The 26·5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body

Abstract

The caldera-forming 26•5 ka Oruanui eruption (Taupo, New Zealand) erupted ~530 km<sup>3</sup> of magma, >99% rhyolitic, <1% mafic. The rhyolite varies from 71•8 to 76•7 wt % SiO2 and 76 to 112 ppm Rb but is dominantly 74–76 wt % SiO<sub>2</sub>. Average rhyolite compositions at each stratigraphic level do not change significantly through the eruption sequence. Oxide geothermometry, phase equilibria and volatile contents imply magma storage at 830–760°C, and 100–200 MPa. Most rhyolite compositional variations are explicable by ~28% crystal fractionation involving the phenocryst and accessory phases (plagioclase, orthopyroxene, hornblende, quartz, magnetite, ilmenite, apatite and zircon). However, scatter in some element concentrations and <sup>87</sup>Sr/<sup>86</sup>Sr ratios, and the presence of non-equilibrium crystal compositions imply that mixing of liquids, phenocrysts and inherited crystals was also important in assembling the compositional spectrum of rhyolite. Mafic compositions comprise a tholeiitic group (52•3–63•3 wt % SiO<sub>2</sub>) formed by fractionation and crustal contamination of a contaminated tholeiitic basalt, and a calc-alkaline group (56•7–60•5 wt % SiO<sub>2</sub>) formed by mixing of a primitive olivine–plagioclase basalt with rhyolitic and tholeiitic mafic magmas. Both mafic groups are distinct from other Taupo Volcanic Zone eruptives of comparable SiO<sub>2</sub> content. Development and destruction by eruption of the Oruanui magma body occurred within ~40 kyr and Oruanui compositions have not been replicated in vigorous younger activity. The Oruanui rhyolite did not form in a single stage of evolution from a more primitive forerunner but by rapid rejuvenation of a longer-lived polygenetic, multi-age ‘stockpile’ of silicic plutonic components in the Taupo magmatic system.