New Zealand Journal of Geology and Geophysics abstracts
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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