Volcanic history of the Mount Misery rhyolite domes, Tauranga Volcanic Centre

Abstract

The Mount Misery rhyolites are a group of three complex lava domes, which are part of the Minden Rhyolites within the Tauranga Volcanic Centre. The lava domes are a previously unstudied volcanic area and have in recent years been partially exposed by a series of quarries, creating an opportunity to understand the volcanic history. A pyroclastic succession flanking the side of the Pukunui dome was studied to compare it to the Otawera ignimbrite and potentially correlate it to the Pukunui succession. This study aims to determine the volcanic emplacement and post-emplacement processes of the Mount Misery series domes and pyroclastics, achieved through field work, thin section microscopy, X-ray diffraction, scanning electron microscopy, and X-ray fluorescence.

Key findings from this study show similar phenocryst assemblages but the groundmass textures are complex. The rhyolite groundmasses analysed varied between glassy, crystalline and devitrified, while key phenocrysts found were plagioclase and quartz, and within Maungatūtū/Mount Misery and Greenpark domes also minor clino- and orthopyroxenes. Quartz polymorphs such as cristobalite were common in the groundmass as needle-like structures, while at Pukunui, pure quartz composed the thin bands. The thin bands found at Pukunui and Greenpark formed as a strain feature that caused segregation of quartz along preferential axial planes. Sanidine was found at all three locations as devitrification of glass within the groundmasses. Maungatūtū/Mount Misery had sanidine present as radiating spherulites, while Greenpark had a new find of sub-circular crystalline inclusions which appeared to be some variation of a spherulite.

The domes have similar geochemical and mineralogical compositions, proving they are linked to the same source magma, but would have erupted at different points in time, likely with Maungatūtū/Mount Misery as the oldest and Pukunui as the youngest. The flow banding and flow folding present at Pukunui and Greenpark, and the lack of shearing features, have been used to identify the domes as endogenous. The Pukunui pyroclastic succession included multiple flow units that were correlated to the Otawera ignimbrite elsewhere, which was likely emplaced as a proximal ignimbrite from a dome collapse event at Pukunui. This was one of the final volcanic events in the Mount Misery series area.