Sedimentary Characteristics And Tectonic Siginifiance Of The Middle-upper Triassic In Lanling Area,Qiangtang Of Tibet

A lot of works focused on the LongmuCo—Shuanghu suture zone due to its significance for exploring the early tectonic evolution history of the Qinghai Tibet Plateau.Furthermore,it is very important to understand the evolution of the Paleo Tethys oceans,determine the collage time between the Eurasian continent and Gondwana continent and collage process and so on.The subduction process of Paleo Tethys oceans between late Paleozoic and Triassic and the tectonic belts formed by collisional orogeny in the north and South Qiangtang blocks have been widely accepted.However,the controversial is still existence on the closure time and tectonic evolution of the paletethys ocean.For a long time,a few works focused on the Triassic strata of this suture zone and a few works focused on association between the subduction zone and the fore arc palaeogeographic pattern.Moreover,the tectonic setting of the sedimentary response has not been discussed and the question about the paleoTethys subduction related fore arc deposition system has not been identified.So it is very important to study about the middle—upper Triassic strata in the Lanling region to determine the closure time and the temporal and spatial pattern of the Triassic oceanic subduction.The author discovered the Latin step of Middle—Norian of Late Triassic sediment of platform facies carbonatite formation on the north of Lanling.A total of 6genera and 6 species of Coral fossils?including undetermined species?were identified in the cross section.Respectively Conophyllia sp.,Omphalopyllia sp.,Coryphyllia sp.Pamirastraea sp.,Margarophylliacf.capitata.,Craspedophylliasp.The coral assemblage age was the middle-late Triassic.three sedimentary subfacies were determined including:restricted platform,open platform and shoal in the platform margin.In the middle-upper Triassic of radiolarian siliceous strata,6 genera and 12species?including undetermined species?including Cenosphaera sp.,Eptingium sp.,Astrocentrus.cf.pulcher,Triassocampe dumitricai,T.scalaris,T.sp.,T.deweveri,Striatotriassocampe sp.,Pseudostylosphaera sp.,P.cf.Japonica.of radiolarian fossils were identified in the east Bank of Gangtangcuo.The sedimentary environment was deepwater deposit.The ratio of rare earth element LaN/YbN in siliceous rocks is between 1.09-1.32and average value is about 1.18.The ratio of LaN/CeN is between 0.882-1.313andaveragevalueis 1.042,and the value of the total amount of rare earth elements is between 27.62 x 10^-6-135.56 x 10^-6 and the average value is 66.46x 10^-6.Geochemical discrimination diagrams and comparison with siliceous rocks formed in typical sedimentary environments are found:the siliceous rocks in the Linling area of the middle section of the Longmuco-Shuanghu suture zone are formed under the interaction of the main biological action and the influence of the hot fluid.The formation environment is between the continental margin and the ocean basin,which is more inclined to the deep-water basin sedimentary environment far from the continental margin.It is fully illustrated that the Paleo Tethys ocean in the middle-late Triassic appeared on the complex sedimentary and oceanic evolution stage,presenting the multi island ocean geographical pattern.