峨眉山大火成岩省晚期酸性火山岩的岩石地球化学特征

Petrology and Geochemistry of the Late-Stage Acidic Volcanic Rocks of the Emeishan Large Igneous Province

峨眉山大火成岩省出露有少量酸性火山岩,它们与基性火山岩共生,表现出双峰式的特征,为研究峨眉山地幔柱晚期岩浆活动提供了重要的窗口。本文通过对双峰式火山岩主、微量元素和斑晶电子探针分析研究表明,基性火山岩属于碱性玄武岩,酸性火山岩主要由粗面岩组成;相对玄武岩,粗面岩中MgO、Fe2O3、P2O5、TiO2、CaO含量明显降低;粗面岩与玄武岩具有相互平行的REE配分模式,但粗面岩出现明显的Eu负异常,以及Sr、Ti等元素的强烈亏损;粗面岩与玄武岩具有同源的特征,通过稀土元素模拟计算表明粗面岩可以由玄武质岩浆经过80%分离结晶作用(辉石、斜长石和Fe-Ti氧化物)而形成。在峨眉山大火成岩省晚期出现双峰式火山岩,可能与地幔柱活动晚期岩浆供给少,在地壳岩浆房中停留时间长,岩浆发生强烈分离结晶作用有关。

The acidic volcanic rocks of the late stage magmatism of the Emeishan Large Igneous Province （ELIP） are sporadically distributed and co-existed with marie rocks in the ELIP, with character of bimodal volcanism. They provide important window for studying the late stage magmatism of the Emeishan mantle plume. The major and trace element compositions of the bimodal volcanic rocks, and the electron microprobe analyses of their phenoerysts indicate that the marie rocks are alkaline basalt, while the acidic rocks are mainly traehytes. The contents of MgO, Fe2O3, P2O3, TiO2 and CaO of the traehytes are obviously lower than those of the alkaline basalts. Though both the traehytes and basalts show parallel REE distribution pattern, the traehytes show obvious negative Eu anomalies. In addition, the Sr and Ti are strongly depleted in the traehytes in comparison with the basalts. On the basis of the linear relationship of the incompatible elements, it is suggested that the basalt and the traehyte are derived from the same mantle source. According to the REE modeling, the traehytes could be generated through 80 % fractional crystallization （pyroxene ＋ plagioelase ＋ Fe-Ti oxides） from the basaltic magma. It is proposed that the bimodal volcanism occurred in the late stage of the ELIP could be related to strong fractional crystallization of the magma due to the long stay of magma in the chamber and low supply rate of magma at the late stage of the Emeisban mantle plume activity.