内蒙古锡林浩特I型花岗岩的时代及构造意义

在内蒙古锡林浩特水库地区出露的花岗岩确定为I型花岗岩，具有较高的Cr、Co、Ni丰度。Ca、Al含量和N2O／K2O比值较高，Fe、Mg含量较低。微量元素蛛网图中显示出明显的Nb、Ta、P、Ti负异常。在SiO2-K2O及AFM图中，花岗质岩石投在钙碱性系列区；在构造环境判别图中，花岗质岩石样品都投在火山弧＋同碰撞花岗岩区。锆石测年结果显示平均年龄为317．0±4．0Ma，属晚石炭世。这套晚石炭世岛弧花岗岩的存在，表明加里东期古亚洲洋并未完全关闭，晚石炭世时仍然存在洋壳的俯冲消减事件。从区域上看，是北侧的贺根山洋盆向南俯冲的结果。

Age and its tectonic significance of I-type granite in Xilinhot, Inner Mongolia

It is determined that the granite outcropped in Xilinhot reservoir area, Inner Mongolia, has been iden- tified as I-type granite with high abundances of Cr, Co and Ni, high contents of Ca and A1, high ratios of N20/K20 and low contents of Fe and Mg. In the trace elements spider grams, the granitic rocks show clear negative Nb, Ta, P and Ti anomalies. In the SiO2-K20 and AFM diagrams, the granitic rocks are mainly plotted in area of calc-alkaline series. In the tectonic setting discrimination diagrams, the granitic rocks are plotted in the area of volcanic arc granites ＋syncollision granites, indicating that the granite formed in an island-arc environment. The granite yields a zircon U-Pb age of 317.0±4. 0 Ma, correspond- ing to the late Carboniferous age. The suite of late Carboniferous island-arc type granitic rocks indicates that the Palo-Asian Ocean was not fully closed in the Caledonian period and the oceanic crust subduction and consumption-related events still occurred in the late Ccarboniferous. It is concluded that the late Car- boniferous granite may be caused by the southward subduction of the North Hegenshan oceanic basin.