Geochronology And Tectonic Setting Of Mafic Rocks In The Dingqing Ophiolite In The Eastern Segment Of The Bangongco-Nujiang Suture Zone In Tibet

The Dingqing ophiolite is located in the eastern section of the Bangong LakeNujiang suture zone and is divided into the eastern and western rock mass.At present,there has not been a systematic chronology and tectonic environment study on the western rock mass.In this paper,basalt,gabbro and diabase in the Zongbai ophiolite mélange on the east side of the western rock mass are selected as the research objects.Detailed field geological survey and indoor mineralogical research are carried out on them,using LA-ICP-MS zircon U-Pb dating and the main and trace elements of the whole rock,combined with the results of the regional survey,explored the time-space frame and tectonic environment of the mafic rocks in the Zongbai ophiolite mélange in Dingqing.The evolution of the Tethys Ocean in the eastern section has been further restricted.The main knowledges gained in this paper are as follows:(1)Basalt is produced by the interlayer of basaltic lava and tuff,which is in structural contact with the early Jurassic sedimentary rocks;Zircon U-Pb age of basalt is 198.7±3.8Ma,which is early Jurassic;It is believed that basalt may be formed in the ocean island environment related to the mantle plume,which is not a member of the Dingqing ophiolite.(2)The gabbro intruded into the early Jurassic sedimentary rocks as veins,and its zircon U-Pb age is 164.3±2.6Ma,which is the Middle Jurassic;It is believed that the gabbro was formed in the continental rift environment.During the formation it was affected by the combination of the LVZ(Low Velocity Zone)enrichment component at the bottom of the lithosphere and the upwelling asthenosphere,which is not a member of the Dingqing ophiolite.(3)The diabase is in tectonic contact with the early Jurassic sedimentary rocks,and its zircon U-Pb age is 114.2±1.3Ma,which is the Early Cretaceous;The diabase has the characteristics of N-MORB and E-MORB,It is thought that it may be formed in the back-arc expansion ridge environment affected by the mantle plume,and it does not belong to the member of the Dingqing ophiolite.This study initially reported the progress of the petrology and chronology in the western rock mass,which filled the gap of the ophiolite research in this area.This result provides new evidence for understanding the history of the formation and evolution of Dingqing ophiolite,but more field investigations and data are needed to support the overall evolution model of the Dingqing ophiolite formation.