青海南山当家寺花岗岩体锆石U-Pb年代学、地球化学及其地质意义

LA-ICP-MS Zircon U-Pb Dating and Geochemistry of the Dangjiasi Granitic Complex in the Qinghai Nanshan Tectonic Zone,and Its Geological Implications

青海南山构造带发育北西—南东向展布的印支期花岗岩带,其研究对厘清古特提斯演化阶段西秦岭造山带与南祁连构造带的衔接转换关系具有重要意义。当家寺花岗岩体是由花岗闪长岩和二长花岗岩组成的复合花岗岩基。本文对当家寺花岗岩体进行了详细的岩石学、岩石地球化学和LA-ICP-MS锆石U-Pb同位素年代学研究。结果表明,花岗闪长岩和二长花岗岩的结晶年龄分别为240.1±2.1 Ma和241.0±2.6 Ma,属于中三叠世。地球化学研究显示岩体相对高硅(SiO_2=66.37%~73.99%)和富钾(K_2O=3.37%~4.73%),属于准铝质—弱过铝质高钾钙碱性I型花岗岩。岩石稀土配分曲线表现为轻稀土富集的右倾型,具中等程度的铕负异常(δEu=0.32~0.64)。微量元素显示富集大离子亲石元素Cs、Rb、K,相对亏损高场强元素Nb、Ta、P、Ti。岩石成因研究表明其形成于下地壳基性岩为主的源岩的部分熔融,同时存在一定程度幔源岩浆贡献。结合区域地质背景资料,认为当家寺花岗岩体形成于洋壳俯冲阶段,可能为宗务隆洋向南俯冲的地球动力学背景下的产物。

An NW-SE trending Indosinian granitoide belt occurs within the Qinghai Nanshan tectomc zone, and the study of this granite zone has great scientific significance for investigating the transitional relation between the West Qinling and the Qilian orogenic belts in Paleozoic Tethys evolution stage. The Dangiiasi granitic complex consists dominantly of granodiorite pluton and monzogranite pluton. In this paper, a detailed study for the LA-ICP-MS zircon U-Pb dating, petrology and geochemistry characteristics was carried out for the Dangjiasi granitic complex. The results show that the crystallization ages of the Dangjiasi granodiorite and monzogranite are 240.1＋2.1 Ma and 241.0±2.6 Ma, respectively, belonging to the Middle Triassic. The whole rock geochemical data show that the Dangjiasi granitic complex is relatively rich in SiO2 （66.37±73. 99%） and K2O （3.37%-4.73%）, suggesting that the granitic rocks are metaluminous to weak peraluminous high-K calc-alkaline I-type granite. The rocks are enriched in LREE and depleted in HREE. The REE patterns show rightward incline and moderate negative Eu anomaly （SEu= 0. 32 - 0. 64）. The trace element geochemistry is evidently characterized by positive anomaly of LILE（e. g. Cs, Rb, K）and negative anomaly of HFSE（e. g. Nb, Ta, P, Ti）. The Dangjiasi grantic complex originated mainly from the partial melting of lower crustal basic rocks, with minor mantle-derived component. In combination with analyses of regional geological setting, we suggest that the Dangjiasi granitic complex was formed in the stage of the southward subduction of the Zongwulong Oceanic crust.