New Zealand Journal of Geology and Geophysics abstracts
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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