A tholeiitic damp parental magma for the Miyi mafic intrusion: Implications for water heterogeneity in the Emeishan large igneous province

Abstract

The debate over the anhydrous or hydrous nature of large igneous provinces (LIPs) continues. In the Emeishan LIP (ELIP), the water content of low-Ti primary magma is known to be hydrous, while that of high-Ti primary magma is not well constrained. We investigated the water content of the high-Ti Miyi mafic intrusion in central ELIP using comprehensive petrological, mineralogical, geochronological, and geochemical approach. The gabbros and diabases in the intrusion were dated at approximately 259–258 Ma, coinciding with the ELIP formation. Petrographic observations, along with whole-rock and mineral compositions, reveal that the gabbros are typical cumulates. Equilibrium tests and rhyolite-MELTS modelling results confirm that the tholeiitic fine-grained diabases of the chilled margins can represent parental magma compositions. Crystallization of such parental magmas under 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 using pMELTS modelling and rare earth elements show that diabase compositions could result from 50 to 70 wt% differentiation from an anhydrous (0.5 wt% H2O) high-Ti primary magma. The water content in the ELIP varies, with an anhydrous high-Ti endmember and a hydrous low-Ti endmember. This heterogeneity may be caused by heterogeneous source lithologies.