藏南罗布莎蛇绿岩壳层熔岩地球化学特征及成因

GEOCHEMICAL CHARACTERISTICS AND ORIGIN OF THE LUOBUSA OPHIOLITE CRUST LAVAS IN XIZANG, CHINA

罗布莎蛇绿岩是雅鲁藏布江蛇绿岩带东段出露较好,也是研究程度较高的蛇绿岩片之一,对其壳层熔岩研究表明,该熔岩属于亚碱性玄武岩与安山岩/玄武岩之间的过渡类型,富集Rb、K、Ba等大离子亲石元素,高场强元素具有轻微左倾正斜率分布特征,并亏损Nb、Ta,反映出该蛇绿岩受到了俯冲作用的影响,稀土元素显示LREE亏损、HREE平缓的球粒陨石标准化分布模式,体现了该蛇绿岩N-MORB的地球化学特征,结合第一过渡系元素地球化学特征,通过对其构造环境的判别,提出罗布莎蛇绿岩属于岛弧型蛇绿岩范畴,其构造环境应为俯冲带之上(SSZ)的弧间盆地。

The Luobusa ophiolite is one of well-outcropped and well-studied rock suite that occurs in the eastern segment of Yarlung Zangbo ophiolite belt. The research results on Luobusa ophiolite crust lavas indicates that it belongs to the transitionary type between the sub-alkaline basalt and the andesite/basah. It is rich in large ion lithophile elements such as Rb, K, Ba. The HFS elements distribution pattern shows a small positive slope, depletion in Nb, Ta, which implies that the ophiolite is affected by subduction. The chondrite-normalized REE pattern shows LREE-depleted and HREE-smoothed distribution, which demonstrates that the ophiolite is geochemically represented by N-MORB. According to the above discussions, and by combining the geochemical characters of the first transitional elements and based on discrimination of tectonic environment, we conclude that the Luobusa ophiolite belongs to the island-arc ophiolite formed in the interarc basin of supra-subduction zone.