An ultramafic-mafic suite of nodules characterises the basic-intermediate shield-forming lavas on Mauritius. A close mineralogical coherence is seen between highly porphyritic host lavas and nodule type, with peridotite occurring only in picrite basalt, olivine gabbro in olivine basalt, bytownite anorthosite and feldspathic gabbro in feldspar-phyric basalt, and kaersutite gabbro, labradorite-andesine anorthosite and titanomagnetite-pyrrhotite assemblages in hawaiite.

Prominent layering fabrics, crystallisation sequences, variations in chemical mineralogy and in mineral assemblages, together with phase equilibria constraints suggest that the nodules are derived from a compositionally and cryptically layered cumulate sequence which crystallised in a subjacent magma chamber at depths of 5–10 km (1.5–3.0 kb). The extensive development of kaersutite-rich assemblages and of marked compositional reversals in plagioclase is interpreted as evidence for the development of significantly hydrous conditions in the magma chamber by the hawaiitic stage of evolution. Combined olivine-clinopyroxene and plagioclase geother-mometry calculations suggest temperatures of c. 1000°C with PH2O/PTot ratios possibly fluctuating between 0.5–1.0 at this stage.

Fractionation of kaersutite-rich assemblages is thought to have maintained the transitional character of the magma until the final stages of evolution. The mineralogical coherence of the nodule/lava association indicates a simple model of magma chamber evolution. A process relating eruption of phenocryst-enriched magma from close to the magma/cumulate interface, with possible incorporation of unconsolidated crystal mushes, and elutriation of only the highest, most recently consolidated cumulates is proposed.

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