内蒙古中部四子王旗地区北极各岩体锆石定年及其岩石化学特征

内蒙古中部四子王旗北极各花岗岩基位于兴蒙造山带的二道井—查干乌拉—红格尔缝合带南缘,主要由正长花岗岩组成。为精确厘定该岩体的形成时代,对其进行了锆石LA-ICP-MS U-Pb定年,获得了(264.0±3.4)M a(MSWD=1.5)的206Pb/238U加权平均年龄,应代表其侵位年龄。岩石地球化学数据显示该岩体具相对富碱、富钾的特征,w(K2O+Na2O)变化于8.0%~9.29%之间,w(K2O)/w(Na2O)比值为1.20~2.59,属于高钾钙碱性岩石系列;w(Rb)/w(Sr)值较低(2.4~4.2),富集大离子亲石元素(LILE),(La/Yb)N介于3.1~13.6之间,亏损高场强元素(HFSE)等特点显示其应为Ⅰ型花岗岩。岩石暗色矿物为黑云母,副矿物出现榍石而未见富铝矿物,富含暗色包体,与过铝质花岗岩相伴生,为较典型的KCG岩石组合,属于由俯冲作用向大陆碰撞演化过程中构造体制转换阶段的产物,而在构造环境判别图解中均落入同碰撞花岗岩区域。同位素定年和岩石地球化学特征表明北极各岩体形成于同碰撞环境,可能是大规模岩浆底侵作用导致地壳熔融的结果。

Dating and Petrochemistry of the Beijige Pluton in Siziwangqi,Inner Mongolia

The Beijige pluton is located in the Siziwangqi area, Inner Mongolia, tectonically belongs to the Xing-Meng orogenic belt, lying on the southern margin of the Erdaojing-Chaganwula-Honggeer suture zone. It is predominantly composed of syenogranite. LA-ICP-MS U-Pb zircon dating and petrochemical analysis have been carried out in order to understand the formation age and natures of the pluton. As a result, zircons from syenogranite give a LA-ICP-MS mean U-Pb age of (264.0±3.4)Ma(MSWD =1.5). Petrochemical data indicate that syenogranite belong to high-alkaline series (K_2O+Na_2O=8.0%-9.29 %, K_2O/Na_2O=1.20-2.59), low Rb/Sr ratios (2.4-4.2), rich in LILE((La/Yb)N =3.1-13.6), deplete in HFSE show an affinity to I-type granite. The main dark mineral and accessory mineral of syenogranite are biotite and sphere, with abundant mafic microgranular enclaves, a common feature of KCG, which formed in the transition stage of continental subduction and collision. In the discrimination diagrams, all samples fall into the syn-collision area. Geochronological and petrochemical data imply that the Beijige pluton is related to a syn-collisional regime, maybe triggered by an extensive underplating.