Sedimentary Characteristics Of The Lower Permian Qudi Formation Of Niema Mountain In Rongma Area, Tibet

The paper relies on the project of“Tibet 1:50000 Rongma area regional geology and mineral resources”,which is undertaken by C hina University of Geosciences(Beijing),We conducted a study of chronology,petrology and geochemistry in the Lower Permian Qudi Formation in N ierma mountain of Rongma area.Through the LA-ICP-MS zircon U-Pb isotope dating in the tuffs sandwic,We can determine the stratum age of the group of the song. Qudi Formation;Through detailed field observation,profile measurement,and thin section statistics,Then combined with the regional background information,We make a detailed study of the gravity flow deposits in Qudi Formation to determine its sedimentary facies and depositional environment;Through the whole analysis of the sandstone in Qudi Formation To determine its provenance background.A set of gravity flow sedimentary assemblage is found in the Lower Permian Q udi Formation in N ierma mountain of Rongma area,Tibet.The weighted average age of LA-ICP-MS zircon U-Pb dating was 283.2+2.6 Ma,in Artinskian. According to the lithology, sedimentary structure and fossil records, 9 lithofacies and 5 lithofacies associations can well be identified. These lithofacies belong to the inner- fan,the middle fan and the outer fan respectively,and can be subdivided into the main channel and inter-channel microfacies of inner fan,the branch channel,natural levee and channel interval microfacies of middle fan,the lobate body and ending channel microfacies of outer fan,as well as the deep-sea basalt and mudstone. Therefore,it is fairly concluded that the Qudi Formation in the study area is a abyssal fan formed in a bathyal to abysmal-bathyal settings. SiO2/Al2O3-Na2O/K2 Odiagram shows that the sandstone of the curve group is lithic sandstone with low maturity;The average of CIA was 62.14,which indicated that the rocks in the provenance area suffered from a certain intensity of chemical weathering.(Fe2O3*+MgO)-TiO2 illustrations, La-Th-Sc discrimination diagrams, Th-Hf-Co discrimination diagrams and REE comparison table of tectonic background contrast,showing the passive continental margin or island arc characteristics;M average was 28.89 and Sr/Ba average was 3.58,That showing a depth marine sediments characterize.In summary,The Deposition age of the Qudi Formation was early Permian Artinskian period,and belong to abysmal-bathyal sedimentary environment,The provenance background was the passive continental margin or island arc.sedimentary environment is different from the previous so called shallow marian Qudi Formation,and it can be illustrated that the sedimentary environment of Q udi Formation in the Southen Q iangtang Basin is obviously different in regional in the West-East direction,it was gradually changed from the shallow marain in Ritu at the West to the abysmal-bathyal in Gaize and N ima at the East. It also shows that the Palaeotethys in the study area was in the mature evolutionary stage at early Permian,and the time that Q iangtang block rifted and drifted from the northern margin of Gondwana is earlier in the east than in the west.