The Radiolarian Biostratigraphic Research Near Zhongba District, Tibet

The closure of Neo-Tethys and India-Eurasia collision are key issues for knowing the evolution of Himalayan Orogen and the formation of the Tibet Plateau. So far, the time of the closure of Neo-Tethys and India-Eurasia collision varies from researchers to researchers, like 65-60 Ma,45 Ma,55 Ma. The disapperance of the Tethys and the latest marine deposit are direct proof of this issue. Radiolarian stratigraphy is essential method to constrain the evolution of paleo-ocean and the collision event.This thesis focuses on the area near Zhongba District which belongs to the weatern part of Yalung-Tsangpo Suture Zone. Based on the radiolarian stratigraphy research of the silicious shale, chert in melange alonge the suture zone and deep-marine sediments on the Indian passive continental margine, we decipher the time and environment of their formation to know more about the evolution of the Neo-Tethys. The main conclusions are listed as follows:Based on the radiolarian biostratigraphy research, we get the formation time of the melange. Combine with lithologies of the matrix and blocks, the tectonic contact, the melange in research area can be divieded into new units. The strata can be divided to four melange slices:Tawa Slice, Erba Slice, Maquanhe Slice, Tanong Slice. The Erba Slice, Maquanhe Slice are silt-mud melange. Their ages are Late Jurassic Kimmeridgian-Early Cretaceous Hauterivian, Late Jurassic Tithonian-Early Cretaceous Hauterivian, Early Cretaceous Valanginian-Hauterivian, respectively. This conclusion indicates that the time of the melange are mainly Late Jurassic-Early Cretaceous, other than Paleogene proposed by the 1:250000 geological mapping program.Based on radiolarian biostratigraphy research on silicious shale and chert in the Weimei Formation and Rilang Formation which belonge to the Indian passive continental margine. We get Late Jurassic Oxfordian and Kimmeridgian radiolarian assemblages in Weimei Formation. This indicates that the Indian continental passive margine are marine environment lying below the CCD.In the quartz sandstone interbeded with chert which sits to the south of the deep marine sediments, we get Late Paleocene Buryella tetradica (RP5) and Bekoma campechensis (RP6) (63-58.5Ma) typical radiolarians. This indicates that, in the Qulong area in Zhongba District, there still existes Tethys. The continental collision began after Palaeocene at least.