New Zealand Journal of Geology and Geophysics abstracts

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Volcanic geology of Rarotonga, southern Pacific Ocean

G. M. THOMPSON
J. MALPAS

Department of Earth Sciences
University of Hong Kong
Pokfulam Road
Hong Kong

IAN E. M. SMITH*

Department of Geology
University of Auckland
Private Bag 92019
Auckland, New Zealand
Email: ie.smith@auckland.ac.nz

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.

Keywords  Rarotonga; ocean island volcanism; alkali basalt; ankaramite; phonolite

New Zealand Journal of Geology and Geophysics, 1998, Vol. 41: 95-104

0028-8306/98/4101-0095 $7.00/0 (c) The Royal Society of New Zealand 1998

PDF file of entire paper: medium quality (972K); (scanned from paper original: notes about this process)

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