峨眉山大火成岩省中高Os苦橄岩的发现及地质意义

Discovery of High-Os Picrites in the Large Emeishan Igneous Province and Its Geological Significance

本文对峨眉山大火成岩省中苦橄岩及其共生的玄武岩进行了铂族元素(PGE)分析,结果表明苦橄岩比玄武岩的PGE含量要高至少一个数量级,并且具有明显高的Os含量,不仅比熔融程度最高的科马提岩要高,而且比原始地幔还要高,另外,还显示出超球粒陨石的Os/Ir比值(2.84～3.88)。其高的Os/Ir比值可能与岩浆上升过程中混入黑色页岩有关。部分熔融计算表明,含有0.01%硫化物的原始地幔+0.5%的外核在7%的熔融程度下,然后又被约10%的黑色页岩混染可以模拟原始岩浆的PGE含量。其Os含量及其他地球化学特征与其同时代的西伯利亚暗色岩系的相似性可能暗示了这两个大火成岩省来自于同一个起源于核-幔边界的超级地幔柱。另外,还根据苦橄岩和玄武岩PGE的含量估算了该地区PGE的成矿潜力。

We report concentrations of platinum group elements （PGE： Os, Ir, Ru, Rh, Pt and Pd） for picritebasalt lavas of the large Emeishan igneous province. The absolute concentrations of Os in most of the Emeishan picrites are much higher than in komatiites, and in estimated primitive mantle. In addition, they have superchondritic Os/Ir ratios （2.84～3.88）. The unusually high Os/Ir ratios suggest that the picritic magmas may have been contaminated by black shales. Modeling shows that a 7% partial melt of primitive mantle containing 0.01% of sulfides can account for the observed Os, Ir and Ru concentrations in the most primitive picrite if the melt is contaminated by 10% of black shale en route to the surface. The similarities in Os concentrations and other geochemical characteristics between the Emeishan province and Siberian Traps imply that they might be derived from a super-plume, which originated from mantle-core boundary. The large Emeishan igneous province has most of the features required to form Noril＇sk-style Ni-Cu-PGE sulfide deposits; therefore, its potential for large magmatic sulfide deposits must be considered.