Evolution of young andesitic–dacitic magmatic systems beneath Dominica, Lesser Antilles

• Dominica andesites–dacites contain 3 mineral equilibrium assemblages.
• Andesite genesis occurs through convective self-mixing within a single reservoir.
• All Dominica magmas undergo similar processes.
• Varying amounts of basaltic–andesitic mush exist beneath the entire island.
• Dominica's volcanic rocks are comprised of a mafic crystal cargo and rhyolitic melt.

The geology of Dominica, Lesser Antilles, is dominated by young (< 200 ka) andesitic–dacitic volcanic deposits. Although previous studies have mostly focused on the Plat Pays Volcanic Centre (PPVC), a volcanic complex located on the south-western tip of Dominica, evolved deposits are found across the entire island. Dominica's andesitic–dacitic volcanic rocks are characterized by wide ranges in mineral compositions and a host of disequilibrium features. Mineral–mineral and mineral–melt equilibrium constraints suggest that three separate equilibrium assemblages are associated with the development of these evolved magmas. While Rayleigh fractionation modeling indicates that these equilibrium groups may be related via fractional crystallization, enclaves with irregular boundaries and chilled margins provide evidence of open-system processes such as magma mingling. To explain the disequilibrium assemblages and textures, we propose that Dominica andesites and dacites are the result of convective self-mixing within magma reservoirs comprised of three distinct components: 1) underplating basalt, 2) basaltic–andesitic mush and 3) interstitial rhyolitic melt. A notable exception is Aux Diables, the northernmost center on the island, which has a strikingly different whole-rock geochemistry than other centers. Trace element modeling indicates that this center is influenced by lower crustal assimilation in addition to fractional crystallization. This study suggests that basaltic–andesitic mush is a necessary component in the generation of evolved magmas beneath the island of Dominica. Available ages suggest that such mush has existed beneath parts of the island for at least 200 ka.