柴北缘双口山石英正长岩锆石U-Pb定年、地球化学及Sr-Nd同位素特征

本文报道了柴北缘西段双口山地区石英正长岩锆石U-Pb年代学,全岩地球化学和Sr-Nd同位素等分析结果。石英正长岩岩体具有高硅(SiO_2=59.47%～68.66%)、碱性?过碱性(全碱含量为11.18%～14.04%),贫CaO(0.31%～3.30%)的特征。其里特曼指数σ为4.87～9.55、铝饱和指数(A/CNK)=1.12～1.36,属钾玄岩系列,过铝质范围。稀土元素总量高(ΣREE=231.31×10^–6～569.26×10^–6),富集轻稀土元素,亏损重稀土元素((La/Yb)_N=24.03～58.33),具弱Eu负异常(Eu/Eu*=0.72～0.95);微量元素亏损Ba、Sr、Ti、P,富集Rb、Th、U、Zr、Y和Ga,具有A型花岗岩特点,根据微量元素组成判别其产出于板内环境。锆石LA-ICP-MS U-Pb定年获得加权平均年龄为243.5±1.6 Ma,属中三叠世。样品的?_Nd(t)介于–1.08～–2.15,二阶段模式年龄t_DM2=1.10～1.19 Ga,表明其具有多源特征,是受到俯冲组分改造或影响的岩石圈地幔熔体和中新元古代变质基底熔体发生岩浆混合作用的结果,其形成与三叠纪古特提斯洋向北俯冲导致弧壳增厚和发生弧后拉裂有关。

Zircon U-Pb Geochronology,Geochemistry, and Sr-Nd Isotopes of Shuangkoushan Quartz-syenite,Northern Qaidam,China

The Shuangkoushan quartz-syenite is situated in the west segment of Northern Qaidam.This paper reports results of LA-ICP-MS zircon U-Pb dating,whole-rock geochemical and Sr-Nd isotopic analyses of the quartz syenite.Geochemically,the Shuangkoushan quartz-syenite shows enrichment in silicon(SiO 2=59.47%-68.66%)and total alkali(Na 2 O+K 2 O=11.18%-14.04%),and depletion in CaO(0.31%-3.30%).The quartz-syenite belongs to peraluminous(σ=4.87-9.55)and shoshonitic series(A/CNK=1.12-1.36).The samples of quartz-syenite are rich inΣREE(231.31×10^-6-569.26×10^-6)with negative Eu anomalies(Eu/Eu*=0.72-0.95)and(La/Yb)N values of 24-58.The quartz-syenite is characterized by enrichment of LILEs and HFSEs such as LREE,Rb,Th,U,Zr,Nb,Y and Ga and depletion of Ba,Sr,Ti and P.The trace element signatures indicate that the Shuangkoushan quartz-syenite is A-type granite.LA-ICP-MS zircon U-Pb dating shows that it was emplaced at 243.5±1.6 Ma.TheεNd(t)values(-1.08 to-2.15)and t Dm^2(1.10 to 1.19 Ga)of the Shuangkoushan quartz-syenite indicate a magma mixing origin,i.e.,the parant magma was likely mixture of melts of the Meso-Neo-Proterozoic metamorphic basement and lithosphere mantle that affected by subduction components.In conclusion,the Shuangkoushan A-type granite(A 1)was related to the thickening and stretching of post arc,which caused by the northward subduction of the Paleo-Tethys Ocean during Triassic period.