Magmatic architecture below the Okataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa New Zealand, inferred from basalt geochemistry

The Okataina Volcanic Centre (OVC) is the most recently active rhyolitic volcanic centre in the Taupō Volcanic Zone, Aotearoa New Zealand. Although best known for its high rates of explosive rhyolitic volcanism, there are numerous examples of basaltic to basaltic-andesite contributions to OVC eruptions, ranging from minor involvement of basalt in rhyolitic eruptions to the exclusively basaltic 1886 C.E. Plinian eruption of Tarawera. To explore the basaltic component supplying this dominantly rhyolitic area, we analyse the textures and compositions (minerals and melt inclusions) of four basaltic eruptions within the OVC that have similar whole rock chemistry, namely: Terrace Rd, Rotomakariri, Rotokawau, and Tarawera. Data from these basaltic deposits provide constraints on the conditions of magma evolution and ascent in the crust prior to eruption, revealing that at least five different magma types (two basalts, two dacites, one rhyolite) are sampled during basaltic eruptions. The most abundant basaltic magma type is generated by cooling-induced crystallisation of a common, oxidised, basaltic melt at various depths throughout the crust. The volatile content of this melt was increased by protracted fluid-undersaturated crystallisation. All eruptions display abundant evidence for syn-eruptive mixing of the different magma types. Rotomakariri, consisting of a mafic crystal cargo mixed into a dacitic magma is the most extreme example of this process. Despite similar bulk compositions, comparable to other basaltic deposits in the region, these four OVC eruptions are texturally distinct as a consequence of their wide variation in eruption style.

Chapter 2.2b Palmer Land and Graham Land volcanic groups (Antarctic Peninsula): petrology

Large-volume rhyolitic volcanism along the proto-Pacific margin of Gondwana consists of three major episodes of magmatism or ‘flare-ups’. The initial episode (V1) overlaps with the Karoo–Ferrar large igneous provinces at c. 183 Ma. A second (V2) episode was erupted in the interval 171–167 Ma, and a third episode (V3) was emplaced in the interval 157–153 Ma. The magmatic events of the V1 and V2 episodes of the Antarctic Peninsula are reviewed here describing major and trace elements, and isotopic (Sr, Nd, O) data from rhyolitic volcanic rocks and more minor basaltic magmatism. An isotopically uniform intermediate magma developed as a result of anatexis of hydrous mafic lower crust, which can be linked to earlier, arc-related underplating. The subsequent lower-crust partial melts mixed with fractionated mafic underplate, followed by mid-crust storage and homogenization. Early Jurassic (V1) volcanic rocks of the southern Antarctic Peninsula are derived from the isotopically uniform magma, but they have mixed with melts of upper-crustal paragneiss in high-level magma chambers. The V2 rhyolites from the northern Antarctic Peninsula are the result of assimilation and fractional crystallization of the isotopically uniform magma. This process took place in upper-crust magma reservoirs involving crustal assimilants with an isotopic composition akin to that of the magma. A continental margin-arc setting was critical in allowing the development of an hydrous, fusible lower crust. Lower-crustal anatexis was in response to mafic underplating associated with the mantle plume thought to be responsible for the contemporaneous Karoo magmatic province and rifting associated with the initial break-up of Gondwana.

Genesis of Recent Mafic Magmatism in the Taupo Volcanic Zone, New Zealand: Insights into the Birth and Death of Very Large Volume Rhyolitic Systems?

Mafic magmatism of the rifting Taupo Volcanic Zone (TVZ) of the North Island, New Zealand, is volumetrically minor, but is thought to tap the material that provides the heat source for voluminous rhyolite production through partial melting of the crust, which ultimately results in very large volume explosive eruptions. We have studied the major and trace element chemistry of 14 mafic samples from across the entire TVZ, and the U isotopic composition of whole-rocks, groundmasses and separates of mafic mineral phases from a selection of nine samples (with the remaining five too sparsely phyric for mineral separation). Some minerals yield significant 234U enrichments despite groundmass and whole-rock close to 238U–234U secular equilibrium, pointing to uptake of variably hydrothermally altered antecrystic minerals prior to the eruption of originally sparsely phyric to aphyric mafic magmas. However, incompatible trace element patterns indicate that there are three chemically distinct groups of samples, and that samples may be used to derive primary melt compositions. We employ the latest version of the Arc Basalt Simulator (ABS5) to forward model these compositions, deriving mantle source parameters including mantle fertility, slab liquid flux, mantle volatile content, degree of melting, and P–T conditions of melt segregation. We show that mafic rocks erupted in areas of old, now inactive calderas constitute low-degree, deep melts, whereas those in areas of active caldera-volcanism are high-degree partial melts segregated from a less depleted source at an intermediate depth. Finally, high-Mg basaltic andesites erupted in the SW and NE of the TVZ point to a fertile, shallow mantle source. Our data are consistent with a petrogenetic model in which mantle melting is dominated by decompression, rather than fluid fluxing, and progresses from shallow to deeper levels with time. Melt volumes initially increase to a tipping point, at which large-scale crustal melting and caldera volcanism become prominent, and then decrease owing to progressive depletion of the mantle wedge by melting, resulting in the dearth of heat provided and eventual cessation of very large volume rhyolitic volcanism. ABS5 modelling therefore supports the notion of a direct link between the chemistry of recently erupted mafic magmas and the long-term activity and evolution of rhyolitic volcanism in the TVZ.

Attenuation in the mantle wedge beneath super-volcanoes of the Taupo Volcanic Zone, New Zealand

SUMMARY The Taupo Volcanic Zone has a 120-km-long section of rhyolitic volcanism, within which is a 60-km-long area of supervolcanoes. The underlying subducted slab has along-strike heterogeneity due to the Hikurangi Plateau's prior subduction history. We studied 3-D Qs (1/attenuation) using t* spectral decay from local earthquakes to 370-km depth. Selection emphasized those events with data quality to sample the low Qs mantle wedge, and Qs inversion used varied linking of nodes to obtain resolution in regions of sparse stations, and 3-D initial model. The imaged mantle wedge has a 250-km-long 150-km-wide zone of low Qs (<300) at 65–85 km depth which includes two areas of very low Qs (<120). The most pronounced low Qs feature underlies the Mangakino and Whakamaru super-eruptive calderas, with inferred melt ascending under the central rift structure. The slab is characterized by high Qs (1200–2000), with a relatively small area of reduction in Qs (<800) underlying Taupo at 65-km depth, and adjacent to the mantle wedge low Qs. This suggests abundant dehydration fluids coming off the slab at specific locations and migrating near-vertically upward to the volcanic zone. The seismicity in the subducted slab has a patch of dense seismicity underlying the rhyolitic volcanic zone, consistent with locally abundant fractures and fluid flux. The relationship between the along-arc and downdip slab heterogeneity and dehydration implies that patterns of volcanism may be strongly influenced by large initial outer rise hydration which occurred while the edge of the Hikurangi Plateau hindered subduction. A second very low Qs feature is 50-km west above the 140-km-depth slab. The distinction suggests involvement of a second dehydration peak at that depth, consistent with some numerical models.