大兴安岭嘎仙蛇绿混杂岩中超镁铁质岩地球化学、年代学特征及其地质意义

Geochemical and chronologic characteristics of ultramafic rocks in Gaxian ophiolitic mélange of the Da Hinggan Mountains and their geological significance

嘎仙蛇绿混杂岩位于新林-嘎仙-吉峰蛇绿混杂岩带中段。岩石主要由镁铁-超镁铁质岩石组成,呈构造岩块的形式与变质细碎屑岩基质混杂产出。对超镁铁质岩系统的岩石学、地球化学和年代学研究结果表明,超镁铁质岩石SiO2含量平均值为44.87%,Mg#平均值为87.06,锆石U-Pb同位素年龄为649.6±4.2 Ma,时代为新元古代。稀土元素配分曲线总体呈平坦型,具有富集型洋脊玄武岩(E-MORB)向洋岛玄武岩(OIB)过渡的趋势。样品亏损Nb、Ta、Ti元素,相对富集Th、U、Zr、Hf元素,具有岛弧岩浆岩的特征。在Nb/Yb-Th/Yb构造环境判别图解中,样品点落入E-MORB和OIB交汇部位。通过与区域构造带岩石组合和地球化学特征对比,认为嘎仙超镁铁岩岩浆源区可能为受俯冲流体交代的富集地幔,属俯冲型(SSZ)蛇绿岩,其就位机制与持续洋内俯冲有关。

The Gaxian ophiolitic melange belt is located in the middle of Xinlin-Gaxian-Jifeng ophiolitic melange belt.This ophiolite mainly consists of ultramafic and mafic rocks, mixed with metamorphic fine clastic rock matrix in the tectonic block form.This paper focuses on the systematic petrology, geochemistry and chronology of ultramafic rocks.The result shows that in the ultramafic rock, the average value of SiO2 is 44.87%, the average value of Mg# is 87.06, and the zircon U-Pb age is 649.6±4.2 Ma, suggesting Neoproterozoic.The curve of REE is of flat type, which shows MORB’s transition to OIB.Geochemical data of the sample show depletion of Nb, Ta, Ti and enrichment of Th, U, Zr, Hf, exhibiting characteristics of the arc magma.In the Nb/Yb-Th/Yb tectonic environment discrimination diagram, it is located at the intersection of E-MORB and OIB.A comparison with the rock association of the regional tectonic and geochemical characteristics shows that the magma source area of the Gaxian ultramafic magma may be an enriched mantle replaced by subductive fluid, belonging to the SSZ type ophiolite, and its emplacement mechanism might have been related to the continuous intra-ocean subduction.