The alkali basalt andesite association of Grenada Lesser Antilles

Abstract

Grenada is the southernmost volcanic island of the Lesser Antilles. A series of volcanic centres ranging from Pliocene to Recent in age are present overlying a folded Lower to Middle Tertiary volcano-sedimentary formation. Eruptions of silica-undersaturated alkali basalt and picrite magmas have occurred repeatedly during the evolution of these centres. Calc-alkaline andesites and dacites show a close field association with the basalts and picrites. Recent activity on the island has been explosive in nature. A model of variable volumes of Upper Mantle partial melting is proposed to account for the diversity of major, trace and Rare Earth element compositions and strontium isotope ratios of the basalts and picrites. Geochemical, petrographic and mineralogical criteria suggest that the andesites and dacites are related to these basic melts by fractional crystallisation processes. In addition the chemical compositions and strontium isotope ratios of the andesites and dacites reflect the diverse compositions of the parental basalt magmas. The petrography and mineralogy of the andesites and dacites is similar to calc-alkaline suites elsewhere in the arc. Some of the basalts and picrites contain abundant olivine and sector and oscillatory zoned clinopyroxene phenocrysts. In some basalts, phenocryst amphibole is present. An origin by partial melting of an Upper Mantle pericotite source is proposed. Alternative sources are examiner: but partial melting of a subducted lithospheric plate does not appear to be a significant petrogenetic process in Grenada, Fractional crystallisation of olivine, clinopyroxene and spinel is mainly responsible for the development of a normal calc-alkaline trend towards increasing silica-saturation in the magmas. Subsequent crystallisation of plagioclase feldspar and then amphibole is also important in the development of a trend towards silica- rather than alkali-enrichment in the residual melts. The significant feature of the Grenada vulcanicity is the occurrence within a restricted geographic range of magmas of contrasted geochemical characteristics. The local volcanic and tectonic history of the southern part of the Lesser Antilles island arc are probably the most important factors in the development of these unusual characteristics.