北秦岭西段黄牛铺岩体锆石U-Pb年代学、岩石地球化学特征及其地质意义

摘要

对北秦岭西段黄牛铺岩体进行了LA-ICP-MS锆石U-Pb年代学与岩石地球化学分析.结果显示,黄牛铺岩体的锆石U-Pb年龄为(449.3 ±4.9)Ma,表明其形成时代为晚奥陶世.黄牛铺岩体属于准铝质高钾钙碱性系列,在成因类型上具有I型花岗岩的岩石矿物学与地球化学特征,其稀土元素配分曲线呈轻稀土元素富集、重稀土元素亏损(LREE/HREE=9.83 ～ 11.55)、铕弱负异常(δEu =0.82 ～0.95)的右倾型,微量元素Rb、Th、U、Sm等富集,Ta、Nb、Ti、Hf等明显亏损,表现为岛弧岩浆岩的地球化学特征.结合区域地质资料,认为黄牛铺岩体形成于晚奥陶世商丹洋俯冲过程中的岛弧环境,是壳幔岩浆混合作用的产物,这为进一步研究北秦岭的构造演化过程提供了更加充分准确的岩浆活动证据.%The research on LA-ICP-MS zircon U-Pb geochronology and petrogeochemistry of Huangniupu plutons is carried out.Results show that the zircon U-Pb age of Huangniupu plutons is (449.3 ±4.9) Ma,which indicates that the formation age of Huangniupu plutons is Late Ordovician.Huangniupu plutons belong to highk calc-alkaline series with quasi-aluminous feature,which has the mineral and geochemical characteristics in genetic type of I-type granite.The REE distribution patterns reveal that Huangniupu plutons have a right-wing type with LREE enrichment,HREE depletion (LREE/HREE =9.83-11.55) and weak negative Eu anomaly (δEu =0.82-0.95),and the trace elements show the enrichment in Rb,Th,U,Sm and obvious depletion in Ta,Nb,Ti,Hf,which is consistent with the geochemical features of magmatic rocks formed in island-arc.Combined with the regional geological data,it is concluded that Huangniupu plutons were formed in the island arc setting during Shangdan Ocean subduction in Late Ordovician and generated by the crustal-mantle magma mixing,which provides more sufficient and accurate evidences of magmatic activities for the further study on the tectonic evolution of the North Qinling Mountains.