Research On Volcanic Rocks Of Duoni Formation In The West Of The Bangong Lake-Nujiang Metallogenic Belt

The Bangong Lake-Nujiang(BN)suture zone is not only an important structural boundary,but also an important metallogenic belt.The in-depth study of the BN metallogenic belt can provide important clues for understanding and inverting the evolution of the Tethyan tectonic domain and the formation and development of the Tibetan Plateau,as well as for finding more reserves of mineral resources to serve the national economy.Volcanic rocks can reflect the characteristics of magma activity,which can help us understand the plate tectonic evolution process.Thus,the study on the origin and dynamics of early Cretaceous volcanic rocks in this area is the key to analyze the geological evolution of the BN metallogenic belt.The study area is located at the middle of the Qinghai-Tibet Plateau,and the tectonic site is located at the northern margin of Gangdise,south to the BN suture zone.Through the field geological survey,the section survey,the analyses on the geological,petrological and geochemical characteristics,and the determination of the geological age of the volcanic rocks of Duoni Group in the study area,the following achievements and knowledge have been achieved:The Duoni Formation in the area of Geji-La Guocuo is a set of coastal-shallow marine carbonate-clastic rock-volcanic rocks,which underwent two or more sedimentary tectonic cycles,forming a basic sequence of a ternary structure.The bottom is dominated by bioclastic limestone,while the middle is mainly composed of conglomerate,fine conglomerate,gravel coarse sandstone,quartz coarse sandstone,feldspar quartz sandstone,siltstone and so on,showing a variety of thick rhythm interbedded output,and the top is mainly the Anshan-rhyolite assemblage.The U-Pb ages of the zircon in the volcanic rocks of Duoni Formation were determined to be 112 ± 2 Ma and 112.3 ± 1.4 Ma.A variety of fossils were collected in the lower bioclastic limestone,including fixed clams,insects,foraminifera,amphipods,sea lilies,bivalves,brains,algae,sponge bone needles,shells,snails,etc.,wherein the biological age of the spherical pseudo-snail Pseudamaura bulbiformis(Sowerby)is in consistence with the formation era of the volcanic rocks,i.e.,the Early Cretaceous.After regional comparison,it was determined that the formation era of the Duoni Formation was Barremian~Albian.The Duoni Formation in the Geji-Laguocuo district has a significantly different construction combination from the Duoni Formation in the Bange-Basu district in the north and the Zenong Formation in the Cuoqin-Shenzha district in the south.In addition,the Early Cretaceous in the Geji-Gerencuo district has three-fold temporal characteristics.Based on these evidences,it was speculated that the northern margin of Gangdise has a three-fold tectonic framework in space.