New Zealand Journal of Geology and Geophysics abstracts
Petrography and geochemistry of lithic fragments in ignimbrites from the
Mangakino Volcanic Centre: implications for the composition of the
subvolcanic crust in western Taupo Volcanic Zone,
New Zealand
STEPHEN J. P. KRIPPNER
ROGER M. BRIGGS
Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
COLIN J. N. WILSON
Institute of Geological & Nuclear Sciences
Wairakei Research Centre
Private Bag 2000
Taupo, New Zealand
JAMES W. COLE
Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand
Abstract The Mangakino Volcanic Centre is the westernmost and
oldest rhyolitic caldera volcano in the Taupo Volcanic Zone, North Island, New
Zealand. The largest eruptions from Mangakino occurred in two periods of
caldera-forming activity during 1.68-1.53 Ma (Period I), and
1.21-0.95 Ma (Period IIA), producing several voluminous widespread
welded and nonwelded ignimbrites and minor fall deposits. Other activity from
Mangakino generated fall deposits and rhyolitic lava domes. Lithic fragments
are common in all Mangakino ignimbrites (1-10 modal %), and consist of diverse
lithologies including: rhyolite, dacite, andesite, and basaltic andesite lava,
welded ignimbrite, tuff, volcanic breccia, biotite granite, granodiorite
porphyry, siltstone, sandstone, greywacke, metagreywacke, metaconglomerate,
biotite and hornblende-biotite schist. Lithic populations in Period I
ignimbrites are dominated by andesite lavas, suggesting that there was a
pre-existing andesite volcano in the Mangakino area, geochemically distinct
from Titiraupenga and Pureora, the nearest roughly contemporaneous andesitic
volcanoes. Later ignimbrites that erupted during Period IIA, contain
predominantly rhyolitic lava lithics, implying that significant dome building
activity occurred at Mangakino, which represented greater volumes of rhyolitic
lava than previously described from the area. Petrographic, geochemical, and
geophysical (density and magnetic susceptibility) data measured from the lithic
fragments are used to propose a model for the shallow crust below Mangakino
Volcanic Centre. This model postulates eruptions through a basement of Mesozoic
biotite schists overlain by metagreywackes, a thin cover of Tertiary sandstones
and siltstones, and an overlying volcanic succession of andesite, dacite and
rhyolite lavas, welded ignimbrites, and lacustrine sediments. Ignimbrite
eruptions incorporated comagmatic biotite granite fragments from the
crystallised margins of the silicic magma chambers, and effectively sampled the
subvolcanic crust and volcanic pile from the vent walls en route to the
surface.
Keywords volcanism; ignimbrites; Mangakino Volcanic Centre;
Taupo Volcanic Zone; lithic fragments; petrography; geochemistry; subvolcanic
crust
New Zealand Journal of Geology and Geophysics, 1998, Vol. 41: 187-199
0028-8306/98/4102-0187 $7.00/0 (c) The Royal Society of New Zealand
1998
PDF file of entire paper: medium quality (1233K); (scanned from paper original: notes about this process)
This year's abstracts |
Journal home page |
All abstracts |
Publishing home page
