雅鲁藏布江北岸基性-超基性岩带的成因

Origin of a basic-ultrabasic belt on the northern bankof the Yarlung Zangbo River

沿雅鲁藏布江北岸大竹卡—谢通门一带,即冈底斯弧岩浆岩带南缘出露了一条超基性-基性岩带,岩带的分布与雅鲁藏布江北岸的强航磁异常带相吻合。岩带主要由零星孤立出露的规模较小的近圆形辉长岩岩体、岩瘤或岩滴组成,在日喀则县曲尼出露了超镁铁堆晶岩、辉长岩和玄武岩连续剖面。它们呈一系列自北而南逆冲推覆的构造岩片,经历了明显不同于弧花岗岩基的形成过程。通过野外地质考察和系统的岩石学研究,认为曲尼基性-超基性岩带是在岛弧—弧后系统形成的蛇绿混杂岩,是新特提斯大洋岩石圈在俯冲消亡过程中的残片。

An ultrabasic-basic belt is exposed in the Dagzhuka-Xaitongmoin area along the north bank of the Yarlung Zangbo River, i.e. on the southern margin of the Gangdise arc magmatic belt. The distribution of the belt coincides with that of the strong aeromagnetic anomaly on the north bank of the Yarlung Zangbo River. It is composed dominantly of small, sparse and isolated subrounded gabbro bodies, bosses or drips, and a continuous section of ultramafic cumulates, gabbro and basalt is exposed at Quni, Xigazê County. These rocks occur as a number of south-vergent thrust nappes and a process of formation which is notably different from that of the arc granite batholith. Through field geological surveys and a systematic petrological study, the authors propose that ophiolites formed in the Quni basic-ultrabasic in the island arc-back arc system are fragments of the Neo-Tethys oceanic lithosphere during its subduction and extinction.