内蒙古苏尼特左旗阿尔善黑云二长花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义

LA-ICP-MS zircon U-Pb geochronology of the biotite monzonitic granite and its geological significance, in the Aershan of Sonid Zuoqi,Inner Mongolia

通过对内蒙古苏尼特左旗阿尔善布拉格一带出露的黑云母二长花岗岩调查及LA-ICP-MS锆石U-Pb年龄测试分析,获得三个样品的锆石U-Pb年龄介于308.0±1.2 Ma^311.8±1.2 Ma,属于晚石炭世。地球化学分析结果显示:该岩石的SiO_2含量为65.22%~73.10%,富碱(K_2O+Na_2O=5.43%~7.71%),A/CNK值0.88~1.03,属准铝质钙碱性-高钾钙碱性系列;轻重稀土之间分馏较明显,轻稀土相对富集,重稀土元素亏损,配分图整体呈现出右倾特征,δEu为0.79~1.05,未见明显负铕异常;微量元素显示大离子亲石元素Rb、Ba、Th、U相对富集,Nb、Ta、P、Ti具有明显亏损。综合最新区域资料及本文研究成果,笔者认为阿尔善黑云二长花岗岩产生于碰撞挤压隆起向后碰撞伸展阶段转换的构造环境,是贺根山洋盆闭合后的后碰撞伸展阶段早期的产物。

This paper carries out some geochemical analysis and geological survey on a biotite monzonitic granite in the Aershanbulag village, Sonid Zuoqi, Inner Mongolia. SHRIMP ziron U-Pb dating shows that the crystallization age of the biotite monzonitic granite are from 308.04-1.2 Ma to 311.84-1.2 Ma, which are in the late Carboniferous. Geochemical researches indicate that the biotite monzogranite is rich in silica （SiO2=65.22% 73.10% ）, alkali （K2O＋Na2O=5.43% - 7.71% ）, suggesting a typical Calc-alkaline to high-K Calc-alkaline series with the metaluminous feature （A/CNK=0.88 - 1.03 ）.The rocks are poor in rare earth elements with enriched in LREE[（La/Yb）N=4.13 - 15.37], LILEs（Rb,Ba,Th,U） and depleted in HFSE, especially Nb,Ta ,P and Ti. The Eu values vary from 0.79 to 1.06 , with almost no obviously negative Eu anomalies. In combination with previous studies, the authors propose that biotite monzonitic granite was produced in the environment from the Post-collision uplift turu to the Post-collision extension phase, and formed by the developmental stage of post- collisional granites after closing of the Hegenshan Ocean.