New Zealand Journal of Geology and Geophysics abstracts

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Geology, petrology, and petrogenesis of Little Barrier Island, Hauraki Gulf, New Zealand

JAN M. LINDSAY*
TIM J. WORTHINGTON
IAN E. M. SMITH
PHILIPPA M. BLACK

Department of Geology
The University of Auckland
Private Bag 92019
Auckland, New Zealand

*Present address: GeoForschungsZentrum Potsdam, P.B. 4.2, 14473 Potsdam, Germany.

Abstract  Little Barrier Island is the emergent part of a large, isolated, dacite-rhyodacite volcano in the active Hauraki Rift, 80 km northeast of Auckland. Two volcanic episodes are recognised: Waimaomao Formation was emplaced as a rhyodacite dome at 3 Ma, whereas the more extensive dacitic lavas of Haowhenua Formation were erupted between 1.2 and 1.6 Ma. All Little Barrier lavas are strongly porphyritic and contain phenocrysts of plagioclase, orthopyroxene, and hornblende. Geochemically, they are subduction related and distinct from the older lavas of the Coromandel Volcanic Group, being Zr rich but Rb and Ba poor. Their Sr and Nd isotope ratios are similar to those of the Tonga-Kermadec arc volcanoes. Modelling of the dacite supports petrographic evidence that recharge and mixing were important in the magmatic system. Little Barrier and two dacite domes of similar age and composition near Whangarei form a northwest-trending lineament subparallel to the Alexandra Volcanics and the Vening Meinesz Fracture Zone.

Keywords  volcano; dacite; K-Ar; geochemistry; Little Barrier Island; Hauraki Rift

New Zealand Journal of Geology and Geophysics, 1999, Vol. 42: 155-168

0028-8306/99/4202-0155 $7.00/0 (c) The Royal Society of New Zealand 1999

PDF file of entire paper: medium quality (1947K); (scanned from paper original: notes about this process)

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