Abstract

The Waimihia pumice deposit was erupted from Taupo volcano about 3.3 ka BP. It comprises a plinian tephra fall deposit divisible into two volumetrically subequal fall units and a late-stage, volumetrically minor, near-vent ignimbrite. About 7.5km³ of magma were erupted at an average rate of c. 8 × 10⁸ kg s⁻¹. Whole rock, glass and mineral compositions define three crystal-poor magma types: rhyolite, rhyodacite and andesite. The proportions of the three magmas varies with stratigraphic position in the two fall units. The lower Waimihia fall unit shows a steady upward increase in rhyodacite from <1% to 5.5% and a near absence of andesite (<0.05%). Rhyodacite in the upper Waimihia fall unit decreases upward from 28% to 2% while andesite increases (0.2% to 0.8%) before decreasing (0.3%). The composition of the ignimbrite coincides with that of the top part of the upper fall unit (consistent with its stratigraphic position), suggesting that partial column collapse occurred towards the close of activity. Rhyolite dominates the total Waimihia deposit (c. 92%) and is similar to other post-22 ka Taupo rhyolites in terms of major element, trace element and Sr isotope composition, mineralogy and Fe-Ti oxide temperature and oxygen fugacity. The rhyodacite (c. 7.3%) formed by hybridization of rhyolite and andesite prior to the eruption, and occurs as grey pumices and with rhyolite in streaky pumices. Andesite (c. 0.3%) occurs as black scoria, sometimes containing traces of rhyodacite; it is an unusual high TiO₂ tholeiitic andesite whose ⁸⁷Sr/⁸⁶Sr ratio of 0.7062 is indistinguishable from that of the rhyolite. The heterogeneous pumice and scoria clasts were formed in a second magma mixing process that was active when the magmas were being transported to the surface during the eruption. The order in which the magmas were erupted, and the decoupling of the peaks in rhyodacite and andesite production, are explained by withdrawal from a three-layer sill that had formed following the injection of c. 0.16 km³ of andesite into a ≥ 7.5 km³ sillshaped rhyolitic magma chamber.