内蒙古东北部额尔古纳地区上护林-向阳盆地中生代火山岩LA-ICP-MS锆石U-Pb年龄和地球化学特征

LA-ICP-MS锆石U-Pb测年结果表明,研究区中生代火山岩可分为2期,分别是早白垩世早期（约144Ma）吉祥峰组流纹岩类和早白垩世晚期（约125Ma）上库力组流纹岩类与梅勒图组玄武岩类。早白垩世早期吉祥峰组流纹岩富硅、富碱、富钾（K2O/Na2O〉1）,富集轻稀土元素和Rb、Th、U等元素,亏损重稀土元素、高场强元素（Nb、Ta、Ti）及Sr、P,具有A型流纹岩的特征,暗示其形成于伸展环境。早白垩世晚期火山岩显示双峰式岩石组合特征,基性端元富碱、高钾,富含轻稀土元素和Rb、Ba,亏损重稀土元素和高场强元素（Nb、Ta、Ti、Y）,类似于钾玄质玄武岩,酸性端元显示A型流纹岩的特征。双峰式火山岩组合的存在暗示其形成于陆内拉张的构造环境。结合区域上中生代火山岩的空间分布规律,认为早白垩世早期火山岩的形成与蒙古-鄂霍次克缝合带的演化有关,早白垩世晚期双峰式火山岩的形成与古太平洋板块向欧亚大陆下的俯冲作用相联系。

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.