内蒙古阿巴嘎旗北部晚奥陶世A型二长花岗岩研究及地质意义

Late Ordovician A-type monzogranites in north of Abaga Banner,Inner Mongolia,and their geological implications

在内蒙古乌里雅斯太活动边缘带的阿巴嘎旗北部发现长约12 km、宽约3 km的花岗岩。对该花岗岩进行了详细的岩相学、元素地球化学和岩石年代学特征分析,表明岩石为二长花岗岩,具有富硅、偏铝到过铝质和高钾特征,属高钾钙碱性岩石系列。二长花岗岩显示高的K_(2)O/Na_(2)O(1.24~1.40)、FeOT/MgO(0.82~0.93)、Zr+Sr+Y、Rb/Nb和Y/Nb值,表现出A2型花岗岩的特征。岩石大离子亲石元素总体富集,高场强元素中Ti和Nb亏损,但Zr、Ta和Hf富集。稀土元素总量偏低,LREE/HREE(1.66~6.99)变化较大,δEu为0.04~0.39,表明该岩石是在相对低压、高温条件下由原有地壳物质部分熔融形成的。两个样品的锆石U-Pb定年显示其年龄分别为444.8±1.5 Ma和445.2±1.1 Ma,表明其形成时代为晚奥陶世,是兴蒙造山带由碰撞挤压的构造环境向碰撞后伸展体制的转变过程。

Late Ordovician A-type granites with the length of 12 km and width of 3 km have been discovered in the north of Abaga Banner in the Uliastai active continental margin,Inner Mongolia.According to the analysis of regional geology,petrography,geochemistry and geochronology,the studied rock is proved to be monzogranites rich in silicon,aluminic-peraluminous and high potassic,belonging to high-K calc-alkaline rock series.With high K_(2)O/Na_(2)O(1.24~1.40),FeO T/MgO(0.82~0.93),Zr+Sr+Y,Rb/Nb and Y/Nb values,the monzogranites show characteristics of A2-type granites.The rocks are enriched in large ion lithophile element(LILE)while Ti and Nb are depleted but Zr,Ta and Hf are enriched in light of high field strength element(HFSE).The total quantity of rare earth elements(REEs)is relatively low;the ratio of light rare earth element(LREE)/heavy rare earth element(HREE)(1.66~6.99)varies greatly;andδEu is 0.04~0.39.All these indicate that the rocks were formed by the partial melting of the crustal materials under relatively low pressure.The zircon U-Pb dating of the two samples shows that they are 444.8±1.5 Ma and 445.2±1.1 Ma respectively,indicating that they were formed in the Late Ordovician as a result of the transition from collision-compression tectonic environment to post-collision-extension system of the Xingmeng Orogenic Belt.