The Lesser Antilles volcanic chain : a study in arc magmatism.

Geology of the Lesser Antilles

The volcanic rocks of St Kitts have uniform 87 Sr/ 86 Sr ratios (c. 0.7037) and high 143 Nd/ 144 Nd 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.

Geochemical evolution of St Kitts and Montserrat, Lesser Antilles

St Kitts forms one of the northern group of volcanic islands in the Lesser Antilles arc Eruptive products from the Mt Liamuiga centre are predominantly olivine + hypersthene-normative, low-K basalts through basaltic andesites to quartz-normative, low-K andesites Higher-Al and lower-Al groups can be distinguished in the suite Mineral assemblages include olivine, clinopyroxene, orthopyroxene, plagioclase and titanomagnetite with rarer amphibole, ilmenite and apatite Eruptive temperatures of the andesites are estimated as 963–950°C at fO2 NNO + 1 (where NNO is the nickel–nickel oxide buffer) Field and mineral chemical data provide evidence for magma mixing Glass (melt) inclusions in the phenocrysts range in composition from andesite to high-silica rhyolite Compositional variations are broadly consistent with the evolution of more evolved magmas by crystal fractionation of basaltic parental magmas The absence of any covariation between 87Sr/86Sr or 143Nd/144Nd and SiO2 rules out assimilation of older silicic crust However, positive correlations between Ba/La, La/Sm and 208Pb/204Pb and between 208Pb/204Pb and SiO2 are consistent with assimilation of small amounts (<10%) of biogenic sediments Trace element and Sr–Nd–Pb isotope data suggest derivation from a normal mid-ocean ridge basalt (N-MORB)-type mantle source metasomatized by subducted sediment or sediment melt and fluid The eruptive rocks are characterized by 238U excesses that indicate that fluid addition of U occurred <350 kyr ago; U–Th isotope data for mineral separates are dominated by melt inclusions but would allow crystallization ages of 13–68 ka However, plagioclase is consistently displaced above these ‘isochrons’, with apparent ages of 39–236 ka, and plagioclase crystal size distributions are concave-upwards These observations suggest that mixing processes are important The presence of 226Ra excesses in two samples indicates some fluid addition <8 kyr ago and that the magma residence times must also have been less than 8 kyr

/pdf/a-complex-petrogenesis-for-an-arc-magmatic-suite-st-kitts-3muqk99yuv.pdf

A Complex Petrogenesis for an Arc Magmatic Suite, St Kitts, Lesser Antilles.

The geology, petrology, and petrogenesis of Saba island, Lesser Antilles

Saba is composed of basic and intermediate members of the calc alkaline series with a broad resemblance to volcanic rocks on other islands of the northern Lesser Antilles. The most abundant rock-type is hornblende andesite with relatively high potassium and associated elements. Notable petrographic features are the persistence of magnesian olivine into relatively siliceous whole-rock composition and the prevalence of hornblende even in the more basic rocks. Quartz xenocrysts also occur in the basalts Clinopyroxene phenocrysts are of an gite composition but groundmass grains are sometimes in the range subcalcic augite to magnesian pigeonite. Disequilibrium features in the petrography, together with anomalies in the chemical trends may be due to incorporation of xenocrysts or perhaps in extreme instances to the hybridisation of contrasting magmas.

F. Buckley

Papers

Petrology of the Volcanic Rocks of Saba, West Indies