Sedimentary Characteristics And Tectonic Significance Of The Upper Triassic Wuga Formation In The Nima Region Tibet

The Bangonghu-Nujiang Suture Zone,which divides the Lhasa and Qiangtang parcels into two parts: Lhasa to the south and Qiantang to the north,is a concerned suture zone in the middle of the Qinghai-Tibet Plateau.Previous studies on the Bangonghu-Nujiang suture zone are controversial,Such as the ocean's opening time,closing time,and the subduction polarity of the Bangonghu-Nujiang Ocean,especially the open time.There exists various opinions about the open time,such as Devonian,Carboniferous-Early Triassic,Late Permian-Early Triassic,etc,Opened before the Late Triassic.And late Triassic,early Jurassic-late Jurassic opened into the ocean.The Upper Triassic Wuga Formation stratum is widely distributed in the Bangonghu-Nujiang suture zone,Is one of the main stratum within the suture zone.In this article,we choose the Upper Triassic Wuga Formation in the west of the Nima area to be the study object,as it developments continuously and has clear depositional sequences.Lithology analysis,lithofacies analysis and the sandstone grain size analysis method are used to analyze the sedimentary facies of the Wuga Formation;the sandstone clastic composition statistics,cathodoluminescence analysis and the Dickinson triangle plot are used to analyze the provenance;then,the property of the Late Triassic Banggonghu-Nujiang Ocean is also been discussed.Thus,we have the following understandings:The Wuga Formation in the Nima area is mainly a set of Shallow sea sediment,furthermore,which can be divided into inland shed sediments that is consisting mainly of sandstone and mudstone,and outland shed sediments that is consisting mainly of sandstone,mudstone and carbonate.Inland shed sediments are mainly medium-fine sandstone,coarse sandstone and a small amount of pebbly coarse sandstone,or a small amount of siltstone and argillaceous siltstone.They are usually yellow-brown and yellow-gray,and develop horizontal and parallel bedding well.Outland shed sediments are mainly still water sediments,such as dark silty mudstones,mudstones,shales,argillaceous limestones,and a small amount of fine-silty sandstone.they develop horizontal and small wavy bedding.Microscopic microscopy analysis shows that the detrital components of sandstone are mainly single crystal quartz with mineral detritus,a small amount of polycrystalline quartz,feldspar dominated by obvious polycrystalline double crystals,and heavy mineral debris such as zircon.The rock debris is dominated by sedimentary rock debris,including sandstone debris,mudstone debris,vermiculite rock debris and a few carbonate rock debris;magmatic rock debris is second most rock debris,it mainly consists of medium-acidic ejected rock;only a few metamorphic rock debris was found,of which metamorphic quartzite is the main debris,in addition,a little phyllite and slate cuttings were included.The statistical results of clastic rock composition show that the types of sandstone in the Wugacuo section are mainly feldspar lithic sandstones,a few lithic sandstones,lithic quartz sandstones and lithic feldspar sandstones.By tracking the contemporaneous volcanic rocks in the adjacent area,we believe that The Nadigangri Island arc volcanic rock of the Tangti block may be the major contributor to the acidic volcanic rock debris of the Wuga Formation.The Dickinson triplot of clastic component in the study area shows the tectonic setting of the provenance area is the reconstructed orogenic belt and the island arc orogenic belt,combining with the comparison between the detrital components of the study section and the neighboring area,the provenance area of the Wuga Formation in the Nima area is considered to be the Qiantang block,which is adjacent to the north of the Nima area.The Bangonghu-Nujiang Tethys Ocean has developed to a mature ocean in late Triassic,the extension time should be obviously earlier than the Late Triassic.The sedimentary facies of Wuga formation in Nima area and the late Triassic South Qiangtang formation are the continuous transitional relationship.It should be the continuous deposition of the sea at the edge of South Qiangtang.