Genesis of the igneous rock suite of Grenada, Lesser Antilles

Abstract

The Grenada igneous rock suite comprises extrusive rocks, coarsegrained, gabbroic cumulate blocks, and peridotite ncxlules ~ The extrusi ve rocks include a variety of basalts and basaltic andesites, together with andesites and dacites. Two basaltic series are distinguished in major and trace element chemistry. 'The generally rnicrophyric, Mseries basalts include picritic types close in composition to primary partial melts of peridotite. The porphyritic, C-series basalts have major element chemistry consistent with derivation by olivine fractionation from these primitive compositions.

The major element variation is explicable by fractionation of phenocryst phases. Ne-norrna.ti ve M-series parents fractionated olivine, clinopyroxene, and spinel at high pressures, producing Hy-nornative derivatives. Poth basaltic series are parental to andesites, amphibole fractionation being restricted to the origin of these evolved magmas. Trace element abundances are highly variable, and cannot be products of variable partial melting and crystallisation processes, a heterogeneous source being required. Different sources are required for the two basaltic series. Combined chemical data and published isotopic analyses are interpreted in a model of variable contamination of the mantle source by a component, rich in water and incornp3.tible elements, derived from the suJ:xiucted lithosphere. This component may include a contribution from sediments as well as from subducted oceanic crust.

Olivine-bearing cumulates have mineral assemblages and canpositions indicative of equilibrium crystallisation from M-series magmas, producing basaltic andesite residua. Olivine-free cumulates equilibrated with andesitic residual liquids. Combined thermodynamic calculations and experimental studies indicate cumulate crystallisation at O. 5 to 6 kbar and 9000 -10000 C, at high oxygen and water activities.

Peridotite nodules in M-series basalts are interpreted as accidental inclusions of depleted mantle material. Comparison of the chemical data with those from other islands in the lesser Antilles and other arcs suggests that , although differing in trace element and isotopic canposi tions, probably as a result of differing source chemistries, many island arc suites are products of fractionation of picritic magmas. The eruption of these in Grenada may reflect unusual thermal characteristics in the underlying mantle, or unusually high melt water contents.