Silicate liquid immiscibility in the ~ 2.5 Ga Fe-rich andesite at the top of the Dongargarh large igneous province (India)

Abstract

Microstructures indicative of silicate liquid immiscibility were identified in the groundmass of plagioclase–pyroxene-phyric andesites from the Mangikhuta Volcanics at the top of the ~2.5Ga Dongargarh large igneous province (LIP), central India. The bulk rock is an Fe-rich, Al-poor andesite (SiO2 57–58wt.%, FeO≤9.86wt.%, Al2O3 ~13wt.%) compositionally similar to icelandite. The Mangikhuta andesites are residual liquids after extensive fractional crystallization along an iron depletion and silica enrichment trend of the magma that produced underlying iron-rich tholeiitic basalts. Evidence for melt unmixing includes devitrified iron-rich glassy globules (10–20μm) and numerous iron-rich, crystalline relict melt structures (up to 50μm) with ferroan augite composition and occasionally with attached tiny magnetite crystals. Further evidence includes Si-rich ocelli and Fe-rich melt structures with Si-rich cores. The formation of immiscible globules and melt structures took place after 75% of the bulk melt had crystallized. This is interpreted to mark the onset of liquid immiscibility.Major element concentrations of the conjugate melts (Fe-rich and Si-rich) closely match compositions measured in natural samples and in experiments. However, it appears that bulk melt compositions influence major element partitioning (Di) between co-existing Fe-rich (LFe) and Si-rich (LSi) melts (Di=CiLFe/CiLSi). DFe, DMg and DCa increase, and DAl decreases in the conjugate melts when the liquid evolves along iron depletion and silica enrichment trends. In contrast, DFe, DMg and DCa decrease, and DAl increases when the bulk liquid evolves along iron enrichment and silica depletion trends.The melt unmixing in the Dongargarh andesite has begun at ~1000°C and an oxygen fugacity between quartz-fayalite–magnetite (QFM) and magnetite–wustite (MW). Late-stage evolution of the Dongargarh large igneous province (LIP) has led the conditions that give rise to liquid immiscibility.