Differentiation of the Emeishan Flood Basalts at the Base and throughout the Crust of Southwest China

Abstract

The Emeishan flood basalt is a large igneous province (LIP) erupted in southwestern China during the Permian Triassic period. Seismic images of the deep crustal structure beneath the Emeishan large igneous province (ELIP) indicate unusually high seismic velocities (Vp = 7.1 - 7.8 km/s) at the base of the crust. The thickness of these high-velocity layers ranges from 8 km in the western section to 24 km in the east, and are generally interpreted as large igneous intrusions. In this paper, we use the petrological code MELTS to quantify the crystal fractionation of Emeishan primary melts at the base of the crust as they ascended to the surface. Using the thickness and the seismic velocity (Vp) of these high-velocity layers as constraints, MELTS rigorously calculates the amount and the composition of the fractionated solid phases and the major oxide concentrations in the residual liquids. The computed compositions of the residual liquids are rich in TiO2 and FeOtot, consistent with the observed compositions of most Emeishan basalts. Moreover, the calculated seismic velocities (Vp) of fractionated solid phases are also consistent with the geophysical observations. Mass balance calculation shows that the entire volume of the Emeishan basalts is about 8.9x106km3. The results also explain the absence of huge layered gabbro intrusions (V-Ti-magnetite deposits) in the west section of ELIP, and indicate that the east section is a potential mineral exploration region for V-Ti-magnetite deposits.