西昆仑上其木干岩体岩石成因及地质意义——地球化学及U-Pb年代学证据

Petrogenesis and geological significance of Shangqimugan plutons in Western Kunlun:Evidence from geochemistry and U-Pb chronology

上其木干岩体位于新疆阿克陶县上其木干,区域上沿乔拉克克-苏盖特-阿加尔东断裂呈NW-SE向带状分布。岩性主要为二长花岗岩,SiO_2含量为66.06%~75.13%,Al_2O_3含量为12.45%~14.78%,(K_2O+Na_2O)平均含量为8.82%,K_2O/Na_2O均值为1.25,铝指数A/CNK(0.89~0.98)<1.1,属高钾偏铝质钙碱性系列。岩石稀土元素总量中等(∑REE=231×10^(-6)~404×10^(-6)),轻重稀土元素分异明显,球粒陨石标准化配分模式为平滑右倾型,δEu平均为0.17,具有明显的Eu亏损;明显富集相容元素Ni、Co、Cr、V,不相容元素Rb、Th以及高场强元素Zr、Hf等元素,强烈亏损Ba、Sr、P和Ti元素,相对亏损Nb和Ta元素。采用LA-ICP-MS锆石U-Pb同位素测年,得到二长花岗岩的^(206)Pb/^(238)U加权平均年龄为225.4±1.9 Ma(MSWD=6.0),认为岩体属于晚三叠世早期的岩浆活动产物。岩石地球化学特征和微量元素判别图解显示岩石为壳幔岩浆混合成因,具有A型花岗岩特点,形成于后碰撞伸展阶段的张性环境,标志着西昆仑造山带在晚三叠世早期造山作用的结束和板内构造演化的开始,为后碰撞环境向板内伸展环境的过渡,同时也标志着古特提斯洋在该区晚三叠世早期恰好完全闭合。

Located in Shangqimugan area of Akto County in Xinjiang, Shangqimugan plutons exhibit NW-SE trending zonal distribution along the Qiaolakeke-Sugaite-Ajiaerdong fracture. The plutons are mainly composed of monzonitic granite. The SiO2 values range from 66.06 % to 75.13 % and A1203 from 12.45 % to 14.78 %. Average （K2O＋ Na2O） value is 8.82%, average K2O/Na2O ratio is 1.25, and aluminum index A/CNK （0.89 ～0.98） 〈 1.1, suggesting that the plutons belong to high-K metaluminous calcicalkaline series. The total REE values of the plutons are 231 × 10^-6 404 × 10^-6, and LREE and HREE show obvious fractionation. Chondritenormalized REE patterns are of smoothly rightoblique type. Average value of 3Eu is 0.17, implying obvious depletion of Eu. Compatible elements （Ni, Co, Cr, V）, incompatible elements （Rb, Th）and HFSE （Zr, Hf, etc. ） are obviously concentrated; Ba, Sr, P and Ti are strongly impoverished; Nb and Ta are depleted in a certain extent. LA-ICP-MS zircon U-Pb dating yielded a 206Pb/238U weighted mean age of 225.4± 1.9 Ma （MSWD = 6.0）, indicating that the plutons belong to early Late Triassic. Combined with geochemical characteristics and trace elements discrimination diagrams, the authors hold that the rocks resulted from the mixing of the crust and mantle magmas, belonged to A-type granite and formed in the tensional environment of a postcollision extensional phase, which marked the end of the orogeny of the West Kunlun orogenic belt and the beginning of intraplate tectonic evolution in early Late Triassic, implying a transition from the collision environment to the intraplate extensional environment. These data also demonstrate the complete closure of Palec-Tethys in this region in early Late Triassic.