The Lesser Antilles volcanic chain: a study in are magmatism

The Lesser Antilles volcanic are is related to subduction of the American p late under the Caribbean plate. The rate of subduction is low, 2-4 cm a(-1) , and this has been reflected, at least over the past 0.1 Ma, in relatively low magma production rates (3-5 km(3) Ma(-1) km(-1) of are). The are is se gmented; a northern segment trends 330 degrees and the Benioff zone dips at 50-60 degrees, whilst the southern segment trends 020 degrees and the dip varies from 45 degrees to 50 degrees in the north to vertical in the south. Pleistocene-Recent volcanism (< 2 Ma) occurs in narrow zones less than 10 km wide and seems to define three segments, the break between the central a nd southern segments being in the same location as the kink in the Benioff zone. Magma production over the past 0.1 Ma has been higher in islands of t he central segment (8-40 km(3)) than in the northern and southern segments (0-5 km(3)); the variations may be related to the degree of obliquity of su bduction along the are. Cenozoic volcanic rocks of the are are divided into low-K and medium-K seri es, each of which contains basaltic (MgO > 6%) members ranging from hyperst hene- to nepheline-normative. It is likely that all the Lesser Antilles eru ptives had picritic (or, more rarely, ankaramitic), possibly silica-undersa turated, primary magmas. The medium-g rocks show wide variations in trace-e lement and isotopic characteristics. A generalised sequence of phenocryst a ssemblages, applicable to both groups, is: olivine + spinel +/- clinopyroxe ne --> olivine + spinel + clinopyroxene + plagioclase --> plagioclase + cli nopyroxene + titanomagnetite + orthopyroxene +/- amphibole +/- quartz. Phen ocryst crystallisation temperatures were: basalts 1180-1130 degrees C; basa ltic andesites 1060-1050 degrees C: and andesites-dacites 960-740 degrees C . Magmas inferred to be primary to the eruptive suites equilibrated within the spinel peridotite facies in the mantle wedge at pressures between 1.5 a nd 3 Cpa. fO(2) conditions of magma crystallisation were rather oxidising ( NNO +0.5 to NNO + 3). Estimates of magmatic water contents, using direct an d indirect methods, give conflicting results. Generalisations, based on mel t inclusion data, are that water contents increased from 1-2 wt.% in primar y/parental magmas to 5-6 wt.% in dacitic and rhyolitic melts. Primary magmas were generated in normal mid-ocean ridge basalts (N-MORB)-ty pe mantle, modified by the addition of a fluid component derived mainly fro m subducted basaltic crust and a component derived from partial melting of subducted sediment. The proportions of each component in the different magm a types are still debated, as is the importance of crustal contamination in producing their trace-element and isotopic characteristics. Compositional variations in the magmatic suites are a result mainly of polybaric fraction al crystallisation, accompanied, particularly in some central islands, by c rustal contamination, and by minor magma mixing. The amount of contaminatio n may be related to volumetric volcanic production. The influence of the sediment component relative to that of hydrous fluids generally increases towards the south. However, neighbouring islands, and d ifferent centres within islands, may show different fractionation histories , indicating that the factors which controlled magma compositions, such as water concentrations in the source rocks and magma ascent rates, vary on th e scale of tens of kilometers. The presence in individual centres on Grenad a of two series with differing major- and trace-element and isotopic charac teristics implies mantle sources which are heterogeneous on the scale of si ngle plumbing systems. (C) 2000 Elsevier Science B.V. All rights reserved.