北祁连造山带窑洞沟地区中奥陶世花岗岩成因——锆石U-Pb年代学和岩石地球化学制约

Petrogenesis of the middle Ordovician granite in the Yaodonggou area of the North Qilian orogenic belt: Constraints from zircon U-Pb geochronology and geochemistry

对北祁连造山带窑洞沟地区花岗岩进行了LA-ICP-MS锆石U-Pb测年以及主量和微量元素地球化学分析,结果表明,窑洞沟花岗岩的结晶年龄为466±2 Ma。窑洞沟花岗岩具有较高的SiO_2(72.45%～80.18%)和全碱含量(Alk)(6.04%～8.12%),且K_2O含量低,K_2O/Na_2O值为0.07～0.71,里特曼指数在0.96～2.16之间,小于3.3,铝饱和指数(A/CNK)为0.94～1.01,为低钾到中钾钙碱性系列的准铝质岩石;在球粒陨石标准化的稀土元素配分模式图上表现出轻稀土元素富集的右倾型,具有中等的负Eu异常,δEu介于0.67～0.81之间,在原始地幔标准化的微量元素蛛网图上明显富集Rb、K等大离子亲石元素和Th、U等放射性元素,亏损Nb、Ta、Ti、P等高场强元素,在P_2O_5-SiO_2协变关系图上与Ⅰ型花岗岩趋势一致,在Y-Rb图上显示出分异Ⅰ型花岗岩的特征。结合区域地质背景,认为窑洞沟花岗岩的形成可能与北祁连洋盆持续向北俯冲导致弧后拉张伸展,诱发幔源岩浆岩底侵上升,致使古老地壳物质重熔形成母岩浆,再经历中等程度分离结晶作用有关,其岩浆具有岛弧岩浆属性,可能是由于弧后盆地在岛弧的基础上裂开或者其岩浆源区受到了俯冲板片流体/熔体的改造作用。

This paper reports LA-ICP-MS zircon U-Pb dating data as well as whole rock major and trace element data from granite pluton in the Yaodonggou area of the North Qilian orogenic belt.The result shows that the pluton was emplaced at 466±2 Ma.The Yaodonggou granite contains high SiO2(72.45%~80.18%)and Alk(6.04%~8.12%).Its K2O content is less than that of Na2O with K2O/Na2O ratio of 0.07~0.71,Ritterman index of 0.96~2.16(less than 3.3),and A/CNK of 0.94~1.01.The granite belongs to low-medial potassium calcium alkaline series quasi-aluminum rocks.The chondrite-normalized REE patterns show the right-inclined type of LREE enrichment,with a medium negative Eu anomaly andδEu of 0.67~0.81.The primitive mantle-normalized spidergram shows depletion of Nb,Ta,Ti,P and enrichment of Rb,K,U.The P2O5-SiO2diagram shows the trend of Ⅰ-type granite.The Y-Rb diagram shows the characteristics of differentiated Ⅰ-type granite.Combined with the regional geological background,the authors hold that the formation of the Yaodonggou granite was probably related to the northward subduction of the North Qilian Ocean Basin,which led to the back-arc extension and mantle-derived magmatic intrusion and uplift.As a result,the ancient crustal material was remelted to form parent magma,then it was subjected to moderate crystallization differentiation.The magma had the property of island arc magma,which might have been attributed to the fact that the back arc basin was split on the basis of island arc or its magma source area was reformed by subduction plate fluid or melt.