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
Authors: Harmon Lydia , Smithies Sarah , Gualda Guilherme , Gravley Darren 
Published: August 2023

The complex volcanology and petrology of the Whakamaru volcanic deposits in Aotearoa New Zealand have thus far obscured the number of eruptive phases and the relative timing of these eruption(s). We investigate pumice clasts from multiple localities to elucidate the relative timing of the eruptions, with a focus on the pre-eruptive conditions of the melt-dominated magma bodies that fed the Whakamaru eruptions and on the mushes from which these magmas were extracted. Paired whole-rock and glass compositions confirm four magma types erupted during the Whakamaru eruptions (types A, B, C, D; originally identified by Brown et al., 1998). Using the glass compositions of the pumice clasts, we calculate pre-eruptive storage temperatures (using zircon saturation geothermometry) and pressures (using rhyolite-MELTS geobarometry). Using matching whole-rock compositions from a subset of pumice clasts, we calculate extraction pressures from magma mush (also using rhyolite-MELTS geobarometry). Pre-eruptive storage pressures estimate the depths where melt-dominated magma bodies were located prior to eruption; extraction pressures, in contrast, estimate the depths at which melt was extracted from magma mush to form melt-dominated magma bodies at shallower levels of the crust. Magmas were stored at shallow depths (~50-150 MPa) prior to eruption. Extraction pressures for types B and C are well constrained to 155-355 MPa (with an assemblage including plagioclase and quartz). Extraction pressures for types A and D depend on oxygen fugacity (fO2), as the extraction assemblage includes plagioclase and orthopyroxene (170-290 MPa for ΔNNO = 0 to +0.5 and 290-360 MPa for ΔNNO = +1 to +1.5). The four magma types likely represent independent magma bodies, with these melt-dominated magma bodies stored shallower than and separate from the mush. At least two different magma subsystems 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. The distribution of magma types recorded in the ignimbrite deposits and in the correlated tephras (Harmon et al., 2024) reveal 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. The 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. This complex pre-eruptive architecture probably contributed to the complex eruptive patterns observed for the Whakamaru eruptions.