A Combined Paleomagnetic And Zircon U-Pb Geochronological Study On The Cretaceous Volcanic Rocks In The Western Part Of The Lhasa Terrane

The collision and continuous convergence between India and Asia have not only created the Tibetan plateau and the extensive deformation within Asia, but also led to a great change of the climate, environment and deep structure in its adjacent area. So the India-Asia collision and intracontinental deformation have always been the frontier and hot issue for geologists to study the formation of the Tibetan plateau and geodynamics. Because the Lhasa terrane was the southern margin of Asia before the India-Asia collision, its pre-collisional paleogeography is a key to understanding the India-Asia collision process, as well as to constraining the amount of the post-collisional convergence within Asia. A combined paleomagnetic and zircon U-Pb geochronological study has been carried out on the Zenong Group volcanic rocks from the Shiquanhe area in the west of the Lhasa terrane, and the obtained achievements are summarized as following:(1) The sampling Duoai Formation volcanic rocks of the Shiquanhe area were dated at about 113-115 Ma.(2) The main magnetic carriers of the Duoai Formation volcanic rocks and the Langjiu Formation rhyolite are magnetite, while those of the Langjiu Formation tuff and basalt are hematite and magnetite.(3) High-quality paleomagnetic results have been obtained from the Zenong Group volcanic rocks of the Shiquanhe area. The site-mean direction for 29 sites is Ds= 339.2°, Is = 35.9°, k = 36.3, α95 = 4.5° after tilt-correction, which not only passes both fold and reversal tests, but also satisfies the model of geomagnetic paleosecular variation. The site-mean paleopole for 29 sites using the Fisherian statistics located at67.8°N, 319.8°E with A95 = 4.5°.(4) New paleomagnetic data, as well as previous Cretaceous paleomagnetic data obtained from the Lhasa terrane, show that the west of the Lhasa terrane remained a stable latitude of 17.3° ± 1.0°N during the whole Cretaceous for the reference point(32.2°N, 80.4°E). A north-south crustal shortening of 960 ± 220 km has taken place between the Lhasa terrane and East Asia since the India-Asia collision.