西藏得几蛇绿岩体中玻安岩的地球化学特征及其地质意义

西藏日喀则蛇绿岩是我国发育最好的蛇绿岩之一,目前有关日喀则蛇绿岩形成的构造背景存在多种不同的认识。作者在研究得几蛇绿岩剖面时发现存在玻安岩,从而为日喀则蛇绿岩具有弧前背景提供了直接证据。得几蛇绿岩中玻安岩类样品具有高硅、高镁、低铁的特征,岩石中Zr、Yb和Y的含量也较低,具有明显的玻安岩特征。根据这些岩石的地球化学性质,认为岩石是由早期经历过较高程度的部分熔融的残余在含水条件下再发生部分熔融形成的。得几蛇绿杂岩体部分形成于弧前环境。

GEOCHEMICAL CHARACTERISTICS AND GEOLOGICAL SIGNIFICANCE OF BONINITES IN THE DEJI OPHIOLITE,TIBET

The Xigaze ophiolite is one of the best preserved ophiolitic section in the Yalung Zangbo River ophiolite belt.At present, there are various views c oncerning the tectonic setting of the ophiolite formation.We have found boninite s in the Deji ophiolite section in this study, thereby providing direct evidence for their fore arc setting. The samples from the Deji boninite are characteriz ed as being high Si, high Mg and low Ti. The contents of Zr, Yb and Y are low , so these rocks have the charateristics of typical boninites. Based on the abov e geochemical evidence, it is demonstrated that this sequence of boninites was d erived from partial melting of depleted mantle peridotite in the presence of a h ydrous fluid, and the Deji ophiolite occurred in the fore arc setting.