青海冷湖盐场北山花岗闪长岩年代学、地球化学及其Hf同位素特征

Geochronology,Geochemistry and Hf Isotope Characteristics of Yanchangbeishan Granodiorite of Lenghu Area in Qinghai

盐场北山花岗闪长岩位于柴北缘西端青海冷湖地区。岩体高Si(71.19%~71.81%),富Na(4.52%~4.85%)。铝饱和指数平均为1.02,属弱过铝质花岗岩。岩体富集大离子亲石元素(K、Rb、Sr、Th),亏损高场强元素(Nb、Ta),具弧型花岗岩特征。LA-ICP-MS锆石U-Pb测年结果表明岩体形成于(265±2)Ma。锆石Hf同位素初始比值176 Hf/177 Hf分布于0.282 962 0~0.283 048 7,并具有的正εHf(t)值(12.54~15.19),其平均两阶段模式年龄TDM2(Hf)为367 Ma,反映岩体源区可能为新生的镁铁质下地壳,另外,εHf(t)基本位于亏损地幔演化线附近,指示该玄武质下地壳来源于亏损地幔。结合区域地质背景和岩石地球化学特征,认为盐场北山花岗闪长岩与泥盆纪镁铁-超镁铁岩有关,由于宗务隆裂谷小洋盆的向南俯冲,引发其上覆玄武质新生地壳发生熔融形成,进而揭示晚二叠世末柴北缘处于火山弧或活动陆缘的构造环境。

Granodiorite develops in Yanchangbeishan of Lenghu area in Qinghai, which is located in the west segment of the north margin of Qaidam. The samples have higher SiOz （71.19%- 71.81%） and Na20 contents （4.52%-4.85%）. The alumina saturation average index is 1.02, indicating that the granodiorite belong to weak peraluminous granites. Moreover, these samples are depleted in HFSEs （Nb, Ta） intensively, rich in LILEs （K, Rb, Sr, Th）, reflecting the charac- teristics of island-arc magma. LA-ICP-MS zircon U-Pb dating shows that the formation age of the rock is 265~2Ma. The zircon Hf isotope analysis reveal 178 Hf/l^7 Hf ratios varying in 0. 282 962 0 to 0. 283 048 7, with high positive cHf （t） （12.54-15.19） and two-stage Hf model ages （averaged at 367 Ma）. In the 176Hf/177Hf-eHf （t） discrimination diagrams, these zircons were plotted nearly on the depleted mantle evolution line, it can be concluded that the rock chiefly derived from the new basaltic lower crust which is originated from depleted mantle. Based on the regional geological background and the geochemical characteristics of the masses, we consider that Yan