Petrology and geochemistry of Rodrigues Island, Indian Ocean

Abstract

Rodrigues Island is composed of a differentiated series of transitional-mildly alkaline olivine basalts. The lavas contain phenocrysts of olivine (Fo88−68)±plagioclase (An73−50), together with a megacryst suite involving olivine, plagioclase, kaersutite, clinopyroxene, apatite, magnetite and hercynite-rich spinels. Troctolitic-anorthositic gabbro xenoliths are widely dispersed throughout the lavas and are probably derived from the upper parts of an underlying layered complex: the megacrysts may originate from coarse, easily disaggregated differentiates near the top of this body. Modelling of major and trace element data suggests that the majority of chemical variation in the lavas results from up to 45% fractionation of olivine, clinopyroxene, plagioclase and magnetite at low pressures, in the ratio 20∶35∶39∶6. The clinopyroxene-rich nature of this extract assemblage is significantly different to that of the xenoliths, and suggests that clinopyroxene-rich gabbros and/or ultrabasic rocks may lie at greater depth. Sr and Nd isotopic data (87Sr/86Sr 0.70357–070406,143Nd/144Nd 0.51283–0.51289) indicate a mantle source with relative LREE depletion, and emphasise an unusual degree of uniformity in Indian Ocean island sources. A small group of lavas with strong HREE enrichment suggest a garnet-poor source for these, while high overall Al2O3/ CaO ratios imply high clinopyroxene/garnet ratios in refractory residua.