西秦岭多哇地区萨日加岩体LA-ICP-MS锆石U-Pb年龄、地球化学特征及其地质意义

LA-ICP-MS Zircon U-Pb Ages and Geochemical Characteristics of Sarijia Intrusion in Duowa Area of West Qinling,China and Their Geological Implications

西秦岭造山带发育大量的印支期高钾钙碱性花岗岩,它们的成岩时代和岩石成因对认识西秦岭岩浆活动和构造演化具有重要意义。对西秦岭多哇地区萨日加岩体进行了详细的野外地质调查、岩相学观察、LA-ICP-MS锆石U-Pb定年和全岩地球化学研究,旨在查明该岩体的年龄、成因类型以及成岩地球动力学背景。结果表明:萨日加岩体由英安斑岩、花岗斑岩及少量石英闪长岩组成;花岗斑岩LA-ICP-MS锆石U-Pb年龄为(238.2±2.6)Ma(平均标准权重偏差(MSWD)为1.7);萨日加岩体属于过铝质高钾钙碱性岩石系列,富集大离子亲石元素Th、Rb、K和轻稀土元素,亏损高场强元素Nb、Ta、Ti等,指示其岩浆源区具有壳幔混合的特征;岩石的主量、微量元素特征显示,英安斑岩属于I型花岗岩,形成于洋-陆俯冲岛弧环境;花岗斑岩倾向于S型花岗岩,形成于大陆碰撞构造环境。由此认为,萨日加岩体形成于晚三叠世西秦岭由洋-陆俯冲岛弧向陆-陆碰撞转变的构造环境。

West Qinling orogenic belt is characterized by large amounts of Indosinian high-K calc-alkali granites,and the age and genesis of granites are essential to understand the tectonic evolution and magma activity in West Qinling.The detailed field geological investigation,petrography observation,LA-ICP-MS zircon U-Pb ages and geochemistry of whole rock for Sarijia intrusion in Duowa area of West Qinling were studied with the aim of ascertaining the age,genetic type and geodynamic background of the intrusion.The results show that Sarijia intrusion is composed of dacite porphyrite,granite porphyrite and slight quartz diorite;granite porphyrite has a LA-ICP-MS zircon U-Pb age of(238.2±2.6)Ma(MSWD is 1.7);Sarijia intrusion belongs to peraluminous and high-K calc-alkali series,which is enriched in large ion lithophile elements(Th,Rb and K)and light rare earth elements(LREEs),and depleted in high field strength elements(Nb,Ta and Ti,etc.),indicating that the magmatic source is characterized by crust-mantle mixing;the characteristics of major and trace elements show that dacite porphyrite belongs to I-type granite and forms in the ocean-continent subduction-related island arc enviroment,while granite porphyrite belongs to S-type granite and forms in the continental collision setting.Therefore,it is proposed that Sarijia intrusion is emplaced at Late Triassic when the tectonic setting is transformed from ocean-continent subduction-related environment to continental collision setting.