Volcanic geology of Rarotonga, southern Pacific Ocean

Abstract

The island of Rarotonga in the southern Pacific is the emergent summit of a Pliocene - Pleistocene volcanic complex built by effusive and pyroclastic eruptions of mainly mafic magma. Petrographically the basaltic rock types are ankaramite and basalt, which range in chemical composition from alkali basalt to nephelinite. Phenocryst assemblages suggest two igneous series, one with a relatively simple equilibrium assemblage of olivine, titanian augite and magnetite, and one with olivine, diopside-augite and titanian augite in which the phenocrysts show disequilibrium textures. These variations reflect fractionation, assimilation, and recharge processes in the upper part of the magmatic system that produced the volcano. The final stages of volcanism at Rarotonga were pyroclastic eruptions of phonolites and effusive eruptions of foidal phonolites, both representing late stage fractionation products. Detailed mapping, together with geochemical work, has prompted a revision of the stratigraphy of the island based on the concept of a single cycle of magmatic activity rather than the Hawaiian style multi-phase evolution favoured by earlier workers.