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蛇绿岩中的地幔橄榄岩是蛇绿岩的重要组成部分,不仅记录了其所经历的板块构造事件,还可以探讨地幔的物质组成和深部地幔作用。位于雅江缝合带西段的普兰蛇绿岩以出露面积约600 km2的特大型地幔橄榄岩体而引人注目。为了探讨普兰蛇绿岩地幔橄榄岩的成因,本文在普兰地幔橄榄岩体的东段完成了一条穿过岩体的长约10km的地质剖面,对地幔橄榄岩进行了系统采样,开展了详细矿物学、地球化学研究,取得以下主要进展和认识:(1)普兰地幔橄榄岩中的二辉橄榄岩经历了9%~15%的部分熔融作用,方辉橄榄岩最终经历了15%~25%的部分熔融作用;(2)基于橄榄石-尖晶石矿物化学平衡计算的普兰地幔橄榄岩氧逸度?log(fo2)FMQ值从-1.11到+0.45,略高于用VY含量估算出的氧逸度(在FMQ到FMQ-1趋势),指示其形成于还原环境;(3)普兰地幔橄榄岩不是简单的地幔残余,还经历了后期熔体交代作用。对普兰地幔橄榄岩交代产生类似“U”型稀土配分模式的熔体可能是洋中脊(MOR)环境产生的低融岩浆;(4)地幔橄榄岩地球化学特征及氧逸度计算结果表明,普兰地幔橄榄岩可能形成于洋中脊(MOR)环境,并没有俯冲带(SSZ)环境的印记。对于普兰地幔橄榄岩以及雅鲁藏布江缝合带其他岩体的成因,前人主要根据地幔橄榄岩一些元素的富集特征提出的MOR→SSZ两阶段演化模式需要重新审视。
The mantle peridotites in ophiolites are important components of ophiolites. They not only record the tectonic events they experienced, but also investigate the mantle material composition and deep mantle effect. The Pulan ophiolite, which is located in the western segment of the Yajiang suture zone, has attracted the attention of the extra large mantle peridotite with an area of about 600 km2. In order to investigate the genesis of the Perlan ophiolite mantle peridotite, a geological section about 10 km long through the rock mass was completed in the eastern section of the Permian pluton. The mantle peridotite was systematically sampled and detailed mineralogy (1) Lherzolite in the Permian Permian experienced partial melting of 9-15%, and the harzburgite finally experienced 15-25% (2) Based on the olivine-spinel mineral chemical equilibrium, the logarithm of oxygen fugacity per logarithm (log 2) FMQ of the Pulan mantle peridotite ranged from -1.11 to +0.45, which was slightly higher than that estimated from the VY content The oxygen fugacity (from FMQ to FMQ-1) indicates that it is formed in a reducing environment. (3) The Permian peridotite is not a simple mantle remnant but also undergoes post-melt metasomatism. The melts produced by similar “U” type REE to the Permian peridotite may be low-melting magma generated in the mid-ocean ridge (MOR) environment; (4) The geochemical characteristics of mantle peridotites and the calculation results of oxygen fugacity It is suggested that the Permian peridotites may be formed in the mid-ocean ridge (MOR) environment without the imprint of the subduction zone (SSZ). For the genesis of the Plenomian peridotites and other rocks in the Yarlung Zangbo suture zone, the MOR → SSZ two-stage evolution model previously proposed mainly by the enrichment of some elements of the mantle peridotite needs to be revisited.