The Sedimentary Features And Paleogeographic Implication Of The Late Jurassic Quartz Sandstones In Tethys Himalaya Of Southern Tibet

The Jurassic was an important geological historical period in the evolution of the Tethys Ocean,accompanied by the cracking of East Gondwana,the opening of the new Tethys ocean basin,and the abundant information of the early evolution of the new Tethys in the geological areas associated with Tethys.The geological region related with Tethys,in Jurassic,contains rich information about the evolution of early NeoTethys ocean.It not only records the evolution history of eastern Gondwana,but also provides the constraints of time for the expansion and cracking of Neo-Tethys ocean by the study of its provenance.The Tethys Himalaya contains a continuous complete quartz sandstone whose tectonic background and depositional environment are still controversial.Therefore,it is possible to solve the above problems in the Jurassic stratum of Tethys,which the quartz sandstone with the source information.The thesis analyzed and compared the sedimentary characteristics and provenance of quartz sandstone in the study area through petrology,sedimentology,heavy mineral geochemistry and geochronology.The main use of statistical analysis of sandstone skeleton particles,scanning electron microscopy-cathodoluminescence identification,U-Pb detrital zircon age spectrum analysis and other scientific methods and methods,and statistics and analysis of samples,collecting and finishing previous research work,Comprehensive paleontological stratigraphy,in-depth analysis of sedimentary environment,etc.,establish sedimentary evolution sequences,and infer and restore the paleogeographic patterns of the Upper Jurassic.The strata of the study area are focused on the Menkadun Formation in the Nyalam area and the Weimei Formation in the Gyangze area,and the analysis of the overlying and underlying strata limits the isochronism of the Menkadun and Weimei Formations in the area.At the same time,through the horizontal comparison of Jurassic strata in the region,it was determined that the north zone of the Tethyan Himalayas was mainly composed of a thick layered slope sedimentary stratum,while the south zone was composed of the outer continental facies sedimentary strata.The Weimei Formations and the Menkadun Formation are two sets of phases that are isochronism and out of phase with each other.The statistical analysis of the skeleton grains of quartz sandstone shows that the quartz component has a high degree of maturity and the source area is likely to be a stable craton.The proportion of stable heavy minerals such as zircon and rutile is relatively high,which also indicates that this set of quartz sandstone is a continental block origin,and there is a re-circulation of the preexisting quartz sandstone.Agematched stratigraphy of Tethys-related geological stratigraphy is matched by U-Pb detrital zircon age spectrum.The age spectrum featuring the 550 Ma and 950 Ma age peaks is the best match with the East Indian stable craton basin.The material source of the S Himalayan quartz sandstone sequence is the East India stable Craton Basin.The bidirectional paleocurrents of the NE-SW direction of the Menkadun Formation and The paleocurrents of the NEE direction of the Weimei Formation show that the direction of the ancient coastline along the northern edge of the passive north-western India shifted from the NNW to the NEE of the east,in the region.There is an angle between the two,which corroborates the submarine expansion of India and Western Australia in late Jurassic.