Zircon LA-ICP-MS U-Pb Ages of the Mesozoic Volcanic Rocks in Nagqu Area of Gangdise in Tibet and Their Geological Significance
西藏冈底斯带那曲地区中生代火山岩的LA-ICP-MS锆石U-Pb年龄和地质意义

Island-like early Cretaceous volcanic rocks are extensively distributed in northern Gangdise, Tibet. Regional geological survey at the scale of 1:250000 has mapped the late Cretaceous and middle-late Jurassic volcanic rocks in Nagqu area. Based on geological data available, the authors investigated the late Cretaceous and the middle-late Jurassic volcanic rocks which are well exposed respectively in Nagqu County town and Haermai Village. They both are andesitic volcanic rocks and exhibit angular unconformity with the underlying bed composed of black sandy slate. Meanwhile, they were sampled to perform zircon U-Pb dating by using the LA-ICP-MS system at the State Key Laboratory of Geological Processes and Mineral Resources in Wuhan for regional comparison. Most zircons separated from these volcanic rocks exhibit striped absorption and obvious oscillatory zoning, with a few ones showing the core-rim structure. The ages of two andesite samples collected respectively from eastern Nagqu and Haermai are 116.3±1.4 Ma and 111.37±0.73 Ma, suggesting that the volcanic rocks were formed basically at the same time. Regional comparison shows that early Cretaceous volcanism might have been the strongest one in northern Gangdise, with its activation time being 110 Ma or so, and the strong emplacement of early Cretaceous island-arc granitoids took place subsequently. The problem whether the early Cretaceous volcanism resulted from Shiquanhe-Lhari ocean subduction or from Bangong Co-Nujiang ocean subduction requires further study for final conclusion.