Petrogenesis of Early Jurassic volcanic rocks in the Amdo area, Tibet: Evidences for the subduction of the Bangong-Nujiang Tethys ocean

It can better understand the geological evolution of the Tethys through studying Mesozoic volcanic rocks widely exposed in the Lhasa Terrane. This paper report LA-ICP-MS zircon U-Pb geochronology and zircon in situ Lu-Hf isotope, whole-rock analysis of major and trace elements and Sr-Nd isotope for the volcanic rocks in the Amdo area. LA-ICP-MS zircon U-Pb analysis suggest that the volcanic rocks formed at 180. 7 +/- 4. 7Ma, and reveal that they formed in the Early Jurassic. The SiO2, MgO and Fe2O3T of these volcanic rocks range from 62. 50% to 76. 15%, 0. 33% to 3. 05%, 2. 35% to 6. 69% , respectively. They are enriched LREE in the chondrite-normalized REE patterns with weak negative Eu anomalies, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements ( HFSEs). Zircon epsilon(Hf),(t) and whole-rock epsilon(Nd)(t) of these volcanic rocks have positive values and are from +15. 1 to +17. 1, from +7. 1 to +13. 2, respectively. These features suggest that Amdo volcanic rocks were probably produced by partial melting of new lower crust by the subduction of Tethys oceanic crust. Combining with regional geologic data, they were not originated from the subduction of Longmu Tso-Shuanghu Paleo-Tethys ocean or Yarlung Zangbo Neo-Tethys ocean, but Bangong-Nujiang Tethys ocean. Combining with the Jurassic magmatic arc in the Gerze and Rutog, they form the magmatic arc belt in the north side of Bangong-Nujiang Tethys ocean. Therefore, we suggest that the Early Jurassic island arc volcanic rocks in the Amdo area are the products of northward subduction of the Bangong-Nujiang Tethys ocean. These results provide direct evidence of the volcanic rocks for the subduction of the Bangong-Nujiang Tethys ocean.