New Zealand Journal of Geology and Geophysics abstracts
Tikorangi Ignimbrite: a 0.89 Ma mixed andesite-rhyolite ignimbrite,
Matahana Basin, Taupo Volcanic Zone, New Zealand
S. C. HILDYARD*
J. W. COLE
S. D. WEAVER
Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand
*Present address: Department of Earth Sciences, Monash University, Clayton,
Victoria 3168, Australia.
Abstract The 0.89 Ma Tikorangi Ignimbrite
(revised name) is a mixed andesite-rhyolite ignimbrite preserved within a
localised area of the Matahana Basin on the western side of the Taupo Volcanic
Zone, New Zealand. The source of the ignimbrite is poorly constrained, but the
location and limited maximum pumice and lithic data available suggest that the
ignimbrite was emplaced by a pyroclastic flow erupted from within the Kapenga
caldera complex. The ignimbrite is the oldest exposed unit sourced from the
complex and offers a window into the early history of the Taupo Volcanic
Zone.
The Tikorangi Ignimbrite can be conveniently divided into three units. The
Lower Tikorangi ignimbrite (LTi) is dominantly rhyolitic and has been subject
to varying degrees of hydrothermal alteration. This progressively grades into
the Middle Tikorangi ignimbrite (MTi), which best illustrates the mixed nature
of the ignimbrite, and has four types of juvenile pumice: black, grey black,
brown black (all andesitic, in various states of oxidation), and white-grey
(rhyolitic). The top of the Middle Tikorangi ignimbrite grades up into a
densely welded, dominantly andesitic lenticulite. The Upper Tikorangi
ignimbrite (UTi) is poor in lithics, crystals, and pumice, but with both
andesite and rhyolite pumice clasts common up to the top of the ignimbrite.
The geochemistry and petrography of vitric fragments and fiamme indicate that
both magma mixing and mingling between andesite and rhyolite have occurred.
Mixing occurred when hotter andesite magma was injected into a reservoir of
more viscous rhyolitic magma and immediately triggered a violent eruption. The
initial stage of the eruption was driven by superheating of the rhyolite magma
and continued degassing. Subsequently, chamber evacuation permitted caldera
block collapse that drove the eruption by pumping out the remaining magma. An
increase in rifting within the Taupo Volcanic Zone, at 0.90 Ma, may have
facilitated the eruption process.
Keywords Tikorangi Ignimbrite; mixed andesite-rhyolite
ignimbrite; Taupo Volcanic Zone; stratigraphy; geochemistry
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43: 95-107
0028-8306/00/4301-0095 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (1202K); (scanned from paper original: notes about this process)
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