内蒙古中部乌拉山地区西沙德盖岩体地球化学-年代学特征及其地质意义

Geochemistry and Geochronology of West Shadegai Granites in Wulashan of Inner Mongolia and Its Geological Significance

内蒙古乌拉山地区西沙德盖岩体位于哈达门沟钼矿田范围内,大地构造位置处于华北克拉通北缘西段,岩性为正长花岗岩。地球化学特征表现为高硅(SiO_2质量分数为74.08%~75.95%)、富钾(K_2O/Na_2O值为1.20~3.39)、富碱(K_2O+Na_2O为7.03%~8.46%)、弱过铝质(Al_2O_3的质量分数为11.22%~13.06%,A/CNK的值为1.002~1.284),里特曼指数指示为钙碱性;轻稀土富集,重稀土亏损(LREE/HREE为19.84~27.83),Eu明显亏损(δEu为0.36~0.53);微量元素富集Rb、Th、U、Pb、Zr、Hf、Sm等,亏损Ba、Ta、P、Ti,高场强元素Nb、Sr含量偏低,具有陆壳改造型花岗岩特征。利用LA-ICP-MS对西沙德盖岩体进行定年,22个锆石^(206)Pb/^(238)U年龄统计权重平均值为(231±0.8)Ma,指示该岩体形成于中三叠世。根据成矿年龄与成岩年龄的对比,成岩与成矿作用均发生于中三叠世末,成矿年龄略小于成岩年龄,属印支期岩浆活动的产物。根据已有年龄资料进行归纳和分析,认为区内在印支期曾发生过重要的构造-岩浆成矿事件。

The West Shadegai granites in Wulashan of Inner Mongolia is located in Hadamengou molybdenum orefield, tectonically located in the west part of the northern margin of North China Craton, whose lithology is syenogranite. Geochemical characteristics show that the granite is characterized by high silicon （the mass frac-tion of Si02 is 74. 08% - 75. 95% ） , rich potassium （ K20/Na20 = 1. 20 - 3. 39） , rich alkali （ K20 ＋ Na20 = 7. 03% -8. 46% ） , weakly peraluminous （mass fraction of A1203 is 11. 22% - 13. 06% , A/CNK = 1. 002 -1. 284）. The Litman index belongs to the calc alkaline. The REEs are of enrichment of light rare earth ele-ments ,loss of heavy rare earth elements （ LREE/HREE = 19. 84 - 27. 83 ） , and obvious loss of Eu （^Eu =0.36 -0.53）. The enriched trace elements include Rb,T h , U , P b , Zr, H f , Sm ; the loss elements in -clude Ba, Ta, P, Ti; high field strength elements including Nb and Sr are slightly low, showing the charac-teristics of continental crust reformation type granite. The LA-ICP-MS 2〇 6P b /238 U weighted average age of 22 zircon samples from Shadegai pluton is （231 ±0. 8） Ma,belonging to Middle Triassic. According to the con-trast of the mineralization age and the diagenetic age, the diagenesis and mineralization both occurred in Late Triassic, and the mineralization age is slightly less than the diagenetic age, showing the pluton belongs to theproduct of extensional tectonic magmatic activity after Indosinian. According to the available age data, we con-cluded that significant tectono-magmatic molybdenite mineralization events had ever occurred in Indosinian.