Paleomagnetic And Geochronological Results From The Risong Formation Rocks In The Western Lhasa Terrane And Their Geological Implications

The Qinghai–Tibet plateau consists of a collage of several terranes which generally accreted to the Palaeo-Asian continent.The Lhasa-Qiangtang collision occurred along the present-day Bangong-Nujiang suture zone,which is located in the central Qinghai–Tibet plateau,separates the Lhasa terrane to the south from the Qiangtang terrane to the north.The closing time of the Bangong-Nujiang oceanic basin is an important metallogenic period.Therefore,the Lhasa-Qiangtang collisional time is crucial to not only understand the tectonic evolution and uplift process of the Tibetan Plateau,but also clarify the metallogenic environment and prospecting direction of the Bangong-Nujiang metallogenic belt.However,the Lhasa-Qiangtang collisional time is still debated.In this thesis,a combined paleomagnetic and geochronological study has been carried on the Risong Formation in the Wuma area of the western Lhasa terrane.Based on new paleomagnetic and geochronologic results,combined with previous Cretaceous paleomagnetic data from the western Lhasa terrane and the western Qiangtang terrane,several main conclusions can be summarized as follows:?1?Zircon U-Pb dating shows that the volcanic interlayer within the Risong ormation in the Wuma area is 120.6±2.2 Ma,which,combined with that the high-temperature magnetization direction of all the redbed samples present a single normal polarity,suggests that the Risong Formation in the sampling area was probably formed during the Early Cretaceous,rather than during the Late Jurassic as given by 1:250,000 scale Wuma regional geological survey report.?2?Two reliable plaoemagnetic poles constrained by positive fold tests have een obtained from the Risong Formation redbeds in the Wuma area.These two poles are located at 38.1°N,161.6°E(A₉₅=2.5°)and 73.7°N,277.7°E(A₉₅=3.1°),respectively.?3?The western Lhasa terrane?32.41°N,83.39°E?probably maintained a relatively stable palaeolatitude at 18.1°±0.9°N during the whole Cretaceous.?4?The Lhasa-Qiangtang collision in the western part had probably occurred y the Early Cretaceous.