青海省赛支寺花岗闪长岩及其暗色包体成因:锆石U-Pb年代学、岩石地球化学和Sr-Nd-Hf同位素制约

祁连造山带分为南祁连、中祁连和北祁连构造带.赛支寺岩体位于中祁连与南祁连构造带的结合部位.首次发现了赛支寺花岗闪长岩及其暗色包体，然而对于暗色包体的成因机理以及与寄主岩石之间的成因联系仍存在很多争议.并对其进行了系统的锆石U-Pb年代学、Lu-Hf同位素、岩石地球化学以及Sr-Nd同位素地球化学研究，探讨赛支寺岩体及其暗色包体的成因及动力学背景.LA-ICP-MS锆石U-Pb年代学表明，赛支寺花岗闪长岩形成于446.1±1.3 Ma，包体形成于446.0±1.0 Ma，两者在误差范围内一致，排除了包体为捕虏体成因.暗色包体具较低的SiO₂含量、较高的Na₂O/K₂O比值，低Sr/Y、La/Yb比值，与寄主岩稀土配分曲线基本一致，但LREE相对较低；86Sr/87Sr=0.706 4~0.706 7，ε_Nd(t)=-7.38~-7.97；发育针状磷灰石，形成于岩浆混合作用.寄主岩SiO₂=66.45%~68.12%，Na₂O/K₂O=0.80~0.97，A/CNK=0.91~1.03，显示准铝质-弱过铝质岩浆特点；富集大离子亲石元素Rb、Th、U、K，亏损Nb、Ta等元素，高Sr/Y、La/Yb比值，轻稀土富集，弱负Eu异常；86Sr/87Sr=0.709 3~0.709 5，ε_Nd(t)=-1.75~-1.03，与祁连造山带I型花岗岩相似；锆石ε_Hf(t)=1.7~6.8，T_DM2=995~1 750 Ma.综上所述认为，寄主花岗闪长岩形成于壳幔岩浆混合.结合区域地质背景，赛支寺花岗闪长岩形成于俯冲背景下，幔源岩浆上涌，侵入到下地壳中，造成下地壳物质熔融，由这种既有幔源物质又有古老地壳物质的花岗岩岩浆形成. 

Origin of Granodiorite and Mafic Microgranular Enclave in Saizhisi,Qinghai Province: Zircon U-Pb Geochronological,Geochemical and Sr-Nd-Hf Isotopic Constraints

The Qilian orogenic belt is divided into the North, Central and South Qilian belts. The Saizhisi granodiorite is located between Central and South Qilian. In order to study the petrogensis and dynamic background of Saizhisi rocks, zircon U-Pb chronology, Hf isotope, whole rock geochemistry and Sr-Nd isotopes are reported for granodiorite and mafic microgranular enclaves (MMEs) in this area. The LA-ICP-MS analyses on zircons from both the Saizhisi granodiorite and MMEs yielded similar ages at 446.1±1.3 Ma and 446.0±1.0 Ma, respectively. The MMEs contain needle-like apatite, and they are characterized by lower SiO₂ contents and higher Na₂O/K₂O ratios. Their REEs are similar to those of Saizhisi granodiorite and ∑REE are lower than those of the host granodiorite; The 86Sr/87Sr of MMEs=0.706 4-0.706 7, the ε_Nd(t) of MMEs=-7.38 to -7.97. The elemental, U-Pb dating and Sr-Nd isotope suggest that they might have been formed by mixing of mafice and felsic member magmas. The Saizhisi granodiorite is metaluminous to weakly peraluminous with SiO₂=66.45%-68.12%, Na₂O/K₂O=0.80-0.97, and A/CNK=0.91-1.03. The granodiorite is enriched in LILEs and depleted in HFSE, enriched in LREE contents and depleted in HREE contents with negative Euanomaly. The 86Sr/87Sr(0.709 3-0.709 5) and ε_Nd(t) (-7.3794--7.9674) of Saizhisi granodiorite is similar to those of I-type granite of the Qilian Orogen belt. The ε_Hf(t) of Saizhisi granodiorite=1.7-6.8, T_DM2=995-1 750 Ma. It is concluded that the Saizhisi granodiorite might have been formed by mixing of mafice and felsic member magmas. Combined with the regional tectonic evolution, it is suggested that the Saizhisi granodiorite was formed in a subduction environment and mostly generated by the mantle which was formed by the lower crust intruded by the upwelling mantle.