Middle Jurassic- Early Cretaceous Radiolarian Chronology Of Yarlung Zangbo Suture Zone: Implications For The Evolution Of The Neo-tethys

Yarlung Zangbo Suture Zone(YZSZ), which lies in the southern Tibetan Plateau, represents the position of the Neo-Tethys Ocean closure. For a long time, the ophiolite in the YZSZ is widely accepted to be the remnant of the Neo-Tethys Ocean lithosphere. Combined with the relevant marine strata in the YZSZ, they record the evolution of the Neo-Tethys Ocean and the collision between the Indian and Eurasian plates.In recent years, geologists have conducted extensive and fruitful researches in the YZSZ, and provided critical evidences for the evolution of the Neo-Tethys Ocean. However, current researches in the YZSZ focus on the isotope geochronology and geochemistry of the ophiolite, and studies on the paleontology and sedimentology of the marine strata are relatively weak, which hinder the systematic understanding of the Neo-Tethys evolution. This thesis focuses on the Jurassic-Cretaceous radiolaria-bearing pelagic siliceous(mud)rocks in the Zhongba area in western YZSZ, and the siliceous rocks at the bottom of the Chongdui Formation in Xigaze Forearc Basin in Central YZSZ, conducts paleontological, stratigraphical and zircon U-Pb dating studies, and gains the following main results.In the mélange belt in Zhongba area, western YZSZ, we measured 7 sections of marine strata. In the radiolarian siliceous rocks, we identified 31 radiolarian genera and 42 radiolarian species of Mesozoic age, and divided 4 Jurassic radiolarian assemblages and 5 Cretaceous radiolarian assemblages. Combined with geological observation and structural geological analysis in the field, this study subdivided the Sangdanglin Formation, Guoyala Formation and Yanduo Formation, which were traditionally thought to be normal marine strata, into 5 structural formations: the Maquanhe Formation, the Tawa Formation, the Erba Formation, the Yisang Formation and the Tanong Formation. According to the radiolarian chronological sequences in the Jiangmuna mélange, this study reconstructed the ocean plate stratigraphic framework, and revealed evolution of the Neo-Tethys Ocean from the Middle Jurassic to the Early Cretaceous. In early Aptian, the strata transformed from radiolaria-bearing, pelagic siliceous rocks to detrital mudstones, which implied the beginning of the Neo-Tethys subduction and coincided with chronological and geochemical records of the ophiolite. The Jurassic-Cretaceous age of radiolaria also denied previous geochronological work which thought the strata was Eocene.In the Xigaze Forearc Basin in central YZSZ, we measured 6 marine sections of the Chongdui Formation in Naxia, Qunrang and Zegapu areas. In the siliceous rocks at the bottom of the formation, 18 genera, 26 species and 5 assemblages of radiolaria were identified with the age of late Barremian to late Aptian in Early Cretaceous, which is consistent with the siliceous mudstones in Zhongba area. In Quartz- Feldspar- Lithic(QFL) diagram, the detrital rocks in upper Chongdui Formation fall into undissected arc to transitional arc areas. Detrital zircons in sandstones show single age peak of 110 Ma, consistent with the age of the Gangdise magmatic arc, indicating that Gangdise is probably the main source area of the Chongdui Formation. Based on these studies, combined with the unconformable contact relationship between the Chongdui Formation and the ophiolite, we conclude that the Neo-Tethys Ocean started to subduct, the Xigaze forearc basin started to develop and receive detrital sediments derived from the Gangdise magmatic arc in late Early Cretaceous.