The volcanic series of Mt Rantop and Robertson's Thumb, Erromango Island, New Hebrides, formed by fractional crystallization of orogenic basaltic magma of near-island-arc tholeiitic type. Differentiation was controlled mainly by separation of plagioclase, olivine and clinopyroxene. The Mt Rantop series is predominantly tholeiitic (plagioclase at the liquidus, late appearance of magnetite, pigeonite in microphenocrysts, and Fe and Ti remaining constant or increasing in the early stages of differentiation); those of Robertson's Thumb are mostly calc-alkaline (magnetite at the liquidus, late appearance of plagioclase, olivine quickly becoming unstable, orthopyroxene in phenocrysts and early decrease of Fe and Ti). The compositional differences reflect higher fO <sub>2</sub> and PH <sub>2</sub> O in Robertson's Thumb during fractional crystallization.

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