The Ruapehu magmatic system : insights from the petrology and geochemistry of lava flows : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand

Abstract

Volcanoes in complex tectonic settings are natural laboratories for exploring how tectonic processes influence plumbing systems, magmatic conditions, and eruptive phases. Recent studies in tephra at the Tongariro Volcanic Centre (TgVC), New Zealand, suggest a dike-dominated plumbing system that facilitates rapid ascent of hot, anhydrous intermediate magmas. Although recent eruptions at Mount Ruapehu have been predominantly phreatic or phreatomagmatic, the processes and timescales responsible for the voluminous andesite-dacite lava flows that built its ~150 m³ edifice over the past ~250 ka remain poorly constrained, yet critical for reconstructing the evolution of its plumbing system. This study integrates geochemical and textural analyses, with thermodynamic modelling from samples of the Mangawhero and Whakapapa formations (<10 to 50 ka) to reconstruct the pre-eruptive magmatic conditions and transport dynamics for effusive eruptions. This provides a better understanding of differences and similarities in the magmatic conditions, timescales and rates of the plumbing system that generated effusive and explosive eruptions. Thermobarometry, hygrometry and MELTS modelling were used to constrain the P T H₂O magmatic conditions and depths of crystallization in orthopyroxene microlites. Residence times were estimated using crystal size distribution (CSD) analyses based on 3D data, minimizing errors associated with the stereological correction of 2D datasets, in combination with a well-known growth rate. The residence times and depth of crystallization were used to determine the magma ascent rates. Magmatic processes and melt-crystal equilibrium were determined through crystal textural analysis and mapping in combination with whole rock and groundmass geochemical analyses. The findings in this study support the hypothesis of a dike-dominated plumbing system, which facilitates the rapid transport of magma (up to 0.08 m/s), limiting crystal residence times (up to 66 days) in magma accumulation zones at variable depths within the crust (4 - 22 km). The active rifting assists the melt production through adiabatic melting, generating hot (970 - 1160 °C) and relatively undersaturated (<2.5 wt%), that maintain the trace elements patterns linked to flux-melting in arc settings. This dike-dominated plumbing system promotes crystal entrainment by ascending melts due to the reactivation of previously emplaced dikes and crystalline bodies, where crystal recycling and accumulation determine the whole-rock trend compositions. The remarkably fast ascent rates for effusive eruptions in volcanic arc settings found at Mount Ruapehu in this study are consistent with those found in rift volcanic systems around the world, confirming the influence of the rifting processes in the magmatic activity. These results exemplify the complex tectonic interaction between an arc volcano and a rift system and indicates that variations in the shallow conduit geometry modulate gas accumulation and overpressure, ultimately influencing the eruptive style.