Petrogenesis And Tectonic Significance Of The Early Jurassic Volcanic Rocks In Sumdo Area,Tibet

The whole process of the opening,evolution and closure of the New-Tethys Ocean,and the docking and collision between India and Eurasia after this process,is an important subject in the study of the pre-Cenozoic evolution of the Qinghai-Tibet Plateau.Magmatism is an important tool for exploring the deep processes and material composition of the Earth,and the Jurassic-Cretaceous magmatism of the Gangdise plate is inseparable from the evolution of the Neo-Tethys Ocean.The magmatism of the Jurassic-Cretaceous in the area is also a relatively important metallogenic period on the Gangdise plate of the Qinghai-Tibet Plateau.Therefore,the study of magmatism during this period also has important prospecting significance and economic value.For a long time,the predecessors have always argued about the genesis of the Early Jurassic volcanic rocks.The understanding is not uniform.In summary,there are two main viewpoints: one view is that the Early Jurassic volcanic rocks are island arc volcanic rocks subducted northward by the New-Tethys Ocean,and the subduction time is before the Early Jurassic.;another view is that the Early Jurassic volcanic rocks are the products of the back-arc basin of the Bangong Lake-Nujiang Tethys Ocean southward subduction.Based on the geological survey and the measured geological section of the Sumdo area in southern Tibet,this paper selects the Early Jurassic volcanic rocks for the whole geochemical analysis of the rocks and discusses the petrogenesis and tectonic significance of the Early Jurassic volcanic rocks.The Early Jurassic volcanic rocks are a series of lithologic continuous calc-alkaline volcanic rocks,including the Yeba Formation volcanic rocks and the Sangri group Bima Formation volcanic rocks.The rock combination is basalt-basalt andesite-andesite-dacite,and the rock is slightly altered.Among them,the volcanic rocks of the Yeba Formation are dominated by medium-acid rocks,and a small amount of slate interlayers are visible locally,and impression fossils are visible in the slate;The volcanic rocks of the Sangri group Bima Formation are dominated by basic basalt and basalt andesite,with a small amount of dacite.The fossil samples found in the slate interlayer of the Yeba Formation volcanic rocks are similarly identified and compared with the bivalve fauna of the Yeba group in the XiaBolang section reported by the predecessors,belonging to the animals of the Early Middle Jurassic Tethys Group.The zircon age of the dacite in the Yeba Formation volcanic rocks is 177 Ma,the zircon age of the Sangri Group tuff is 189 Ma,and the zircon age of the Andesite is 195 Ma,it can be seen that the Yeba Formation and the Sangri Group are both Early Jurassic.The Yeba Formation volcanic rocks and the Sangri Group Bima Formation volcanic rocks have similar geochemical properties and all exhibit typical island arc magma characteristics,enrichment of LREE and LILE(Th,U),loss of MREE and HFSE(Nb,Ta).The volcanic rocks of the Yeba Formation may be derived from the partial melting of the lower crust rich in hornblende.There is a certain source of mantle source in the source area,and the crystallization evolution process undergoes a strong crystallization differentiation effect.The volcanic rocks of the Sangri Group may be derived from the crystallization differentiation of basic magma.Based on the previous data,this paper believes that the Early Jurassic volcanic rocks formed in the northward subduction environment of the Neo-Tethys,in which the Yeba Formation volcanic rocks were formed in the continental marginal arc environment,while the Sangri Group was formed in the Oceanic Island arc environment.And the time limit for subduction should be no later than Early Jurassic.