LA-ICP-MS zircon U-Pb chronology and geochemistry of Mesozoic volcanic rocks from the Shanghulin-Xiangyang basin in Ergun area, northeastern Inner Mongolia

LA-ICP-MS zircon U-Pb dating results indicate that the Mesozoic volcanic rocks in the study area can be divided into two stages, i.e., early Early Cretaceous (~144 Ma) and late Early Cretaceous (~125 Ma). The early Early Cretaceous volcanic rocks, represented by the rhyolites from the Jixiangfeng Formation, have high SiO2 and alkali content(especially K2O, with K2O/Na2O ratios higher than 1), and are enriched in light rare earth elements (LREEs), Rb, Th, U,and relatively depleted in heavy rare earth elements (HREEs) and such high field strength elements as Nb, Ta and Ti, as well as Sr and P, similar chemically to A-type rhyolites, suggesting that they might have been formed in an extensional setting. The late Early Cretaceous volcanic rocks display a bimodal volcanic rock association. The basaltic rocks are characterized by high alkali (especially K2O) content, enrichment of LREEs, Rb and Ba, relative depletion of HREEs and HFSEs such as Nb, Ta, Ti and Y, belonging chemically to the shoshonitic series, whereas the rhyolitic rocks are similar to the A-type rhyolites. The bimodal volcanic rocks were probably formed in an extensional setting. Combined with spatial and temporal distribution of the Mesozoic volcanic rocks in Northeast China, it is suggested that the formation of the early Early Cretaceous volcanic rocks might have been related to the evolution of the Mongolia-Okhotsk orogen, whereas the formation of the late Early Cretaceous bimodal volcanic rocks might be attributed to subduction of the Paleo-Pacific plate (Izanagi) beneath the Eurasian continent.