青海南山构造带印支早期基性杂岩体年代学、地球化学特征及地质意义

青海南山构造带是衔接宗务隆构造带、南祁连构造带和西秦岭造山带的重要结合带.该地区印支早期岩浆作用强烈，虽然该期基性岩分布少而分散，但其成因研究对探讨青海南山构造带印支期演化过程具有重要意义.本文对构造带中段由辉长岩和辉石岩组成的拉日基性杂岩体开展了详细的岩石学、矿物学、岩石地球化学及LA-ICP-MS锆石U-Pb同位素年代学研究.结果表明，辉长岩和辉石岩的结晶年龄分别为247.7±2.8 Ma和247.9±2.5 Ma，说明岩体侵位于早三叠世晚期.岩石具有低Si、富Na、低K和高Mg、Fe的特征；其中，辉长岩属拉斑玄武岩系列，同时，高Al₂O₃/TiO₂比值和低TiO₂含量与玻安岩和玻玄岩相似.岩石的地球化学特征与洋脊和板内玄武岩差异明显，其富集LREE和大离子亲石元素LILEs（Cs、Rb、K、Sr），亏损Nb、Ta、Zr、Hf、Ti、P等高场强元素，显示与岛弧岩浆岩相似的地球化学特征.矿物电子探针分析表明，斜长石为类似于岛弧或活动大陆边缘辉长岩中的高钙斜长石.辉长岩和辉石岩均为尖晶石二辉橄榄岩部分熔融的产物.综合分析表明，研究区可能存在晚古生代?早中生代有限小洋盆，拉日基性杂岩体形成于洋盆俯冲早期阶段. 

Zircon U-Pb Geochronogy,Geochemistry and Its Geological Implication of the Early Indosinian Basic Complex in the Qinghai Nanshan Tectonic Belt

The Qinghai Nanshan tectonic belt, situated in the northern margin of Gonghe basin, is the conjunction area among the West Qinling orogenic belt, South Qilian tectonic belt and Zongwulong tectonic belt. A large quantity of intrusive rocks outcropped in the Qinghai Nanshan tectonic belt, but the distribution of basic rocks is less and dispersed. The petrogenesis of basic rocks is significant to reveal the tectonic framework and evolution history in that period.The Lari basic complex is composed of gabbro and pyroxenite, which situates in the middle of the Qinghai Nanshan tectonic belt. In this paper, a detailed study on petrology, mineralogy, geochemistry and LA-ICP-MS zircon U-Pb dating was carried out for the Lari basic complex. The results show that the crystallization ages of the gabbro and pyroxenite are 247.7±2.8 Ma and 247.9±2.5 Ma, respectively, suggesting that the complex intruded in the late Early Triassic. The whole rock geochemical data show that the rocks of Lari basic complex are relatively rich in Na, Mg and Fe, but poor in Si and K, suggesting that the gabbros belong to the tholeiitic series. What's more, they have high ratios of Al₂O₃/TiO₂, low contents of TiO₂, which are typical features of boninite and boni-basalts. The rocks are enriched in LREE, LILEs (Cs, Rb, K, Sr) and depleted in HREE (Nb, Ta, Zr, Hf, Ti, P), which are similar to the arc magma, but different from ridge and intraplate basalt. The mineral electron microprobe analysis shows that plagioclase in the rocks of Lari basic complax is similar to the high calcium plagioclase in gabbro which originates in the island arc or active continental margin. The gabbro and pyroxenite of the Lari basic complex were formed by partial melting of the spinel lherzolite. In combination with analyses of regional geological setting, we suggest that there existed a Late Paleozoic-Early Mesozoic limited oceanic basin in the Qinghai Nanshan area. The Lari basic complex formed during the early stage of the oceanic basin subduction.