Th, Nb and Zr characteristics and plume causes identification of Emeishan basalts

Abstract

Having compared with the characteristics of world typical mantle plume basalts and analyzed Th, Nb and Zr characteristics of Emeishan basalt, this paper points out that although there were some differences about the formation environment of the three rock areas (WEST, CENTRE and EAST rock area), main rocks of them shared common geochemistry characteristics of ocean island basalts, implying that rocks came from metasomatic enriched mantle. Further studies showed that Emeishan basalt is generated from the mantle plume, but the crustal hybridism, the fractional crystallization of basaltic magma and different magma source regions lead to the fact that rock is generally rick in Th, Th/Nb ratio. In the tectonic setting discrimination diagram, their samples mostly fall into the tholeiite area of intracontinental rift and epicontinental rift, as opposed to the mantle plume area the samples of typical basalt of the world would fall into. The paper also indicates that the data collection must have certain criteria, this means that the samples must have low crustal contamination, little magma fractional crystallization, namely as representative of the rock of forming original magma as possible when using Th/Hf - Ta/Hf double logarithmic diagram (proposed by Wang et al. (2001)) and discrimination diagrams of basalt tectonic settings (proposed by Sun (2003)) to discriminate the tectonic setting of magmatic rock formation.