北山洋晚志留世—早泥盆世构造演化:内蒙古白云山蛇绿混杂岩带南部侵入岩年代学、地球化学的制约

内蒙古北山白云山蛇绿混杂岩带南部中酸性侵入岩主要以脉岩形式产出。采用LA-ICP-MS锆石U-Pb定年和岩石地球化学分析,对脉岩的岩石类型、年代学、构造背景等进行研究,为北山洋的拼合碰撞时限提供年代学约束。研究结果显示:石英闪长岩的年龄为(420.0±1.0) Ma,花岗闪长岩的年龄为(404.6±2.6) Ma,表明脉岩形成时期为晚志留世—早泥盆世。中酸性侵入岩脉SiO₂含量为58.38%~69.46%,K₂O含量为0.74%~1.38%,Al₂O₃含量为15.54%~16.95%,Na₂O含量为4.15%~5.36%,岩石类型为钙碱性石英闪长岩和过铝质低钾拉斑系列的花岗闪长岩。石英闪长岩与花岗闪长岩源于陆缘火山弧区和同碰撞区,二者轻稀土元素相对重稀土元素明显富集,δEu负异常,高场强元素Ta、Nb亏损,显示出岛弧环境的典型特征,表明由于洋壳俯冲作用,研究区在晚志留世—早泥盆世发育了岛弧岩浆岩侵入体。随着俯冲作用深入,北山洋在晚志留世—早泥盆世早期完成闭合碰撞,并在研究区形成同碰撞花岗岩,碰撞时间应早于(404.6±2.6) Ma。

Late Silurian to Early Devonian Tectonic Evolution of Beishanyang: Constraints from Geochronology and Geochemistry of Intrusive Rocks in Southern Baiyunshan Ophiolitic Mélange Belt,Inner Mongolia

Intermediate-felsic intrusive rocks occur mainly as dykes in the southern part of the Baiyunshan ophiolite mélange belt in the Beishan Orogen, Inner Mongolia. LA-ICP-MS zircon U-Pb dating and whole-rock geochemical analysis were performed to study the rock type, geochronology and tectonic setting of the dykes. Meanwhile, it provides chronological constraints for the timing of the Beishan ocean closure. The results show that the intermediate-felsic dykes have SiO₂=58.38%-69.46%, K₂O=0.74%-1.38%, Al₂O₃=15.54%-16.95%, and Na₂O=4.15%-5.36%. The rocks are calc-alkaline quartz diorite and peraluminous tholeiitic granodiorite. Zircon U-Pb ages of the quartz diorite and granodiorite are (420.0±1.0) Ma and (404.6±2.6) Ma, respectively, suggesting the dykes were intruded in the Late Silurian-Early Devonian. The quartz diorite and granodiorite were likely derived from a continental-margin volcanic arc and syn-collisional setting. The clear LREE/HREE enrichments and high field strength element (HFSE, e.g., Ta and Nb) depletions are typical of island arc affinity, which shows the occurrence of island arc magmatism in the Late Silurian-Early Devonian led by oceanic crust subduction. With the progress of subduction, the Beishan ocean may have closed completely in the Late Silurian-Early Devonian to form syn-collisional granites in the study area. The collision time should be before (404.6±2.6) Ma.