Issue 1: February 1996

Abstract

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

  • 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

    • TE. Smith, MF. Thirlwall2 and C. MacPherson2 1Department of Geology, University of Windsor, Windsor, Ont., Canada N9B 3P4, 2Department of Geology, Royal Holloway, University of London, Egham TW20 0EX, UK and To whom correspondence should be addressed

    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 [delta]18O values (~5.5 in clinopyroxene). The isotopically diverse suite (IDS) is characterized by much wider ranges of radiogenic isotopes (87Sr/86Sr 0.7048-0.7077, 143Nd/144Nd 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 [delta]18O values, up to 7.6 in clinopyroxene. The Sr and Pb isotope ratios are too high, and 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 (MORB), 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 fractionated LREE patterns and higher [delta]18O 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.

    KEY WORDS. trace elements; radiogenic isotopes; arc lavas; Lesser Antilles

    Pages: 117 - 143

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