A Combined Paleomagnetic And Zircon U-Pb Geochronological Study On The Late Jurassic Volcanic Rocks In The Lhasa Terrane

The Bangong-Nujiang Suture Zone,which is an important geological boundary across the central Qinghai-Tibetan Plateau,was formed by the collision of the Qiangtang terrane on the northern side and the Lhasa block on the southern side during the Mesozoic.The closure time of the Bangong-Nujiang Ocean is vital to understand the formation and evolution of the Tibetan Plateau.Although geological and geophysical investigations have been carried out in the Lhasa and Qiangtang terranes,a hot debate on the closure time of the Bangong-Nujiang Ocean still remains,and the proposed closure time ranges from the Middle Jurassic to the Late Cretaceous.Therefore,the reconstruction of the Middle Jurassic-Late Cretaceous paleogeographic positions of the Lhasa and Qiangtang terranes is very important to study the closure time of the Bangong-Nujiang Ocean.To constrain Late Jurassic paleogeographic position of the Lhasa terrane,a combined paleomagnetic,rock magnetic and zircon U-Pb geochronological study has been performed on the Zenong Group volcanic rocks in the Gaize area of the Lhasa terrane.Based on these new results,as well as the previous Jurassic and Cretaceous paleomagnetic data from the Lhasa and Qiangtang terranes,several main conclusions can be summarized as follows:1)Two new Zircon U-Pb dating results show that the sampling Zenong Group volcanic rocks in the Gaize area formed at ?147 Ma,supporting that these volcanic erupted during the Late Jurassic,rather than during the Early Cretaceous as given by 1:250,000 scale Cuoqin regional geological survey report.2)A new Late Jurassic plaoemagnetic pole for the Lhasa terranes has been obtained from the Zenong Group volcanic rocks in the Gaize area.The pole is located at 1.2°N,14.3°E,(dp/dm=4.5°/8.0°),showing that the Gaize study area at ?147 Ma should lie at 17.4°±4.5°N.3)Combining Jurassic and Cretaceous paleomagnetic data obtained by their predecessors at the blocks on both sides of the suture band,the closure of the Bangong-Nujiang Ocean might occur before 147 Ma.