Abstract

The volcanic rocks of St Kitts have uniform 87Sr/86Sr ratios (c. 0.7037) and high 143Nd/144Nd ratios (c. 0.5130) in a suite which ranges from basalt to rhyolite. Trace element abundances and the flat chondrite-normalized REE patterns resemble those of island-arc tholeiitic suites. In comparison, the Montserrat lavas have higher concentrations of alkaline elements and are LREE enriched. Geochemical differences may be related to partial displacement of the axis of the volcanic arc about 9 Ma. This would have led to the development of the St Kitts’ suite in an essentially new and immature island-arc environment. Being closer to the junction with the former arc axis, the composition of the Montserrat lavas may reflect previous subduction-related mantle enrichment and/or the influence of a sedimentary component. Isotopic ratios and most trace elements in the Redonda basalt indicate island-arc tholeiite or in some respects MORB affinities, but it has a high Sr content and is LREE-enriched. Despite its distinctive composition, Redonda is situated on the main axis of the volcanic arc and its lavas were evidently contemporaneous with the younger eruptions of St Kitts and Montserrat.

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