Geology and geochemistry of the alkali basalt—andesite association of Grenada, Lesser Antilles island arc
Basanitoids and alkalic basalts that are strongly undersaturated in silica occur on the island of Grenada in the Lesser Antilles. Several volcanic centers have erupted basic lava of these compositions together with subalkalic basalt, andesite, and dacite from Miocene to Holocene time. The volcanic rocks overlie a folded volcanic-sedimentary formation of Eocene to Miocene age. Tuff rings and maars of explosive origin are present. Andesite and dacite are less significant volumetrically on Grenada in comparison with other islands in the Lesser Antilles. The variable trace-element geochemistry of the basanitoids and alkalic basalts is related, on the basis of rare-earth-element data, to a model of variable degrees of partial melting of an upper-mantle garnet peridotite source. It is suggested that fractional crystallization of olivine, clinopyroxene, and spinel, observed as the phenocryst assemblage in the basanitoids and alkalic basalts, takes place at high temperatures; at lower temperatures, these phenocrysts are joined by amphibole and plagioclase. A trend toward increased silica saturation is the result of this fractional crystallization process. The presence of alkalic lava rocks together with variable trace-element abundances and Sr isotope ratios are unusual features of the volcanicity.