班公错—怒江缝合带西段纳屋错蛇绿岩特征

Characteristics of Nawucuo's Ophiolite in the Western Segment of the Bangongcuo-Nujiang Suture Zone

通过对纳屋错地区蛇绿岩的地质背景、岩石学、地球化学特征及构造环境分析,发现超基性岩以高Cr、Ni、Co、稀土总量,低Mg'值为特征,主量元素及稀土元素特征表明其与蚀变玄武岩具有同源岩浆的特点。蚀变玄武岩的主量元素显示其具有拉斑玄武岩向碱性玄武岩过渡的特征;微量元素结果分析表明它与典型大洋中脊玄武岩明显不同,分配型式近似平坦型,相对富集大离子亲石元素Rb、Ba、Sr和高场强元素Th、Ta、Hf,但Nb亏损;在稀土元素配分模式中为轻稀土富集的向右缓倾型式,无明显的Eu异常,但负Ce异常,与洋岛碱性玄武岩的特征相似。根据蛇绿岩的地球化学特征并应用构造环境判别图解,确定该地区蛇绿岩形成于洋脊扩张环境,但不是典型的大洋型蛇绿岩,类似于边缘海型蛇绿岩。

Through analyzing the geological setting, petrology, geochemistry characteristics and tectonic setting of ophiolite in Nawucuo,it is discovered that the ultrabasic rock are characterized by high Cr, Ni, Co, EREE, and low Mg value. Its major element and REE indicated that the ultrabasic rock has the comagmatic feature with altered basalt. The major elements of altered basalt indicated the characteristics of transition to alkali basalt from tholeiite. The trace elements analysis show that the distribution pattern of trace elements of altered basalt is nearly flat and the altered basalt are relative enrichment of LILE and HFSE, such as Rb, Ba, Sr, Th, Ta, Hf, but Nb is loss, which is significantly unlike the typical mid-ocean ridge basalt. The REE distribution pattern are light REE enrichment in right gently-dip type, with no obvious Eu anomaly, but negative Ce anomaly, which is similar to the characteristics of the oceanic island basalts. According to the geological characteristics of ophiolite and adopting the tectonic environment discrimination diagrams,it is determined that the geotectonic setting of ophiolite of Nawucuo is formed in spreading ocean basin. Besides that, the ophiolite is similar to the edge of the sea ophiolite, but not a typical ocean- ic ophiolite.