程先钰, 滕学建, 田健, 段霄龙. 内蒙古北山造山带三个井组地球化学特征、同位素年龄及其构造环境[J]. 地质通报, 2020, 39(9): 1461-1473.
    引用本文: 程先钰, 滕学建, 田健, 段霄龙. 内蒙古北山造山带三个井组地球化学特征、同位素年龄及其构造环境[J]. 地质通报, 2020, 39(9): 1461-1473.
    CHENG Xianyu, TENG Xuejian, TIAN Jian, DUAN Xiaolong. Geochemical characteristics, isotopic ages and tectonic environment of Sangejing Formation in Beishan orogenic belt, Inner Mongolia[J]. Geological Bulletin of China, 2020, 39(9): 1461-1473.
    Citation: CHENG Xianyu, TENG Xuejian, TIAN Jian, DUAN Xiaolong. Geochemical characteristics, isotopic ages and tectonic environment of Sangejing Formation in Beishan orogenic belt, Inner Mongolia[J]. Geological Bulletin of China, 2020, 39(9): 1461-1473.

    内蒙古北山造山带三个井组地球化学特征、同位素年龄及其构造环境

    Geochemical characteristics, isotopic ages and tectonic environment of Sangejing Formation in Beishan orogenic belt, Inner Mongolia

    • 摘要: 北山造山带中段白云山蛇绿岩带南部出露的三个井组火山-沉积地层,主要由枕状玄武岩与气孔杏仁状玄武岩组成,空间上与凝灰质碎屑岩、薄层粘土质硅质岩及厚层状碳酸盐岩整合接触。获得三个井组玄武岩年龄加权平均值为396.6±3.3 Ma,为早泥盆世。岩石地球化学研究表明,玄武岩主量元素特征显示其主体属于亚碱性系列拉斑玄武岩;稀土元素球粒陨石标准化配分曲线主体呈现轻稀土元素富集的右倾型,稀土元素总量为35.88×10-6~129.41×10-6,LREE/HREE值为1.62~3.84,(La/Yb)N值为1.02~3.42。结合区域上大面积发育的S型、A型花岗岩共同构成双峰式岩浆组合特征,推断三个井组处于造山后伸展的构造环境。结合前人资料及区域对比推断,北山地区北山洋闭合于早泥盆世之前。锆石Lu-Hf同位素研究表明,三个井组玄武岩εHft)为(+4.67~+8.02),tDM1=692~826 Ma,结合微量元素Dy/Yb值(1.77~2.13),推断三个井组玄武岩是新元古代地幔源区的亏损尖晶石相地幔橄榄岩向石榴子石相地幔橄榄岩过渡的较高程度部分熔融的产物。

       

      Abstract: A set of island arc basalts is exposed in Sangejing Formation in the south of Baiyunshan ophiolite belt in the middle part of Beishan orogenic belt.It is mainly composed of pillow basalt and porous almond basalt, conformable contact with tuffaceous clastic rocks, thin-layered clayey siliceous rocks and thick-layered carbonate rocks.The weighted average age of basalts in Sangejing Formation is 396.6±3.3 Ma, suggesting Early Devonian.A study of the rock geochemistry shows that the major elements characteristics of the basalts of the Sangejing Formation indicate that the main body of the basalts belongs to the subalkaline tholeiite series.The main part of the normalized distribution curve of chondrite-normalized REE patterns of the basalts exhibit the right-oblique type of enrichment of LREE, with ∑REE being 35.88×10-6~129.41×10-6, LREE/HREE being 1.62~3.84, and (La/Yb)N being 1.02~3.42.The bimodal magmatic assemblage consists of Sangejing basalts, and S-type and A-type granites are developed extensively in this region.It is inferred that the Sangejing Formation occurred after an orogenic extensional tectonic environment.Based on previous data and regional comparison, it is inferred that the Beishan Ocean in Beishan area was closed before the Early Devonian.A study of Lu-Hf Isotope of zircon indicates that the Sangejing basalt has the data εHf(t)=(+4.67~+8.02), tDM1=692~826 Ma and Dy/Yb=1.77~2.13, and that the Sangejing basalts were formed by partial melting of the depleted spinel-facies mantle peridotite to garnet-facies mantle peridotite in the Neoproterozoic mantle source area.

       

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