A Tholeiitic Damp Parental Magma for the Miyi Mafic Intrusion: Implications for the Water Heterogeneity in the Emeishan Large Igneous Province

Abstract

The anhydrous or hydrous scenario of large igneous provinces (LIPs) remains controversial. The low–Ti primary magma is hydrous but the water content of high–Ti primary magma is not well constrained in the Emeishan LIP (ELIP). We investigated this with a comprehensive petrological, mineralogical, geochronological, and geochemical study of the Miyi mafic intrusion in central ELIP. The gabbros and diabases were dated at ca. 259–258 Ma, coeval with the ELIP. Petrographic observations, along with whole-rock and mineral compositions, indicate that the gabbros are typical cumulates. Equilibrium tests and rhyolite-MELTS modelling results proved that the tholeiitic fine-grained diabases in the chilled margins could represent the compositions of parental magmas. Crystallization of such parental magmas at damp (1.5–1.6 wt.% H2O) and low pressure (< 1 kbar) conditions would reproduce the sequence and compositions of early cumulus minerals in the gabbros. The gabbros and diabases show similar EMI-like Sr-Nd-Pb isotopes to the high–Ti picrites in the ELIP. Calculations based on pMELTS modelling and rare earth elements (REEs) show that the compositions of the diabases could be produced by 50–70 wt.% differentiation from an anhydrous (0.5 wt.% H2O) high–Ti primary magma. The water content in the ELIP is heterogeneous with an anhydrous high–Ti endmember and a hydrous low–Ti endmember. Such heterogeneity might be caused by source lithological heterogeneity.

Keywords: water, lithology heterogeneity, gabbro, damp parental magma, Emeishan LIP