New Zealand Journal of Geology and Geophysics abstracts

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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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