Trace element and isotope geochemistry of the volcanic rocks of Bequia, Grenadine Islands, Lesser Antilles arc: A study of subduction enrichment and intra-crustal contamination

Abstract

Pliocene volcanics on the island of Bequia comprise two interbedded suites of basalts and andesites. The isotopically homogeneous suite (IHS) has a limited range of Sr - Nd - Pb isotopes (87Sr/86Sr 0·7040-0·7046, 143Nd/144Nd ∼0·5130 and 206Pb/204Pb 19·36-19·51), and mantle-like δ18O values (∼5·5 in clinopyroxene). The isotopically diverse suite (IDS) is characterised by much wider ranges of radiogenic isotopes (87Sr/86Sr 0·7048-0·7077, 143Nd/144 Nd 0·5128-0·5123 and 206Pb/204Pb 19·7-20·2), in which all of the Sr and Pb ratios are higher and Nd ratios are lower than those of the IHS. The IDS is also characterized by high δ18O values, up to 7·6 in clinopyroxene. The Sr and Pb isotope ratios are too high, and the Nd isotope ratios are too low in the IDS for any of these lavas to be derived from unmodified depleted mantle. Both suites are petrologically very similar and their major element compositions and phenocryst contents suggest that they were formed largely by fractional crystallization of a hydrous tholeiitic melt at pressures <3 kbar. The isotopic ratios and enrichments in large ion lithophile elements (LILE), and to some extent light rare earth elements (LREE), as compared with mid-ocean ridge basalts (MORE), of the IHS lavas suggest that they were derived from a depleted mantle source which had been re-enriched by the addition of 1-4% of a subduction component. This component probably comprised a mixture of dehydration fluids, and perhaps minor siliceous melts, released from subducting sediments and mafic crust. The extreme isotopic ranges, large enrichments in incompatible elements, more fractionated LREE patterns and higher δ18O values of the IDS lavas are interpreted as resulting from ∼10-55% assimilation-fractional crystallization of sediments, derived from the Guyana Shield, which are present in the arc crust, by IHS type melts.