The Research Of Geochemical Characteristics And Geochronology Of The Dengshan Group Volcanic Rocks In Northeastern Jingxi Area And It Is Geology Significance

The Northeast Jiangxi fault zone is an important tectonic boundary of the southeast margin of Yangze plate,which is the boundary between the Jiuling terrane and the Huaiyu terrane in the eastern section of the Jiangnan orogenic belt.Along the fault zone,the Meso-Neoproterozoic mafic-ultramafic rock blocks are widely distributed,and they form NNE Northeast Jiangxi ophiolite belt.It is one of the few Precambrian ophiolites in south China,which contains important information about the tectonic evolution of jiangnan orogenic belt and the Precambrian in south China.Some of the Dengshan Group are also involved in the Northeast tectonic belt,showing the strong characteristic of melange.Its ductile shear is developed,and the tectonic track is consistent with the ductile shear zone in the ophiolite melange,and a large number of intermediate-basic volcanic tectonic lenses are mixed in the shear zone.Clastic rocks of phyllite has undergone a strong metamorphosis.The metamorphic characteristics of the Dengshan Group are similar to the ophiolite melange in the area of the Lieqiao,and they have the melange rocks structural characteristics on the macro level.This article is based on the previous data,the geological survey of Dengshan Group that exposed in northeast Jiangxi Province is carried,which combined with the identification of rock microsection,geochemical analysis and LA-ICP-MS zircon U-Pb dating.According to the characteristics of petrological,geochemical and geochronology,The source area and it's tectonic evolution of Dengshan Group in northeast Jiangxi are discussed.The main research are as follows:(1)The stratum of Dengshan Group in northeast can be divided into three sections: The lower Zhujia Formation is composed of Purple red tuffaceous lithic sandstones,tuffaceous fine sandstone,tuffaceous slate and a rhythmic interlayer,grayish purple conglomerate at the bottom.In the middle Bazhukeng Formation,the lithological combination include of medium fine grained lithic sandstone,tuffaceous siltstone,sedimentary tuff and silty phyllite with basic lava intercalation.The upper Yejia Formation is a set of continental volcanic rocks,composed of a large number of lava and pyroclastic rocks,which is mainly composed of basalt,andesite,and mixed rhyolite,tuff and sedimentary tuff.(2)Geochemistry shows,the Dengshan Group volcanic rock belongs to calcium alkaline basalt series,which is characterized by high potassium,low sodium,medium titanium and high aluminum.In the rare earth elements curves,the number of ?LREE/?HREE is between 3.82 to 4.58,with inclines to the right deviation of rich in light rare earth element and lack of heavy rare earth element.The aggregate performance of microelement is large ion lithophile element,with the characteristic of high field strength element depletion are not insignificant,such as Nb?Ta and significant depleted in Eu?Sr?Ti.The dilution of Ti shows the characteristic of the origin of upper mantlel.The positive abnormal of Pb is maybe produced from earthcrust union dyeing.The ratio of relatively stable elements(Ta/Yb-Th/Yb)shows all samples are falling into the continental margin(continental arc)area,Therefore,the trace elements of volcanic rocks samples are roughly equivalent to the characteristics of active continental arc volcanic rocks.(3)The age of the zircon U-Pb of tuff LA-ICP-MS is 853±10Ma,which is obtained from the lower Zhujia Formation of Dengshan Group.The age zircon U-Pb of rhyolite is 788±10Ma,which is obtained from the upper Yejia Formation.Both of them represent the crystal age of rhyolite and tuff.Therefore,it can conclude that the time limit for Dengshan Groups should be between 790Ma-850 Ma.(4)Between 860-790 Ma is the continuous dive hyperplasia period.The oceanic crust is further dived to the Yangze plate,the oceanic arc region is transformed into a continental arc environment.The continental arc magmatism occurred during the period between 860-790 ma and deposited the volcanic sedimentary sequence of Dengshan Group in the accretary wedge basin.Dengshan Group volcanic rocks occurred.Combined with the geochemical characteristics of the volcanic rocks in the Dengshan Group,the oceanic crust is still in the subduction stage,and the time of the subduction end of the oceanic crust can be further considered to be delayed to 790 Ma.