波密地区帕隆藏布残留蛇绿混杂岩带的发现及其意义

The discovery and significance of the relicts of ophiolitic mélanges along the Parlung Zangbo in the Bomi region, eastern Xizang

关键词:  波密  帕隆藏布  蛇绿混杂岩  西藏

Key words:  Bomi  Parlung Zangbo  ophiolitic mélange  Xizang

摘要:

在进行1:25万墨脱幅地质调查中,笔者首次在波密地区发现和填绘出了帕隆藏布残留蛇绿混杂岩带。帕隆藏布残留蛇绿混杂岩呈串珠状产出于花岗岩类侵入岩中,其岩石组合为橄榄辉石岩、辉石岩、辉长岩、辉长辉绿岩、辉绿岩、石英岩和大理岩,局部可见条带状硅质岩。上述组分之间的相互关系表明,蛇绿岩在花岗岩类岩石侵入之前发生过构造混杂和变形。根据沉积岩所记录的盆地演化过程、蛇绿岩的Rb-Sr年龄值以及残留蛇绿混杂岩带两侧花岗岩类岩石的特征和生成时代综合分析认为:帕隆藏布残留蛇绿混杂岩带形成于石炭-二叠纪的弧间盆地中,至少在晚三叠纪之前出现洋壳;在消减过程中向北俯冲并在中侏罗世之前闭合(弧-弧碰撞)。

Abstract:

The relicts of ophiolitic mélanges were discovered and mapped, for the first time, during the 1:250000 geological survey along the Parlung Zangbo in the Bomi region, eastern Xizang. These relicts of ophiolitic mélanges as a string of beads are enveloped by granitic intrusives, and consist dominantly of olivine pyroxenite, pyroxenite, gabbro, gabbro diabase, diabase, quartzite, marble and locally banded siliceous rocks. The study of the interrelations between the above mentioned rocks shows that the ophiolites had been subjected to green schist facies metamorphism and deformation prior to the intrusion by the granites. The integration of basin evolution recorded by the sedimentary rocks, Rb Sr ages of the ophiolites, geochemical signatures and ages of the granites on either side of the relicts of ophiolitic mélange zones indicates that the ophiolitic mélanges along the Parlung Zangbo in the Bomi region were formed in the interarc basins during the Carboniferous to Permian. The oceanic crust occurred at least prior to the Late Triassic. The arc arc collision took place prior to the Middle Jurassic due to the northward subduction of the oceanic crust.