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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。
The study of the Indosinian granite belt spreading from northwest to southeast in the Nanshan tectonic belt of Qinghai is of great significance to clarify the cohesion and transformation relations between the West Qinling orogenic belt and the South Qilian tectonic belt in the Guteethis evolution stage. Dangjiashan granite body is a composite granite base composed of granodiorite and monzogranite. In this paper, detailed geochronology, lithochemistry and LA-ICP-MS zircon U-Pb isotopic dating have been carried out on the Dangjiashan granite. The results show that the crystallization ages of granodiorite and monzogranite are 240.1 ± 2.1 Ma and 241.0 ± 2.6 Ma, respectively, belonging to Middle Triassic. Geochemical studies show that the relatively high-Si (SiO_2 = 66.37% -73.99%) and potassium-rich (K_2O = 3.37% -4.73%) rocks belong to the quasi-Al-weak permafrost calc-alkaline I-type granites. The REE distribution curves show a right-inclined light LREE enrichment with a moderate negative Eu anomaly (δEu = 0.32-0.64). Trace elements show enrichment of large ion lithophile elements Cs, Rb, K, relatively depleted high field strength elements Nb, Ta, P, Ti. The study on the genesis of rocks shows that it is partially melted in the source rocks mainly formed by the basic crust in the lower crust, and there is a certain degree of mantle-derived magma contribution. Combined with regional geological background data, it is considered that the Dangjiasi granite body formed in the subduction stage of the oceanic crust may be the product of the geophysical background of the southward subduction of the Chongzongyang ocean.