Primitive magmas in the Emeishan Large Igneous Province, southwestern China and northern Vietnam

Abstract

There are several places in the Emeishan Large Igneous Province (ELIP) where highly magnesian volcanic or intrusive rocks accompany more abundant flood basalts. The high-Mg rocks vary in composition from LREE-depleted, low-Ti komatiites to LREE-enriched, high-Ti picrites. These magmas carried cognate olivine phenocrysts with Fo content higher than 91 mol% indicating that these olivines crystallized and equilibrated with high-Mg melts. The maximum measured Fo content from the Dali area is 93.5 mol% which, depending on the KD value used, indicates a MgO content of 23–25 wt.% for the parental melt. The high estimated MgO wt.% values provide evidence for a hot portion of the Emeishan mantle-plume head with a potential mantle temperature of ca. 1700 °C. Melt inclusions trapped within olivine phenocrysts have compositions similar to those of the host lavas but with a higher variability in the abundance of incompatible trace elements. The inclusions yield a calculated maximum MgO content of ca. 22 wt.% for the melt. The compositional and isotopic variability of the high-Mg magmas in the ELIP is akin to that observed in basaltic rocks, which are divided into low-Ti and high-Ti types, suggesting that most of the basalts were developed in deep crustal magma chambers by fractional crystallization and mixing processes from primitive magmas.