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Volcanology and secondary alteration of the 1.6 Ma Ngaroma Ignimbrite, Upper Waipari Valley

The 1.6 Ma Ngaroma eruption from the Mangakino Volcanic Centre (MVC) is the first known rhyolitic event of the Taupo Volcanic Zone (TVZ), and variations of primary components recorded throughout the Ngaroma Ignimbrite provide insight(s) into pre-eruptive processes and post-emplacement mechanisms. A field study of the Ngaroma Ignimbrite was undertaken proximal to the source caldera (~25 km) around the township of Ngaroma. Two primary facies (a lithic- rich and lithic-poor facies) and four subfacies have been identified and are presented in four stratigraphic logs, which vary both vertically and laterally throughout the field area. The facies reflect syn- and post-deposition process that are identifiable from one another based on macroscopic observations including changes in primary componentry, the degree of welding and nature of secondary alteration. Variations in depositional structures and textures highlight the intense welding, limited incorporation of vent-derived lithics, and the absence of a pumice/ash fall deposit underlying the ignimbrite, which suggest that the parent pyroclastic flow formed from the immediate collapse of a highly explosive, caldera-forming eruption column. Petrographic observations and mineralogic analyses involving optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) record high concentrations of sanidine and cristobalite, followed by clay minerals (kaolinite), which suggest the deposit endured significant post-emplacement processes such as vapour-phase alteration and devitrification. The ignimbrite matrix varies from a eutaxitic texture comprising (a) visible, well-formed, but devitrified, glass shards typically 300 to 800 μm, to (b) a fine-grained, homogenous, optically unresolvable medium; the latter can be resolved under SEM and comprises intergrown <3 μm- sized K-feldspar (sanidine), amorphous/nano-crystalline silica (cristobalite ± tridymite), and pore cavities. Geochemical analyses of major and trace elements on bulk ignimbrite and pumice were determined using x-ray fluorescence (XRF) spectrometry and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and on individual minerals by electron microprobe analysis (EPMA). Samples used in these geochemical analyses have been pervasively altered, and that due to the lack or absence of primary material (fresh glass and pumice), some of the patterns observed here are presumed to in part reflect post alteration processes (vapour-phase alteration and devitrification) and therefore, are of limited use for understanding primary magmatic processes. The major and selective trace elements of bulk rock ignimbrite and pumice confirm a rhyolitic composition and are consistent with the effect of syn- and post-emplacement alteration processes. Geochemical signatures of rare earth elements (REE) observed on multielement diagrams are associated with subduction related magmas and indicative of crustal contamination. Major element compositions of both feldspar and pyroxene phenocrysts range from An₁₅₋ An₂₈ and Or₅₋Or₁₂ and from En₃₂ to En₄₁, respectively, and are typical composition of rhyolitic magmas.
Type of thesis
Ross, C. (2020). Volcanology and secondary alteration of the 1.6 Ma Ngaroma Ignimbrite, Upper Waipari Valley (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13764
The University of Waikato
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