Basaltic rocks occurring in the Lesser Antilles are directly comparable in chemical composition and mineralogy with basalts from other calc-alkaline suites of the circum-oceanic islands and from orogenic belts on the continental margins. Basalts of this type, characterized by a high alumina and low alkali content, are distinct from olivine tholeiites of the ocean basins and from alkali basalts of the intraoceanic islands.
Available geochemical and geophysical data place certain limitations on the composition of the magma-source area (?upper mantle). The slightly higher values for K, Rb, and Sr, the higher Rb/K, Rb/Sr, Fe/Mg, Co/Ni and Sr87/Sr86 ratios, and the plagioclase enrichment found in circum-oceanic basalts, compared with oceanic tholeiites, indicate that circum-oceanic basalts are derivative types.
It is suggested that the common circum-oceanic basalts are fractionation products of olivine tholeiite, or represent partial melts from a basic-ultrabasic zone with the appropriate chemistry developed beneath the arc and corresponding with the anomalous low velocity zone. Variations in the composition of the aluminous circum-oceanic basalts may reflect physico-chemical heterogeneities within the source area related to the tectonic or chemical evolution of the arc, or both.
Present data indicate that Lesser Antillean basalts have probably gone through at least two and possibly three types or stages of fractionation at different levels and under different physical conditions. The primary liquid, derived by melting of a peridotite mantle, probably has the composition of an olivine tholeiite. The first stage of fractionation takes place at depths of 30 km, where forsterite and possibly enstatite separate, increasing the A12O3 content and the Fe/Mg and Co/Ni composition ratios. Alternatively, the primary magma was generated from a source approximating basalt with comparatively high Fe/Mg and Co/Ni ratios, in which case fractionation would commence with stage two. The second stage occurs at depths of about 6 to 8 km and involves the separation of anorthite-olivine-salite-amphibole-magnetite cumulates under high-water vapor pressure. Fractionation of this type enriches the liquid in silica, giving andesitic magmas. The third and final stage, occurring at shallow depths of 2 to 3 km, involves the separation of the assemblage labradorite-olivine-augite-hypersthene-titanomagnetite, that is, the dominant phenocrystic assemblage found in the surface basalts and basaltic andesites. Accumulation of one or more of the phases will give rise to accumulative basalts and andesites.