Unravelling magma generation, storage, and ascent processes from the crystal cargo and their host lavas : a case study of Taranaki Volcano, New Zealand : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University (Manawatū campus), Palmerston North, New Zealand
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2021
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Massey University
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Abstract
Mt. Taranaki is a back-arc stratovolcano situated in the North Island of New Zealand, c. 400 km west of the Hikurangi trench. This thesis investigates the Holocene lavas of Taranaki volcano to constrain the processes of melt generation and evolution before these magmas were erupted. The crystal cargo carried in these eruptives is dominated by plagioclase, clinopyroxene, and amphibole, which with exception of the crystal rims are in chemical disequilibrium with the carrier melt represented by the groundmass. Compositional overlap of mineral data between the crystals in lavas and from xenoliths, along with numerous glomerocrysts and fractured crystals, indicate an antecrystic or xenocrystic origin of the crystals. Thermohygrometric data derived from chemical equilibrium between crystal rims and groundmass reveal hot and occasionally
H2O-undersaturated felsic melts (55–68 SiO2 wt%). These cannot be related to deep crustal hot zones and are thus interpreted to be sourced from subduction melange diapirs that rise through the mantle wedge. Repeated, heterogeneous intrusion of these diverse melts into the crust, dominated by plutonic rocks of the 120 Ma Median Batholith, results in a subvolcanic crystal mush zone. The melts entrain mafic crystals in various proportions (40–55 vol.%), reducing magmatic silica contents by 5–11 wt%. Mineral phases display a complex and varied history observed through elemental concentration maps that suggest repeated resorption and recrystallisation in varied environments. These crystals are not exclusive to lavas of any stratigraphic unit and can be found within the same sample over very short length scales (within centimetres from each other).
Eruption-triggering injections pick up this complex crystal cargo. Amphiboles break down during ascent and develop distinct reaction rims that vary little in width (± 20%, 1σ, on average) within individual thin sections but show a large variation between samples from different lava flows, from less than 5 to more than 450 µm. The combined evidence indicates that Taranaki lavas are a product of high temperature, aphyric to sparsely phyric subduction melange partial melts, and remobilised colder, mafic mush zone crystals. Together they ascend through the shallow crust on timescales of the order of hours to days.
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Lava, Magmatism, New Zealand, Taranaki, Mount