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

Abstract

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.