Evidence for Cold, Hydrous Parental Magma on Dominica: Petrology of the Foundland Basalts

SMSU Event Center BC

Dominica is dominated by andesite, dacite, and ignimbrite deposits. Foundland, in the southeast, is dominated by basalt flows, which are likely the most primitive magmas on Dominica. Here, we conducted a study of field relations and detailed petrology of those basalts. Each sample from Foundland is saturated in the same phases: plagioclase + clinopyroxene + titanomagnetite ± ilmenite ± olivine ± orthopyroxene. Sample FB-9 also contains highly reacted amphibole. Most samples are porphyritic with phenocrysts of plagioclase and clinopyroxene. Compared to other basalts on the island, the basalts of Foundland contain little to no olivine. Plagioclase composition ranges from An60 to an extraordinarily calcic An97. The absence of olivine as a phenocryst phase and the presence of anorthite-rich plagioclase, in the context of phase equilibrium experiments on basaltic liquids from the literature, suggest that this phase assemblage is consistent with cold pre-eruptive temperatures (~1000 °C) and elevated H2O contents (≥6wt%). Moreover, the sample containing the most calcic plagioclase also featured large grains of reacted amphibole, further supports these basalts having formed under extremely hydrous conditions. The phase assemblage of Foundland basalts is compared with a series of mafic to intermediate magmas erupted in the northern part of Dominica, proto-Morne aux Diables (pMAD); an outstanding question in the literature is whether both eruptive events had the same source. Samples from both pMAD and Foundland contain amphiboles, little to no olivine, and an abundance of pyroxene, which suggests that magmatism in both Foundland and pMAD originated from a similar cold, hydrous source.