北祁连直河蛇绿岩的地质和地球化学特征

Discovery of ophiolites belts and tectonic implication around Zhihe in the North Qilian

直河蛇绿岩是在北祁连造山带中新发现的,该蛇绿岩由地幔橄榄岩、辉长辉绿岩、玄武岩、硅质岩等组成,以构造岩片为接触关系．蛇绿岩中岩石地球化学特征表明,该蛇绿岩具有大洋拉斑玄武岩的特征,同时具有某些岛弧特征,基性岩类的REE球粒陨石标准化配分模式为平坦型和LREE亏损型,超基性岩为亏损地幔源和富集地幔物质加入的混合成因,稀土分配型式不同于典型洋脊玄武岩,源区为亏损地幔和富集地幔的混合源．北祁连自寒武纪末大陆裂解,完成大陆裂陷向洋盆的转化,至奥陶纪形成沟弧盆体系．直河蛇绿岩属于弧后盆地扩张环境的产物．

The ophiolites belts of Zhihe was discovered in the orogen of the North Qilian. It is composed of mantle-peridotites, dolerltes, gabbros, basalts and silicon rocks. All the rocks units has contact with structure slice. The major and trace elements characteristics indicate that the ophiolites contain ocean tholeiitic basalt, but also some character of island arc. REEs of mafic rocks in distribution patterns are basically of plain style and LREE with a little depletion. REEs of ultra-mafic rocks indicate that the rocks are composed of deplete mantle and rich mantle at the same time. The REEs distribution patterns are different from that of typical ocean ridge basalts, suggesting the formed environment in a small ocean basin or an unmatured ocean basin. According to our study and by combing regional geology and former study, we can outline the tectonic evolution of this area. The North Qilian continental split in later cambrian period had intensively extended and formed a broader ocean. And towards Ordovician it became a trench-arc-basin system. The opholitess belts of Zhihe belong to back-arc basin extended production. This study of the ophiolites belts has great geological implications for Qilian orogen.