西藏泽当—罗布莎蛇绿岩的地球化学特征及其构造意义

西藏泽当—罗布莎蛇绿岩是雅鲁藏布江蛇绿岩带的重要组成部分,受控于板块碰撞结合带。该蛇绿岩主要由变质橄榄岩、辉长辉绿岩及玄武岩等组成。变质橄榄岩具有低的A l2O3和CaO含量,以富Mg,贫Ti、∑REE为特征,与世界上典型蛇绿岩中方辉橄榄岩的特征值一致。辉长岩和玄武岩的主量元素、微量元素特征显示其含有洋脊拉斑玄武岩和大洋岛弧拉斑玄武岩的双重成分,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,具有大洋岛弧玄武岩的特点。辉长岩在球粒陨石标准化稀土元素配分模式图上为LREE亏损的平坦型,无负Eu异常,与洋中脊玄武岩类似。根据上述特征并结合区域地质构造特征,认为泽当?罗布莎蛇绿岩可能形成于边缘海盆地的海底慢速扩张环境。

Geochemical Characteristics of Ophiolites in Zedang-Luobusha,Tibet and Their Tectonic Significance

The Zedang-Luobusha ophiolite in Tibet is an important component part of the Yarlung Zangbo ophiolite belt and controlled by the plate junction zone.It may be well correlated with some typical sequential ophiolites at home and abroad.Ophiolites exposed from Zedang to Luobusha are mainly composed of metamorphic harzburgites,gabbros and basalts.From the analysis of major and trace elements of ophiolite in Zedang-Luobusha,the characteristics of its metamorphic harzburgites have low Al_(2)O_(3),CaO contents,high MgO contents and Mg~* values,and are lower in Ti and ∑REE,and belong to the depleted oceanic lithosphere mantle.They are consistent with those of harzburgites from the typical ophiolites of the world.The diabases and basalts are the transitional MORB/OIT-type basalt.According to the trace elements in the ophiolite,on one hand,the gabbros and basalts show the same characteristics as the island arc basalt,for the depletion elements of Nb,Ta and its relative enrichments of Rb,Sr and Ba.Meanwhile,according to the REE distribution patterns with no Eu(anomaly),the gabbros are similar to mid-ocean ridge basalt.It is concluded that the geotectonic setting of ophiolite of Zedang-Luobusha is formed in marginal sea or small oceanic basin by the comparison with the ophiolites of typical areas and discrimination diagrams of the tectonic setting.This indicates that ophiolite complex of Zedang-Luobusha has a more complex evolution history than hitherto considered.