Paleogene Detrital Zircon U-Pb Chronology Of The Nangqian Basin,eastern Tibetan Plateau And Its Implications

The collision and extrusion between the Indian plate and the Eurasian continent formed the highest altitude and largest area on the earth,known as the"third pole"of the Tibetan Plateau.The formation process of the Tibetan Plateau not only records the continental collision process,lithosphere and crustal deformation law(continental dynamics law),but also its uplift has greatly changed the landform,geomorphology and climate characteristics of inland Asia,and finally formed the present pattern of mountain landform and river system distribution.Therefore,the formation,evolution,deformation process and uplift mechanism of the Tibetan Plateau have always been the focus of geoscience research.The Nangqian Basin is a typical representative of a series of Cenozoic continental basin groups distributed along the Hengduan Mountain-Jinshajiang suture zone in the eastern part of the Tibetan Plateau.The thick Paleogene Gongjue formation is widely distributed in the basin,which contains the information of basin evolution and tectonic activities of adjacent orogenic belts.According to basin-coupling relations,this article mainly through to the eastern Tibetan plateau capsule modest basin in the Cenozoic Gong Jue the different formation of detrital zircon U-Pb chronology study,combined with basin sedimentary facies,paleocurrent direction,gravel composition and predecessors'studying results,discusses the capsule modest basin provenance change and regional tectonic evolution in paleogene,mainly from the following:(1)Based on field investigation,stratigraphic correlation and six controlled measured sections in Nangqian Basin,the Cenozoic filling sequence of the basin has been sorted out,and the Cenozoic Gongjue Formation strata can be divided into three sections from bottom to top:The lower(E_1-2a),mainly outcropped in the north and west of the basin,is composed of a set of red conglomerate,sandy conglomerate and sandstone,which is unconformable over the Carboniferous system and belongs to alluvial fan-fluvial facies.The middle(E₂b),widely distributed in the basin,is mainly composed of lacustrine purplish mudstone,siltstone intercalated with gray-green paste rock and marl,and belongs to lacustrine sedimentary products.The upper(E_2-3c),mainly exposed in the southern part of the basin,belongs to the alluvial fan facies,and belongs to the Late Eocene to Early Oligocene(after 35Ma).(2)The detrital zircon U-Pb ages of the Gongjue Formation in the Nangqian Basin are mainly distributed in the range of 200-500Ma,with a small amount of 52-35Ma and>600Ma.The dominant peaks are concentrated in the range of 200-300Ma and 400-500Ma,and the provenance is mainly from the eastern Qiangtang terrane.Combined with ancient flow;during capsule modest in paleogene basin changed three source:52Ma before early Eocene source mainly basin in the west-southwest of carboniferous-Triassic strata,Eocene mainly for 52-35Ma source mountain and volcanic eruption products during this period,the basin peripheral late Eocene to Oligocene provenance came mainly from northern basin after 35Ma mountain.(3)The Nangqian basin experienced three tectonic events in Paleogene.The Yushu-Nangqian fold belt was deformed and uplifted by regional tectonic activities around 63 Ma in Paleocene,and the foreland basin of Nangqian basin began to be deposited by near-source diluvial fan-braided river.The Nangqian Basin was transformed from a foreland basin to a pull-away basin by regional tectonic activity around 52Ma in the Early Eocene,and the sedimentary environment was transformed from diluvial fan-braided river to lacustrine sediment.The provenance of 52-35Ma was mainly from the basin periphery and volcanic eruptions in the same period.The basin was uplifted by regional tectonic activities during the late Eocene and early Oligocene,resulting in the transition from lacustrine facies to near-source diluvial fan-braided fluvial deposits.