Sedimentary Records And Provenance Analysis Of The Early Jurassic Basin In Tangjia Area,Tibet

The volcano-sedimentary formation of the Mesozoic in the southern Lhasa Terrane are closely related to the subduction of Neo-Tethys.In recent years,domestic and foreign scholars have conducted in-depth research on the Lhasa Terrane,and achieved a lot of pioneering achievements especially in the mesozoic-cenozoic magmatic rocks and Tectonic evolution.However,there are few studies on the sedimentary records of the Early Jurassic period.The strata in the study area are located in the northern part of the south Lhasa.The Jialapu Formation records the information of sedimentary environment,material origin,development history and tectonic evolution of the early Jurassic basin,which is of great significance for determining the tectonic properties of the basin and limiting the subduction time of Neo-Tethys.Based on the detailed field geological survey,in the context of the regional tectonic background and evolutionary,we have identified the basin tectonic properties,sedimentary environments,the time of deposition and the main source area of the Jialapu Formation by using the method of geology,sratigraphy,detrital zircon geochronology and geochemistry and so on.The research has shown that the formation of the Jialapu Formation was formed in the retroarc basin at the back of the Neo-Tethys ocean subduct to the northern terrane,and the sedimentary environment is a Marine transitional phase dominated by Fan delta.The rock assemblage is mainly composed of clastic volcanic rocks,and the volcanic rock interlayer preserved record of volcanic arc in the southern Lhasa area are.The presence of a limestone layer in a local area may represent a small scale of Marine invasion.The youngest U-Pb Detrital zircon age of the Jialapu Formation through the LA-ICP-MS is ~181Ma,which constrained the time of deposition,and the age of U-Pb zircon of andesite is 197.1 + 1.5Ma(MSWD=0.77).The characteristics of the main trace elements show that the volcanic rock is a potassium calc-alkaline series,rich in Rb,Ba,Pb and other large ionic lithophile elementsand the loss of Nb,Ta,Ti and other elements relatively,the Eu is weak and negative,showing the affinity of arc volcanic rocks,and the tectonic environment is close to the continental island arc on the edge of the active continent.Sedimentary rocks are rich in Rb,Nb,U and other large ionic lithophile elements,and losses Ba,Nb,Ta and other high field strength elements;Chondrite standardized distribution curve has the right mode,rare earth elements indicate source area is related to volcanic island arc;It is also indicated that the sedimentary rocks have the characteristics of volcanic rocks with island arc properties.Based on all of above,the provenance of Jialapu Formaion mainly come from volcanic magmatic activityies of the late Triassic and early Jurassic material in sourthern Lhasa Terrane,at the same time accompanied by a small amount of ancient basement of sedimentary recycle material mix.The Neo-Tethys subducted northward not later than the early Jurassic.