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    内蒙古东部马鞍山地区早石炭世高镁安山岩特征及地质意义

    Characteristics and geological significance of Early Carboniferous high-Mg andesites in Ma'anshan area, east Inner Mongolia

      摘要
    • 摘要: 内蒙古东部马鞍山地区发育早石炭世高镁安山岩，其LA-ICP-MS锆石U-Pb测年结果为346.4±1.4Ma，指示其形成于早石炭世。马鞍山高镁安山岩属于钙碱性岩石系列，SiO2含量为53.22%~54.22%，MgO为7.21%~10.03%，Al2O2为14.37%~15.94%，CaO为4.81%~5.94%，富Na、贫K（Na2O为3.87%~4.34%、K2O为0.49%~0.93%）、低TFeO/MgO（< 1.5）、高Cr（364×10-6~429×10-6）、Ni（204×10-6~211×10-6）。所有样品均显示轻稀土元素富集，具轻微的负Eu异常，富集大离子亲石元素Rb、K、Ba、Sr，亏损高场强元素Nb、Zr、Ti、Y、Yb、Lu等，地球化学特征与赞岐岩相似。由此可知，马鞍山地区早白垩世高镁安山岩是由地幔橄榄岩与消减洋壳板片部分熔融产生的富Si质熔体平衡反应的产物，暗示其形成于与俯冲消减有关的构造背景，是古亚洲洋闭合过程中洋陆转化的产物。

       

      Abstract: The Early Carboniferous high-Mg andesites in Ma'anshan area of eastern Inner Mongolia have LA-ICP-MS zircon U-Pb age of 346.4±1.4Ma, indicating that they were formed in Early Carboniferous. Ma'anshan high-Mg andesites belong to calc-alkaline series, with SiO2 53.22%-54.22%, MgO 7.21%-10.03%, Al2O3 14.37%-15.94%, CaO 4.81%-5.94%, rich Na, poor K (Na2O 3.87%-4.34%, K2O 0.49%-0.93%), low TFeO/MgO(< 1.5), high Cr(364×10-6-429×10-6), and Ni (204×10-6-211×10-6). Similar to sanukite, all samples show enrichment of LREE and LILE (e.g., Rb, Ba, Sr and K), slight Eu negative anomaly and depletion of HFSE such as Nb, Zr, Ti, Y, Yb and Lu. They are equilibrium products of Sirich melt derived from partial melting of mantle peridotite and subducted oceanic crust, indicating a subduction background and suggesting that they were formed by ocean-continent transformation during the Palaeo-Asian Ocean closure.

       

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