西藏雅鲁藏布江缝合带西段东波蛇绿岩的构造背景特征

摘要

Located in the western part of the Yarlung Zangbo Suture Zone (YZSZ) of southern Tibet, the Dongbo ophiolite is composed of mantle peridotite, pyroxenite, gabbro and some other kinds of rocks, with harzburgite, dunite and a small amount of lherzolite constituting dominant lithologies. Basalts and siliceous rocks were found at the edge of the mantle tectonites. Occasionally, a small amount of pyroxenite and gabbro dikes with a width of about one meter crosscut the mantle tectonites in NW direction, which resembles the tectonic line of mantle tectonites. Geochemical characteristics of the different lithofacies in Dongbo ophiolite show remarkable differences, and geochemical characteristics of basalt are similar to features of ocean island basalt (OIB); however, pyroxenite and gabbro dikes in the peridotite are similar to MORB in geochemical characteristics, suggesting that they were derived from mid-ocean ridge (MOR) but affected by the subduction fluids at the second stage. The lherzolite in Dongbo ophiolite is consistent with the abyssal peridotite in the feature of LREE depletion; however, geochemical characteristics of harzburgite and dunite show that they were formed at the mid-ocean ridge (MOR) and reformed in a subduction zone setting. The lithogeochemical characteristics of Dongbo ophiolite indicate a tectonic setting of ocean island overlaid with ocean ridge.%西藏东波蛇绿岩位于雅鲁藏布江缝合带西段,由地幔橄榄岩、辉石岩和辉长岩等组成。地幔橄榄岩主要为方辉橄榄岩、纯橄岩和少量二辉橄榄岩。岩体的边界出露玄武岩和硅质岩等。地幔橄榄岩中有少量辉石岩和辉长岩的脉岩,宽约1 m,走向北西,与岩体的构造线方向基本一致。各岩相岩石地球化学研究结果表明,东波蛇绿岩的岩相存在较大的差异,玄武岩具有与洋岛玄武岩(OIB)相似的地球化学特征,而地幔橄榄岩中辉石岩、辉长岩脉与洋中脊玄武岩(MORB)相似,形成于洋中脊环境,并受后期俯冲流体作用的改造。东波岩体中二辉橄榄岩具有与深海地幔橄榄岩较一致的轻稀土亏损特征,而方辉橄榄岩和纯橄岩的地球化学特征显示出岩体形成于 MOR 环境,后受到 SSZ 环境的改造。东波蛇绿岩的岩石地球化学特征显示其洋中脊叠加洋岛的构造背景。