内蒙古石哈河地区黑云母二长花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义

LA-ICP-MS zircon U-Pb dating of the biotite monzonitic granite from Shihahe area in Inner Mongolia and its geological significance

通过对乌拉特中旗石哈河地区黑云母二长花岗岩的锆石U-Pb年代学、岩石地球化学的系统研究,探讨了其形成时代及构造环境。LA-ICP-MS锆石U-Pb年龄加权平均值为（275.0±0.7）Ma,代表了其成岩年龄。岩石的A/CNK值为0.93-1.10,属于偏铝质-弱过铝质I型花岗岩,σ值为2.01-2.88,属于钙碱性系列。岩石ΣREE偏低,轻稀土富集而重稀土亏损,LREE/HREE为9.86-15.63,轻重稀土分异程度较高,Eu异常不明显或呈弱的负异常。原始地幔标准化蛛网图上富集大离子亲石元素Rb、K等,而亏损高场强元素Nb,蛛网图特征与正常弧花岗质岩石基本一致。根据石哈河地区黑云母二长花岗岩所处大地构造位置、侵位时代及岩石地球化学特征,推断其形成于活动大陆边缘构造环境,这也标志着研究区由洋壳俯冲阶段转入陆-陆碰撞构造演化阶段的时间上限应晚于早二叠世晚期。

Zircon U-Pb chronology and geochemistry of the biotite monzonitic granite from Shihahe area of Urad Middle Banner were studied systematically to investigate its formation age and tectonic background. The weighted average age obtained by zircon LA-ICP-MS U-Pb method is（274.99±0.71）Ma, which represents its formation age. The biotite monzonitic granite belongs to subaluminous-weakly peraluminous I- type and calc-alkaline series, with its A/CNK being 0.93-1.10 and σ being 2.01-2.88. Thetotal REE（ΣREE） is lower, LREE values are higher and HREE is lower, with LREE/HREE ratio varying from 9.86 to 15.63,indicating that LREE and HREE differentiated intensely. Eu has insignificant anomalies or weakly negative anomalies. In the primitive mantle normalized trace elements spider diagram, the biotite monzogranite is enriched in large ion lithophile elements Rb and K while depleted in high field strength element Nb, similar to features of normal arc granitic rock. According to tectonic setting as well as emplacement time and characteristics of geochemistry of the biotite monzogranite from Shihahe area, it is inferred that the rocks were formed in an active continental margin environment, and the upper limit of conversion from ocean crust subduction to continent collision was later than late Early Permian.