西藏永珠—果芒错蛇绿岩的地球化学特征及其构造意义

永珠—果芒错蛇绿岩位于西藏永珠藏布—纳木错蛇绿岩带西段,主要由变质橄榄岩、辉长辉绿岩及玄武岩等组成。变质橄榄岩以富Mg,贫Ti、∑REE为特征。辉绿岩和玄武岩的主量元素、微量元素分析显示其含有洋脊拉斑玄武岩和岛弧拉斑玄武岩的双重成分特征,其中高场强元素(Nb、Ta、Zr、H f等)亏损,大离子亲石元素(Rb、Sr、Ba等)相对富集,具有岛弧玄武岩的特点;在球粒陨石标准化稀土元素配分模式图上显示稀土元素特征为LREE亏损的平坦型,无负Eu异常,与洋中脊玄武岩的特征类似。通过与典型地区作对比和应用构造环境判别图解,推断永珠—果芒错蛇绿岩形成于弧后盆地的构造环境。

Geochemical Characteristics of Ophiolites in Yongzhu-Guomangcuo, Tibet and Its Tectonic Significance

Ophiolites exposed from Yongzhu to Guomangcuo are mainly composed of metamorphic harzburgites,gabbros and basalts.From the analysis of major and trace elements of ophiolites in Yongzhu-Guomangcuo,the (characteristics) of its metamorphic harzburgites are high Mg content,and lower in Ti and ∑REE,and belong to the depleted oceanic lithosphere mantle.The diabases and basalts are the transitional MORB/IAB-type basalt.According to the trace elements in the ophiolite,the gabbros and basalts show the same characteristics as the island arc basalt,for the depletion elements of Nb,Ta and its relative enrichment of Rb,Sr and Ba.Meanwhile,according to the REE distribution patterns with no Eu anomaly,the gabbros and basalts are similar to mid-ocean ridge basalt.It is concluded that the geotectonic setting of ophiolite of Yongzhu is formed in back arc basin by the comparison with the ophiolite of typical areas and discrimination diagrams of the tectonic setting.This indicates that ophiolite complex of Yongzhu has a more complex evolution history than hitherto considered.