Paleozoic To Mesozoic Geologic Evolution Of Xainza Area Inthe North Of Lhasa Terrain, Tibet

This study chose Xainza area in the north of Lhasa terrain as the study area. Through field observation, petrographic analyses with point-counting by Gazzi-Dickinson method, and zircon geochronology study, some new insights into several significant issues about the geological evolution of the study area from late Paleozoic to Mesozoic formed. The age spectrum of the detrital zircons in Early Permian Angjie formation, with the results of petrographic analyses, suggests that the Lhasa terrain was adjacent to the north of Indian Gondwana, and received sediments from Tethyan area. The distribution of Triassic strata is limited in the north of Lhasa terrain. In Xainza area, the age spectrum of detrital zircons from Late Triassic Duoburi formation is similar to that of Paleozoic stratum in Middle and Northern Lhasa. Integrating with the results of petrographic analyses, this study points out its main provenance is the recycling material of Paleozoic stratum. Little material is from Qiangtang terrain to the north and Permian-Triassic Songduo orogenic belt and igneous rocks to the south. Yongzhu ophiolite belt is part of Shiquanhe-Namu Co mélange zone. In this thesis, it is pointed out that the Yongzhu ophiolite zone represents a suture zone within Lhasa terrain, and is the relic of a series of back-arc or inter-arc oceanic basins. The mean age of 27 analysis spots of a gabbro sample from the sheeted dike swarm in Yongzhu ophiolite belt is 153± 2.3 Ma, indicating that the oceanic basins in Xainza area were not closed in Late Jurassic. In Xainza area, Late JurassicEarly Cretaceous Rila formation overlies the Yongzhu ophiolite, and its sedimentary process seems to be affected by the movements of faults. Magmatic activities were extensive in the study area in Early Cretaceous. Zricon U-Pb dating results indicates that the potassium granite, andesitic porphyrite, granite porphyry samples are of Early Cretaceous ages. They might be generated in the same petrogenesis setting with the prevalent Early Cretaceous igneous rocks in the north of Lhasa terrain. Many thrust faults developed along the Shiquanhe-Namu Co mélange zone. They juxtaposed the Paleozoic stratum and ophiolites northward onto the Cretaceous sediments. The structural relationships, geochronology study of igneous rocks, and 40Ar-39 Ar thermochronologic analyses reported in the adjacent region, indicate that these faults were initially active from early stage of Early Cretaceous, and extended into Late Cretaceous. The age spectrum of detrital zircons in Early Cretaceous sandstones north of the thrust faults indicate their main provenience is the recycling material of the Paleozoic stratum to the south and igneous rocks on the north of Lhasa terrain. This sedimentary process may be controlled by the thrust system in Xainza area. In this thesis, it is proposed that the Geren Co-Namu Co thrust system in Xainza area developed along a preexisting tectonic weak belt represented by Shiquanhe-Namu Co mélange zone. It might be formed in a combined tectonic setting of Qiangtang-Lhasa collision to the north and Neotethys oceanic listhosphere’s northward flat subduction under Lhasa terrain to the south.