The Whakamaru Magmatic System (Taupō Volcanic Zone, New Zealand), Part 2: Evidence from ignimbrite deposits for pre-eruptive distribution of melt-dominated magma and magma mushes

Abstract

The Whakamaru volcanic deposits in Aotearoa New Zealand make up the largest eruptions in the young Taupō Volcanic Zone (TVZ). The complex volcanology and petrology of the multiple mappable ignimbrite units have obscured the number of eruptive phases, the relative timing of these eruption(s), and the pre-eruptive conditions of the magma bodies. To address these complexities, we use pumice clasts from multiple ignimbrite localities in conjunction with tephra deposits (Harmon et al., Part 1). Analysis of whole-rock and glass compositions from individual pumice clasts reveals the different magma types that participated in the eruptions. We confirm four main types of magma (types A, B, C, D; originally identified by Brown et al., 1998), which likely represent independent magma bodies. By examining the distribution of magma types in the ignimbrite record and corresponding tephra record (Harmon et al., Part 1), we establish the sequence of eruption for the four different mappable ignimbrites. The ignimbrites to the east of the caldera (Rangitaiki ± Te Whaiti) erupted before the Whakamaru ignimbrite (sensu stricto) to the west of the caldera. The youngest Whakamaru ignimbrite eruptions likely deposited to the northwest of the caldera contemporaneously with the Manunui ignimbrite to the west of the caldera. We calculate pre-eruptive storage temperatures (via zircon saturation geothermometry) and pressures (via rhyolite-MELTS geobarometry) using the glass compositions of the pumice clasts. We determine pressures of extraction from magma mush (via rhyolite-MELTS geobarometry) using matching whole-rock compositions. Melt-dominated magmas were stored at shallow depths (~50-150 MPa) prior to eruption. Types B and C extraction pressures are well constrained to 155-355 MPa. For types A and D, extraction pressures depend on the modeled oxygen fugacity (fO2), exhibiting a narrower range given a specific fO2 (overall range 170-360 MPa). Our results suggest there are at least two different magma subsystems that fed the Whakamaru eruptions – one subsystem sourced the type A and type D magmas, while the other sourced the type B and type C magmas. Both subsystems show gaps between storage and extraction pressures, suggesting separation between melt-dominated magma bodies and the magma mush bodies from which they were extracted. Combination of petrological data from the ignimbrites and associated tephras suggest a complex system that included laterally juxtaposed melt-dominated magmas as well as laterally juxtaposed magma mushes that spanned much of the shallow crust, but with regions in which magma appeared in low concentration or was entirely absent. Ignimbrite deposition across the landscape follows patterns revealed by the chronology observed in the tephra sequence.