upwelling and the transition from orogenic tectonic environment to post-orogenic tectonic environment.Comprehensive analysis shows that the tectonic evolution of the Jiangnan Orogen reflects the tectonic regime transition from the Early Neoproterozoic Cathaysia Block to the Yangtze Block subduction（860～825 Ma）,to the Middle Neoproterozoic collision（825～805 Ma）,and finally to the intra continental rift magmatic activity（780～740 Ma）.The variation of trace element content and ratio of Neoproterozoic detrital zircons in the detrital rocks suggests the evolution and transformation of tectonic system in the western Jiangnan orogen,and the determination of diagenetic age and diagenetic tectonic setting of diabase have defined the dynamic environment in the late Early Neoproterozoic Jiangnan Orogen.The tectonic evolution of the Jiangnan Orogen in the Neoproterozoic was constrained and limited by detrital zircons and diabase,which provided a new window for re-examining the tectonic evolution pattern of the Jiangnan Orogen in the Neoproterozoic.