Multiple rhyolite magmas and basalt injection in the 17.7 ka Rerewhakaaitu eruption episode from Tarawera volcanic complex, New Zealand

Abstract

The 17.7 ka Rerewhakaaitu eruption episode (volume ∼ 5 km 3 DRE rhyolite magma) was the second of five major episodes that have built the Tarawera volcanic complex in the Okataina Volcanic Centre over the past 22 kyr. The Rerewhakaaitu episode produced a widespread tephra fall deposit, associated proximal pyroclastic flow deposits, and voluminous rhyolite lava extrusions. Two different rhyolite magmas (T1 and T2) were simultaneously erupted from the main vent area throughout much of the eruption episode. T1 magma was a crystal-poor orthopyroxene-hornblende rhyolite that is highly evolved (whole rock SiO 2 = 77 wt.%), with a moderate temperature (∼ 760 °C, based on Fe–Ti oxides). T2 is a crystal-rich biotite-hornblende rhyolite that is less evolved (SiO 2 = 75 wt.%), with a Fe–Ti oxide temperature of ∼ 700 °C. Ejecta from the simultaneous and sequential eruption of these two magmas include some pumice clasts with mixed (hybrid) and mingled glass compositions and crystal populations. A third rhyolite magma (T3) was extruded from another vent 3 km distant to form an apparently contemporaneous lava dome. T3 was the least evolved (SiO 2 = 74 wt.%) and hottest (∼ 820 °C) of the three magmas. Saturation pressures calculated using dissolved H 2O and CO 2 contents of melt inclusions in quartz crystals indicate that T2 magma stagnated and crystallised at about 12 km depth, while small quartz crystals in T1 magma grew during ascent through ∼ 8 km depths. Some T1 and T2 rhyolite clasts contain vesicular brown blebs with widely variable (andesite to rhyolite) glass compositions, accompanied by olivine, clinopyroxene and calcic plagioclase crystals that are interpreted as xenocrysts derived from injected basalt. Temperatures over 1000 °C estimated from pyroxene phase equilibria in these clasts reflect intrusion of the more mafic magma, which is now identified as the priming and triggering mechanism for three of the four post-22 ka Tarawera rhyolite eruption episodes. However, the rhyolite magma bodies and conduits modelled for each episode have considerable differences in characteristics and geometry. Our preferred model for the Rerewhakaaitu episode is that eruptions occurred from three laterally and vertically isolated rhyolite magma bodies that were initially primed and triggered by basalt intrusion during a regional rifting event. The ascending hotter and less viscous T1 rhyolite magma intersected and further invigorated a stagnant pond of cooler, denser and more viscous T2 magma, and lubricated its transport to the surface.