New Zealand Journal of Geology and Geophysics abstracts

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Volcanic geology and geochemistry of Motuhora (Whale Island), Bay of Plenty, New Zealand

R. M. BURT
J. W. COLE

Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand

P. Z. VROON

Department of Geology
Royal Holloway and Bedford New College
Egham Hill, Egham
Surrey, TW20 0EX, United Kingdom

Abstract  Motuhora (Whale Island) lies c. 11 km offshore from Whakatane in the Bay of Plenty, New Zealand, and comprises tuffaceous marine sediments of the Camp Bay and Motuhora Formations separated by lavas, volcanic breccias, and slope-wash deposits of the Whale Volcanics. Whale Volcanics can be divided into East Dome, Central Dome Complex, and Pa Hill Dome. East Dome is a flow banded, chaotically jointed dacite that is probably extrusive. Central Dome comprises lava flows, and extensive volcanic breccias and tuffs which thicken into a local depression to the north of the central high, suggesting rapid growth and erosion of the dome. Pa Hill Dome is largely intrusive into Camp Bay Formation, although blocks of Pa Hill dacite in an upper slope-wash cobble bed suggest it was partially extrusive.

The lavas are porphyritic with phenocrysts of plagioclase, orthopyroxene, and titanomagnetite with subordinate clinopyroxene and amphibole (particularly in Pa Hill Dome), and rare biotite. Rounded or broken and embayed quartz crystals are found in the Central Dome Complex and Pa Hill domes. Magmatic xenoliths are common in all lavas. Chemically the lavas are medium-K, calc-alkaline andesites and dacites, and show relative LILE enrichment and HFSE depletion typical of arc volcanics. Isotopically, samples tend to have more radiogenic Sr and less radiogenic Nd than volcanics from neighbouring White Island. It is likely that Motuhora lavas were formed by a multi-stage process involving partial melting of N-MORB-type mantle that had been fluxed by fluids rich in incompatible elements derived from the dehydrating downgoing slab and followed by crystal fractionation of the magma. As the magma rose through the lower continental crust it was contaminated, probably by Torlesse metasediment. Petrographic textures and mineral chemistry indicate that magma mixing, while in an upper crustal magma chamber, is the norm for Motuhora lavas.

Keywords  Motuhora (Whale Island); andesite; dacite; volcanology; petrology; geochemistry; isotopes; magma genesis; Taupo Volcanic Zone

New Zealand Journal of Geology and Geophysics, 1996, Vol. 39: 565-580

0028-8306/96/3904-0565 $2.50/0 (c) The Royal Society of New Zealand 1996

PDF file of entire paper: medium quality (2621K); (scanned from paper original: notes about this process)

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