青海南祁连科克岩体地球化学特征及其形成时代

Geochemical characteristics and formation age of Keke Pluton in South Qilian Mountain,Qinghai Province

为探讨南祁连喀克图蒙克山科克岩体的构造属性,对南祁连喀克图蒙克山中奥陶世科克岩体进行了详细的地球化学和定年研究。研究结果表明,该岩体主体岩性为正长花岗岩,具有高SiO_2(68.41%～73.10%)、高Al_2O_3(13.03%～14.35%)的特征,亏损高场强元素Nb、P、Ti和重稀土元素,发育明显的负Eu异常,反映岩石经历了强烈的分离结晶作用,其稀土元素配分曲线类型与壳源花岗岩相似;构造成因分类图解表明,这些花岗岩属同碰撞环境的产物。LA-ICP-MS锆石U-Pb测年结果表明,岩体形成于461±2.7Ma,时代为中奥陶世,表明该岩体的源岩为壳源或壳幔混合的产物。结合区域地质特征认为,该岩体为同碰撞作用的产物,佐证了南祁连地块在奥陶纪处于挤压状态,对进一步研究南祁连造山带的演化具有重要意义。

Lying in the middle of Kaketumengke Mountain, the Keke pluton which was formed in Middle Ordovician is located inthe southeastern part of South Qilian massif. Geochemical and geochronological studies were conducted to investigate the tectonic at-tribute of the Keke pluton. The pluton is mainly composed of syenogranite and is characterized by high content of SiO2 （68.41%-73.10%） and high content of Al2O3 （13.03%-14.35%）. Geochemical data also show depletion of Nb, P, Ti and HREE with Eu nega-tive anomaly. The REE patterns show the characteristics of crustal anatectic granite,and the genetic classification diagram shows thatthe granite was formed in a syn-collisional setting. LA-ICP-MS zircon U-Pb dating shows that gneissic granite was formed at 461±2.7Ma. Combined with regional geology, the authors hold that Keke pluton was probably derived from the source of the crust and re-sulted from syn-collision, which supports the argument of compressive regime of the Qilian block during the Mid-Ordovician and isof great significance for the further study of the evolution of the Qilian block.