The Petrogenesis and Tectonic Significance of Early Cretaceous Volcanic Rocks in Nixiong Area from the Central and Northern Lhasa Terrane

Many studies have been focused on the central and southern Lhasa terrane, but it remains controversial as to the genetic mechanism of the Mesozoic volcanic rocks in the central and northern Lhasa terrane due to less attention paid to the distribution characteristics of volcanic magmatism, the origin of volcanic rocks, and the tectonic significance of the volcanic rocks. Early Cretaceous volcanic rocks are widely exposed in Nixiong area, which record abundant pre-Cenozoic evolutionary geohistory of the Tibetan Plateau. Petrological and zircon U-Pb dating analyses show that the volcanic rocks are mainly composed of basalitic andesite, trachyandesite and rhyolites. They have variable SiO 2 contents ranging from 55.76% to 77.78%, and alumina saturation index (A/CNK) of 0.89-3.04, indicative of high-K calc-alkaline to alkaline-calc and metaluminous to peraluminous. They are characterized by the enrichment of Th and U, and the depletion of HFSEs (such as Nb and Ta), typical of A-type granitoids. In addition, rhyolites show distinct high SiO 2 , but low MgO, TiO 2 , P 2 O 5 and δEu, and display fiercely negative Ba, Nb, Ta, Sr and Eu anomalies, suggesting that they are highly fractionated A-type rocks. LA-ICP-MS U-Pb dating of magmatic zircons from one basalitic andesite, one trachyandesite and two rhyolites samples indicate that they were formed at 117 Ma, 127 Ma and 126-127 Ma, respectively. It is proved that results of previous studies are wrong in that the volcanic rocks in Nixiong area are all Eocene Nianbo Formation of Lingzizhong group or Oligocene Rigongla Formation. In addition, it is found that the basalitic andesite, trachyandesite and rhyolites are likely derived from partial melting of a crust-mantle mixed source, and have experienced significant fractional crystallization. We speculate that the studied rocks have been affected by double subduction of southward subduction of Bangong Co-Nujiang Tethys oceanic crust, and northward subduction of Yalung-Zangbo oceanic crust. © 2018, Editorial Department of Earth Science. All right reserved.