The Lesser Antilles volcanic chain: a study in are magmatism

The Lesser Antilles volcanic are is related to subduction of the American plate 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 segmented; 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 and 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 the 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 subduction along the are. Cenozoic volcanic rocks of the are are divided into low-K and medium-K series, each of which contains basaltic (MgO > 6%) members ranging from hypersthene- to nepheline-normative. It is likely that all the Lesser Antilles eruptives had picritic (or, more rarely, ankaramitic), possibly silica-undersaturated, primary magmas. The medium-g rocks show wide variations in trace-element and isotopic characteristics. A generalised sequence of phenocryst assemblages, applicable to both groups, is: olivine + spinel +/- clinopyroxene --> olivine + spinel + clinopyroxene + plagioclase --> plagioclase + clinopyroxene + titanomagnetite + orthopyroxene +/- amphibole +/- quartz. Phenocryst crystallisation temperatures were: basalts 1180-1130 degrees C; basaltic 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 and 3 Cpa. fO(2) conditions of magma crystallisation were rather oxidising (NNO +0.5 to NNO + 3). Estimates of magmatic water contents, using direct and indirect methods, give conflicting results. Generalisations, based on melt inclusion data, are that water contents increased from 1-2 wt.% in primary/parental magmas to 5-6 wt.% in dacitic and rhyolitic melts. Primary magmas were generated in normal mid-ocean ridge basalts (N-MORB)-type mantle, modified by the addition of a fluid component derived mainly from subducted basaltic crust and a component derived from partial melting of subducted sediment. The proportions of each component in the different magma 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 fractional crystallisation, accompanied, particularly in some central islands, by crustal contamination, and by minor magma mixing. The amount of contamination 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 different 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 the scale of tens of kilometers. The presence in individual centres on Grenada of two series with differing major- and trace-element and isotopic characteristics implies mantle sources which are heterogeneous on the scale of single plumbing systems. (C) 2000 Elsevier Science B.V. All rights reserved.