Abstract:
The caldera-forming 26·5 ka Oruanui eruption (Taupo, New Zealand) erupted 530 km3 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 % SiO2. 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 87Sr/86Sr 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 % SiO2) formed by fractionation and crustal contamination of a contaminated tholeiitic basalt, and a calc-alkaline group (56·7–60·5?wt % SiO2) 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 SiO2 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.
