西藏南部米拉山地区早侏罗世花岗岩地球化学特征及其地质意义

中南拉萨地块内部早侏罗世时期岩浆岩的成因差异对新特提斯洋的早期演化具有指示意义，本次工作选取位于洛巴堆－米拉山断裂带两侧的宗沃花岗岩体和仲达花岗岩体作为研究对象，进行详细的岩相学、年代学和全岩地球化学分析.锆石U-Pb定年结果分别为193.8±2.2 Ma和197.5±1.8 Ma，指示了区内的早侏罗世岩浆事件.宗沃花岗岩与仲达花岗岩样品均具有较高的SiO₂含量(69.80%~74.64%)与较低的A/CNK值(0.98~1.07)，且富集轻稀土元素及Rb、Th、K等大离子亲石元素，亏损Nb、Ta、Ti等高场强元素.全岩地球化学特征指示两处样品均属典型的Ⅰ型火山弧岩浆岩.结合前人的研究成果表明，中南拉萨地块内部的早侏罗世岩浆岩形成于新特提斯洋北向俯冲下的陆内弧环境.本次工作的研究结果结合区域内早侏罗世岩浆岩的全岩地球化学和同位素数据，指示南拉萨地块内早侏罗世岩浆岩主要来源于新生下地壳，而中拉萨地块内早侏罗世岩浆岩的岩浆源区存在着更多古老下地壳成分的加入. 

Geochemical Features and Geological Significance of Early Jurassic Granites in Milashan Area,Southern Tibet

The petrogenetic differences of the Early Jurassic magmatic rocks within the central and southern Lhasa subterranes are indicative of the early-stage evolution of the Neo-Tethyan Ocean. In this study, we carry out petrography, geochronology and geochemistry analysis on Zongwo granitic pluton and Zhongda granitic pluton on both sides of the Luobadui-Milashan Fault. The zircon U-Pb dating results from two plutons are 193.8±2.2 Ma and 197.5±1.8 Ma, indicating the Early Jurassic magmatism in the study area. Granites samples from the Early Jurassic plutons are characterized by high SiO₂ (69.80%-74.64%) contents, low A/CNK (0.98-1.07), enrichments in lighter rare earth elements (LREEs) and the large-ion-lithophile elements (LILEs) such as Rb, Th, K, and depletions in high-field strength elements (HFSEs) such as Nb, Ta, Ti. Geochemical characteristics show that both samples are typical Ⅰ-type volcanic arc magmatic rocks. Combined with previous research results, it is shown that the Early Jurassic magmatic rocks in the central and southern Lhasa subterranes were formed by the northward subduction of the Neo-Tethyan oceanic crust. Our data, in combination with geochemical and isotopic data indicate that the Early Jurassic magmatic rocks in the southern Lhasa subterrane were mainly derived from the juvenile lower crust, while the magmatic source of the magmatic rocks in the central Lhasa subterrane had more involvement of ancient lower crustal components.