内蒙古锡林浩特小乌兰沟早白垩世二长花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义

通过1∶5万区域地质调查,对中亚造山带南缘内蒙古锡林浩特小乌兰沟地区发育的二长花岗岩进行了岩石学、锆石U-Pb同位素年代学、地球化学等研究。LA-ICP-MS锆石U-Pb测年表明,该花岗岩的侵位年龄为(121.29±0.63)Ma,其形成时代为早白垩世。岩石地球化学研究表明,二长花岗岩具有高硅、富钾、贫镁、贫钙、低磷和低钛的特征,属于高钾钙碱性、过铝质系列岩石;微量元素特征表现为富含大离子亲石元素Rb和高场强元素Th、Ta,而亏损Nb、Sr、Ba;稀土元素以略富集轻稀土、Eu亏损为特征。地球化学特征反映其为A型花岗岩,是陆壳岩石部分熔融的产物。构造环境判别图解表明该岩体具有典型的后造山A2型花岗岩特征,说明早白垩世期间锡林浩特小乌兰沟地区已经处于后造山岩石圈伸展构造环境。

LA-ICP-MS Zircon U-Pb Age and Geological Significance of the Early Cretaceous Monzogranite in Xiaowulangou,Xilinhot,Inner Mongolia

Based on 1∶50,000 regional geological survey, we studied the monzogranite in the southern Central Asian Orogenic Belt (CAOB) at Xiaowulangou (Xilin Hot, Inner Mongolia)based on its petrology, zircon U-Pb age and geochemistry. LA-ICP-MS zircon U-Pb dating yielded Early Cretaceous age of (121.29±0.63) Ma. Petrological and geochemical data reveal that the monzogranite is characterized by high Si and K, low Mg, Ca, P and Ti. The rocks are high-K calc-alkaline and peraluminous. The rocks are enriched in large ion lithophilic elements (LILEs, e.g.Rb) and some high field strength elements (HFSEs, e.g.Th and Ta), but depleted in Nb, Sr and Ba. Rare earth element (REE) compositions are characterized by slight LREE enrichments and negative Eu anomalies. Geochemical characteristics suggest that the monzogranite is A-type, and may have been a product of partial melting of continental crustal rocks. Tectonic discrimination diagrams show that these rocks resemble post-orogenic A₂-type granite, indicating that the region was likely in a post-orogenic extensional setting during the Early Cretaceous.