A petrological experiment on Emeishan basalt: Implications for the formation of syenite from the Baima igneous complex

Abstract

The Baima igneous complex (BIC) consists of a cumulate layered gabbroic unit, an Fe-Ti oxide ore zone, and an isotropic quartz syenite. The formation of the BIC is attributed to a variety of petrological processes including: silicate immiscibility, fractional crystallization, and fluxing of fO 2 by either internal or external factors. This study attempts to determine if a parental magma similar to high-Ti Emeishan basalt can produce the syenitic rocks of the BIC. The experimental results at atmospheric pressure show that the liquidus temperature and solidus temperature of the basaltic melt are estimated to be 1303 and 1120°C. The crystallization sequence is: titanomagnetite, clinopyroxene (Wo 43-47 En 32-45 Fs 11-23 ), and plagioclase (An 65-31 ). The residual liquid composition, represented by quenched glass, evolves from lower SiO 2 (~45 wt%) values to higher values (~60 wt%) with corresponding decreases in Ti, Fe, Mg, Ca and increases of Na and K. The results show that a starting composition of high-Ti basalt from the Emeishan large igneous province can produce evolved silicic liquids that resemble the compositions of microgranular enclaves from the Baima syenitic unit.