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 chronology and geochemistry of Mesozoic volcanic rocks from the Shanghulin-Xiangyang basin in Ergun area,northeastern Inner Mongolia
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作者：
Wang Feng
La-ICPMS锆石U-Pb定年结果表明，研究区中生代火山岩可分为早白垩世早期(~144 Ma)和早白垩世晚期(~125 Ma)两个阶段。以鸡翔峰组流纹岩为代表的早白垩世早期火山岩具有较高的SiO_2和碱含量(尤其是K2O，K_2O/Na_2O>1)，富含轻稀土元素(LREE)、Rb、Th、U，相对亏损重稀土元素(HREE)和高场强元素Nb、Ta、Ti、早白垩世晚期火山岩具有双峰式火山岩组合特征。玄武岩具有高碱(特别是K2O)含量，富LREE、Rb、BA，相对亏损HREE和HSE等特征，化学上属于玄武岩系列，而流纹岩与A型流纹岩相似。双峰式火山岩可能形成于伸展环境。结合东北地区中生代火山岩的时空分布，中国早白垩世早期火山岩的形成可能与蒙古-鄂霍次克造山带的演化有关，而早白垩世晚期双峰式火山岩的形成可能与欧亚大陆之下的古太平洋板块(Izanagi)俯冲有关。
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.