Genesis And Tectonic Setting Of Basic Rocks From Ophiolite And Intermediate-acid Rocks In Dongqiao, Tibet

The study of the Bangong Nujiang suture zone is very important to the structural evolution of the Qinghai Tibet Plateau,and there are still arguments about the subduction direction,basin nature and closure time of the middle Tethys ocean represented by the ophiolite in the middle of the BNSZ.In addition,there are few reports of island arc type volcanic rocks under subduction and subduction environment in BNSZ.In this study,diabase and basalt of Dongqiao ophiolite,andesite and granite in subduction environment are selected as research objects to carry out detailed field geological survey,indoor mineralogy and petrology research.Using LA-ICP-MS zircon U-Pb dating,major and trace element analysis of the whole rock,combined with previous research results in BNSZ,the genesis and tectonic environment of Dongqiao ophiolite and intermediate acid rock are discussed,and the closing time of ocean basin is limited.The main knowledge of this paper is as follows:?1?The ages of zircon U-Pb dating in diabase and basalt of Dongqiao ophiolite show a large range of changes.Combined with previous research results,they are divided into three groups.The first group?156-239Ma?represents the age of the rock itself,and the second group?37-141Ma?is later than the age of the rock formation.It is speculated that the formation is due to the later magmatic events,and the third group?277-2454Ma?It is speculated that zircon in the early subducted crust remained in the primitive mantle is responsible for the formation of the rocks.?2?According to the geochemical data of ophiolite in Dongqiao basalt,the source area is spinel lherzolite,diabase has the characteristics of IAT and N-MORB,and basalt has the characteristics of E-MORB.Based on the comparison of Dongqiao mantle peridotite and other ophiolites with similar characteristics in Bannu belt,and the comprehensive geochemical analysis,it is considered that Dongqiao diabase and basalt are produced in the back arc basin of the ocean and affected by subduction fluid / melt to some extent.?3?The andesite are rich in large ion lithophile elements,deficient in high field strength elements and characterized by arc magmatic rocks.Zircon shows that the age of its formation may be 122 ma.Combined with geochemical diagrams,it is believed that the overlying mantle wedge was partially melted when the Qiangtang block on the north side of the ocean crust of Bannu belt subducted,and mixed with crustal materials during magma upwelling.?4?The A/CNK value of Dongqiao granite is less than 1.1,and the main element P₂O₅ is negatively correlated with SiO₂,showing the characteristics of I-type granite.The rock samples are rich in large ion lithophile elements,lack of high field strength elements,and have the characteristics of arc magmatic rocks.?5?Based on the results of predecessors and this paper,it is inferred that the middle part of the middle Tethys ocean opened before the Late Triassic and began to subduct northward in the early Jurassic.The back arc basin was formed in the northernmost Dongqiao area.It may have been closed in 114.5-117.5ma until about 110 mA entered the post collision extension stage.