柴达木盆地北缘果可山石英闪长岩LA-ICP-MS锆石U-Pb定年及其成因

LA-ICP-MS Zircon U-Pb Dating and Petrogenesis of the Quartz-Diorites from the Guokeshan Area in the Northern Margin of the Qaidam Basin

柴达木盆地北缘果可山地区石英闪长岩LA-ICP-MS锆石U-Pb定年获得的206 Pb/238 U加权平均年龄为247±3 Ma。石英闪长岩由斜长石(55%~60%)、石英(15%~20%)、角闪石(10%~15%)和黑云母(5%~10%)组成,属准铝质-弱过铝质系列(A/CNK=0.95~1.02),具有I型花岗岩特征。样品富集轻稀土元素和大离子亲石元素(K、Rb、Cs、Ba、Th等),相对亏损重稀土元素和高场强元素(Nb、Ta、P、Ti等),类似弧岩浆岩。样品中MgO含量(2.04%~2.44%)和Mg#值(48.9~51.9)均高于变玄武岩/变泥岩在1~4GPa压力下部分熔融形成的熔体,且包含大量暗色包体,这暗示它们可能是由角闪岩相下地壳部分熔融产生的岩浆和富集地幔来源的岩浆发生混合而成。低的锆石Ti温度和高Ce^(4+)/Ce^(3+)比值,表明其结晶自低温、高氧逸度岩浆,岩浆形成过程中可能有富水熔体/流体的加入。在花岗岩构造环境判别图解上,样品均落在火山弧环境;同时,样品La/Nb比值(2.27~2.64)与活动大陆边缘区高La/Nb比值(>2.0)特征相吻合。结合区域地质背景分析,我们认为果可山石英闪长岩形成于古特提斯洋向北俯冲的活动大陆边缘弧环境。

LA-ICP-MS zircon U-Pb dating yields a weighed mean age of 247±3 Ma for quartz-diorite at Guokeshan area. These rocks consist of plagioclase （55% to 60%）, quartz （15% to 20%）, hornblende （10% to 15%） and biotite （5% to 10%） and belong to rnetaluminous to weakly peraluminous with A/ CNK values varing from 0. 95 to 1. 02, indicating I-type granite affinity. They are characterized by enrichment of LREEs and LILEs （eg. , K, Rb, Cs, Ba, Th） and depletion of HREEs and HFSEs （eg. , Nb, Ta, P, Ti）, which are consistent with typical features of arc-magmatite. In addition to the presence of abundance mafic microgranular enclaves in samps, they exhibit higher MgO contents （ranging from 2.04% to 2.44%） and Mg^# values （ranging from 48. 9 to 51. 9） than experimental melts generated by partial melting of metabasalts/pelites at 1 to 4 GPa, suggesting that they were derived from by magmas mixing between magma produced by partial melting of amphiboite in lower crust and magma derived from enrich mantle. The low Ti-in-zircon temperatures and high zircon Ce4＋/Ce3＋ ratios indicate zircons were crystallized from low temperature and high oxygen fugacity magmas, and the involvement of water-bearing melts/fluids during the formation of the magmas. All samples fall into the fields of volcanic arc granite in the granite tectonic environment discrimination diagrams, and show high La/Nb ratios （ranging from 2.27 to 2.64）, which are consistent with active continental margin area （〉2.0）. Combined with regionally geological setting, we suggest that the Guokeshan quartz-diorites are formed as a continental margin arc by the northern subduction of Palaeo-Tethys ocean.