Tikorangi Ignimbrite: a 0.89 Ma mixed andesite-rhyolite ignimbrite, Matahana Basin, Taupo Volcanic Zone, New Zealand

The 0.89 Ma Tikorangi Ignimbrite (revised name) is a mixed andesite-rhyolit e 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 ig nimbrite is poorly constrained, but the location and limited maximum pumice and lithic data available suggest that the ignimbrite was emplaced by a py roclastic flow erupted from within the Kapenga caldera complex. The ignimbr ite 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 subje ct to varying degrees of hydrothermal alteration This progressively grades into the Middle Tikorangi ignimbrite (MTi), which best illustrates the mixe d nature of the ignimbrite, and has four types of juvenile pumice: black, g rey 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 Tikor angi ignimbrite (UTi) is poor in lithics, crystals, and pumice, but with bo th andesite and rhyolite pumice clasts common up to the top of the ignimbri te. The geochemistry and petrography of vitric fragments and fiamme indicate th at both magma mixing and mingling between andesite and rhyolite have occurr ed. Mixing occurred when hotter andesite magma was injected into st reservo ir of more viscous rhyolitic magma and immediately triggered a violent erup tion. The initial stage of the eruption was driven by superheating of the r hyolite magma and continued degassing. Subsequently, chamber evacuation per mitted caldera block collapse that drove the eruption by pumping out the re maining magma. An increase in rifting within the Taupo Volcanic Zone, at 0. 90 Ma, may have facilitated the eruption process.