内蒙古科尔沁右翼前旗地区黑云母二长花岗岩的岩石地球化学、年代学特征及其地质意义

兴蒙造山带的构造格局、分布规律和演化过程十分复杂,特别是古亚洲洋“剪刀式”闭合的最后时限及随后的陆-陆碰撞过程仍未达成统一共识。文中研究区位于兴蒙造山带东段,大地构造位置夹持于贺根山—黑河断裂带和索伦—西拉木伦—长春断裂带之间,为研究中亚造山带东段的地质演化提供重要窗口。内蒙古科尔沁右翼前旗黑云母二长花岗岩岩体呈岩株状产出,长轴约400 m,短轴150~250 m,长轴呈NE向展布,北侧侵入大石寨组,南部与林西组呈断层接触关系。岩石为中-细粒花岗结构,块状构造,主要由钾长石(40%)、斜长石(25%)、石英(20%)和黑云母(14%)及少量的绿泥石(1%)等组成。锆石LA-ICP-MS U-Pb测年结果为(221.1±2.1) Ma,表明黑云母二长花岗岩形成于晚三叠世。岩石地球化学表现为高SiO₂(68.46%~70.38%),高K₂O+Na₂O(9.02%~9.39%),低P₂O₅(0.09%~0.17%),低MgO(0.60%~0.80%),低TiO₂(0.54%~0.58%),中等K₂O/Na₂O(0.90~1.54),轻重稀土元素分馏明显,轻稀土相对富集(∑LREE含量为(163.55~226.55)×10^-6),重稀土相对亏损(∑HREE含量为(11.96~21.67)×10^-6),明显富集大离子亲石元素Rb、Th、Ba和K,亏损高场强元素Nb、Zr和Hf,表现出过铝质高钾钙碱性I型花岗岩特征,同时与华北陆块北缘二叠纪末—三叠纪岩浆岩形成于后碰撞/后造山的构造环境相符。综合前人研究成果和区域地质资料,推断内蒙古科尔沁右翼前旗地区黑云母二长花岗岩形成于岩石圈拆沉及软流圈上涌的环境中。

Petrogeochemistry and geochronology of biotite monzonitic granites in the Horqin Right Front Banner area,Inner Mongolia and the geological significance

The Xing-Meng orogenic belt has very complex structural and distribution patterns. Its evolutionary process, especially the final time limit for the closure of the “scissors” type of the Paleo-Asian Ocean and the subsequent land-land collision process have not been determined. The study area is located in the eastern section of the Xing-Meng orogenic belt, where the geotectonic position is clamped between the Hegenshan-Heihe and Solon-Siramulon-Changchun fault zones. It provides an important window for studying the geological evolution of the eastern section of the Central Asian orogenic belt. The Qianqi biotite monzogranite body in the right wing of Horqin, Inner Mongolia has the shape of a rock plant, with a 400 m NE-trending long axis and a 150-250 m short axis. The north side intrudes into the Dashizhai Formation, and the south part is in fault contact with the Linxi Formation. The granite rocks had medium to fine-grained granitic and massive structures. They were mainly composed of potash feldspar (40%), plagioclase (25%), quartz (20%), biotite (14%) and a small amount of chlorite (1%), and had the LA-ICP-MS zircon U-Pb age of 221.1±2.1 Ma, consistent with formation in the Late Triassic. The geochemical characteristics of rocks were high in SiO₂(68.46%-70.38%) and K₂O+Na₂O(9.02%-9.39%) and low in P₂O₅(0.09%-0.17%), MgO(0.60%-0.80%), TiO₂(0.54%-0.58%) and medium K₂O/Na₂O(0.90-1.54) contents, with obvious fractionation of light and heavy rare earth elements (LREE and HREE), relative enrichment of LREE (ΣLREE content (163.55-226.55)×10^-6) and depletion of HREE (ΣHREE content (11.96-21.67)×10^-6). The granite type is peraluminous high-k calc alkaline I-type, indicating a post collisional/orogenic tectonic environment of the Late Permian Triassic magmatic rocks on the northern margin of the North China block. Based on previous research results and regional geological data, we infer that the Qianqi biotite monzogranite was formed under a lithosphere delamination and asthenosphere upwelling environment.