Subaqueous eruption environment of the central Emeishan large igneous province: insights from palagonite in hyaloclastite

Volcanic glass that preserves syn-eruptive hydration products is an important proxy for eruption dynamics, and palaeovolcanic settings. From the central part of the Emeishan Large Igneous Province (ELIP) in SW China, we identified well-preserved palagonitized vitriclasts within mafic-dominated volcaniclastic sequences based on field, petrographic, and geochemical investigations. These hydrated glass fragments exhibit polyhedral morphology, compositional homogeneity, and minimal vesicularity. These characteristics, together with the highly brecciated texture of the strata, absence of stratification, jigsaw-fit fractures among adjacent clasts, and the coexistence with intact pillow lavas, suggest that they are composed of hyaloclastite. Electron probe microanalysis (EPMA) and the mineralogy of pore-filling cements indicate that chemical exchange between magma and external water might have commenced during the quenching process that formed the hyaloclastite. The cementing minerals precipitated within the vapour film simultaneously during quenching of the lava and brecciation. The rapid and dense crystallization of interclast cements prevented further alteration of the quenched palagonite during post-volcanic diagenesis. Based on the macroscopic and microscopic characteristics of palagonite and EPMA data, we elucidate the petrogenetic mechanisms during the early eruptive stage of the ELIP, offering a refined analysis of the early eruptive processes, and proposing a schematic model. The results indicate that the early stage of the ELIP was characterized by extensive subaqueous effusive eruptions, accompanied by intense hyaloclastification. The findings provide crucial evidence that enhances our understanding of the early eruptive dynamics of the ELIP, and underscore the utility of palagonite geochemistry in reconstructing subaqueous volcanism in ancient LIPs.