内蒙古阿尔塔拉中三叠世A型花岗岩锆石U-Pb年龄、地球化学特征及构造意义

Zircon U-Pb dating,geochemistry and tectonic implications of the Artala Middle Triassic A-type granite in Inner Mongolia

在内蒙古二连-贺根山缝合带新发现西乌旗阿尔塔拉A型花岗岩,岩性为二长花岗岩。锆石LA-ICP-MS U-Pb测年结果显示,该二长花岗岩的侵位年龄为242.9±1.5Ma,形成时代为中三叠世。阿尔塔拉A型花岗岩地球化学特征为高钾钙碱性系列,具有较高的SiO2(75.89%~76.79%)、K2O(4.18%~4.30%)和Na2O+K2O(8.23%~8.57%),贫CaO、MgO、Sr、Ba、Eu、Ti和P,相对富集Ga、Rb和Th。该岩石稀土元素配分曲线为海鸥式,具有显著的负Eu异常(δEu=0.11~0.14)。在地球化学判别图解上,阿尔塔拉A型花岗岩显示A2型后造山花岗岩特征,其形成于后造山伸展环境。结合二连-贺根山缝合带蛇绿岩、岛弧型-后造山型岩浆岩的时空演化关系,古亚洲洋二连-贺根山洋盆可能在二叠纪晚期闭合,并在三叠纪进入后造山伸展构造演化时期。

The newly recognized Artala A-type granite occurs along the Erenhot-Hegenshan suture in Xi Ujimqin Banner of Inner Mongolia and is composed of monzogranite.LA-ICP-MS zircon U-Pb dating yielded a weighted average age of 242.9±1.5Ma.The age suggests that the granite was emplaced in the Middle Triassic.The granite belongs to high-K Calc-alkaline series,and is characterized by rich SiO2(75.89%~76.79%)and K2O(4.18%~4.30%),high alkali(Na2O+K2O=8.23%~8.57%),low CaO,MgO,Sr,Ba,Eu,Ti,P,and relatively high Ga,Rb and Th.In addition,it has lower total rare earth element(REE)content with obviously negative Eu anomalies(δEu=0.11~0.14),showing a typical flat gull-wing shaped REE distribution pattern.The Artala monzogranite exhibits the typical geochemical characteristics of A-type granites.According to the chemical subdivision diagrams of the A-type granitoids,the Artala A-type granite belongs to A-type granitoid,which was formed and emplaced in a post-collisional extension setting.Based on the previous studies of the time-space evolution of the ophiolites,arc granitoids,and post-collisional granitoids in the Erenhot-Hegenshan suture,the authors suggest that the Erenhot-Hegenshan oceanic basin of the Paleo-Asian Ocean was probably closed at the end of the Permian and that the Erenhot-Hegenshan suture entered the post-collisional extension tectonic evolution period in Triassic.