Zircon U-Pb Geochronology and Mineral Chemistry of Xinhe Mafic Rocks: Petrological Constraints on Water Heterogeneity in the Emeishan Large Igneous Province

Abstract

The debate of hydrous and anhydrous mafic magmatism in the Emeishan large igneous province (ELIP) requires more robust petrological and mineralogical evidence. This paper reports zircon U–Pb ages, clinopyroxene and hornblende major element compositions, whole-rock geochemical and Sr–Nd–Pb isotopic data of hornblende-rich mafic plutonic rocks from the Xinhe igneous complex in the central ELIP to address this issue. Analytical results indicate that the Xinhe mafic rocks were emplaced at 260–259 Ma, consistent with the main eruptive phase of the ELIP. The Xinhe mafic rocks can be divided into hornblende gabbro and gabbroic diorite based on petrographic characteristics and MgO content. The Sr–Nd–Pb isotopic signatures of Xinhe mafic rocks are indicative of an EMI-type enriched mantle source, which is similar to the adjacent contemporaneous Miyi layered intrusions and basalts in the ELIP. The Xinhe mafic rocks exhibit a hydrous-type crystallization sequence (clinopyroxene–hornblende+plagioclase), which contrasts markedly with the anhydrous-like olivine+plagioclase–clinopyroxene sequence documented in the Miyi layered intrusions. This difference records contrasting magmatic water conditions during crustal magmatic evolution, although water enrichment during differentiation, differing pressure–redox histories, and fluid interaction may also contribute to the observed assemblages. Mantle-source water heterogeneity remains a plausible interpretation that requires independent confirmation.