Geochemistry and LA-ICP-MS zircon U-Pb age of volcanic rocks of Wangguoshan Formation in the Gangmar Co area of Qiangtang, Tibet

A set of early carboniferous volcanic rocks has been found in the Gangmar Co area of Qiangtang for the first time. The volcanic rocks are composed of acidic to basic volcanic rocks (rhyolite, dacite, basaltic andesite, basalt and andesite) intercalated with pyroclastic rocks. The LA-ICP-MS zircon U-Pb ages of two samples (dacite and basaltic andesite) are 358.2Ma±1.2Ma and 350.5Ma±1.4Ma, which indicates that the volcanic rocks formed in the early Carboniferous. Rock geochemical characteristics show that the A/CNK indexes are 0.62~1.08, with an average of 0.81, and Rittmann indexes (δ) are 1.04~2.67, with an average of 1.84. These volcanic rocks are characterized by high LILE (Th, U and Pb) and obvious depletion of HFSE (Nb, Ta and Ti). The geochemical compositions of volcanic rocks belong to the calc-alkaline island arc volcanic rocks. It is indicated that the Lungmu Co-Shuanghu-Lancang River Ocean entered into its subduction stage in the Early Carboniferous.