内蒙古北山马山花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义

LA-ICP-MS zircon U-Pb ages of the granites from Mashan of Inner Mongolia and their geological significances

对内蒙古北山南带东段的马山花岗岩体进行了LA-ICP-MS锆石U-Pb年代学和岩石地球化学研究,结果表明,马山花岗岩以花岗闪长岩为主,偏铝质-过铝质,中钾钙碱性特征,具有较高的SiO_2(64.85%~79.17%)、Na_2O+K_2O(5.13%~6.62%),富钠(Na_2O/K_2O>1);在球粒陨石标准化配分模式图上,配分曲线相对平缓,富集轻稀土元素,重稀土元素分馏不明显且相对亏损,δEu=0.65~0.91,具有弱Eu负异常;在微量元素原始地幔标准化蛛网图上,亏损Ba、Nb、Ta、P、Ti,富集Rb、Th、U、K。通过LA-ICP-MS锆石U-Pb测年,马山花岗岩体的侵位年龄为281.8±3.2Ma。结合区域地质背景,认为马山花岗岩体是壳幔混合成因的,形成于早二叠世后碰撞伸展体制之下,北山南带于早二叠世晚期进入后碰撞裂谷伸展发育阶段。

This paper reports LA-ICP-MS zircon U-Pb ages and geochemistry of the granite in Mashan, east of the southern Beishan, Inner Mongolia, with the purpose of constraining its formation age and petrogenesis. The results show that the granitic body mainly consists of granodiorite which belongs to middle-K calc-alkaline series with metaluminous-peraluminous characteristics and high content of SiO_2(64.85%~79.17%), Na_2O+K_2O(5.13%~6.62%) and Na_2O/K_2O>1. In addition, the granite invariably exhibits relatively gentle light rare earth elements(LREE) enrichment with flat heavy rare earth element(HREE) and weak negative Eu anomalies(δEu=0.65~0.91) in the chondrite-normalized REE patterns, depletion of Ba, Nb, Ta, P, Ti and enrichment of Rb, Th, U, K in the spidergram. The LA-ICP-MS zircon U-Pb age of the granite is 281.8±3.2Ma. Based on regional geology, the above characteristics indicate that the Mashan granite was the mixing product of crustal and mantle derived magmas and was formed under the tectonic setting of post-collisional extension in Early Permian. This implies that the east of southern Beishan Mountain turned to the stage of rift in post-collisional extensional period during Early Permian.