The Tauranga Volcanic Centre (TgaVC), located in the western Bay of Plenty, Aotearoa New Zealand was active between 2.9–1.9 Ma. Silicic volcanism of this period occurred spatially and temporally between the defined locations of the Taupō Volcanic Zone (TVZ) and its predecessor, the Coromandel Volcanic Zone (CVZ), but despite their prominence, the volcanic origin, eruption dynamics and petrogenetic relationships of the volcanic deposits of this period have not previously been studied in detail. This study combines a comprehensive field study with whole-rock, glass and mineral geochemistry and isotopic (Sr, Nd and O) analysis to interpret the origin, evolution and petrogenetic relations of the volcanic deposits of the TgaVC, with particular emphasis on the deposits of the Waiteariki Ignimbrite.
This thesis provides an enhanced chronology for explosive volcanism of the TgaVC with four locally widespread units named and defined as follows: Welcome Bay and Wharo ignimbrites (together formerly known as Lower Pāpāmoa Ignimbrite); Otawera Ignimbrite; and Arateka Ignimbrite (formerly known as Upper Pāpāmoa Ignimbrite). These eruptives were followed by the eruption of the voluminous (870 ± 87 km³ DRE) Waiteariki Ignimbrite at 2.1 Ma which has been correlated with the distal Hikuroa Pumice Member in northern Hawke’s Bay (150-170 km southeast of TgaVC). The integrated proximal and distal records show that at least eight eruptions occurred in the TgaVC between 2.4 and 1.9 Ma and provide a maximum repose period between explosive eruptions of ca. 50 kyrs.
The magnitude 8 (M8) Waiteariki super-eruption consisted of the eruption of crystal-rich rhyodacite magma from the newly defined Omanawa Caldera, identified here on the basis of ignimbrite thickness variations, textural features and the presence of numerous silicic lava domes in association with a prominent negative gravity anomaly situated at the southern end of an asymmetrical rifted graben underlying the northern Mamaku Plateau. The Waiteariki Ignimbrite is a monotonous intermediate deposit, characterised by crystal-rich (37 %), rhyodacite (ave. 70.7 wt. % SiO₂) magma with a consistent mineralogical assemblage of plagioclase, hornblende, pyroxene, quartz ± biotite; restricted ranges in whole-rock geochemistry and matrix glass compositions; and the complete absence of compositional gradients and a precursory Plinian phase. Phenocrysts, however, record a complex evolutionary history, providing evidence for a highly heterogeneous mush-source zone.
Individual lavas of the Minden Rhyolite Subgroup of the TgaVC can be grouped into either low- and high-⁸⁷Sr/⁸⁶Sr magmas exclusively erupted in the west and east, respectively. Juvenile products within the Waiteariki and Welcome Bay ignimbrites are mineralogically, geochemically, and isotopically consistent with the high-⁸⁷Sr/⁸⁶Sr Minden Rhyolites, which connects these domes to the magmatic system underlying the Omanawa Caldera. Nd and Sr isotopic values across the TgaVC are comparable to those of the broader TVZ. Calculated δ¹⁸Oₘₑₗₜ values for the TgaVC magmas indicate a temporal trend of decreasing δ¹⁸Ovalues in the TVZ through time. Upper crustal differentiation was dominated by fractional crystallisation (i.e western domes) or assimilation-fractional crystallisation followed by mixing of magmas (Omanawa magma body and eastern domes) while segregated melts (e.g. Bowentown, Mt Maunganui, Maungatūtū/Mt Misery) have unique late-stage differentiation histories.
As volcanism associated with the Omanawa Caldera and the broader TgaVC occurred within the defined structural boundaries of the Taupō Rift, volcanism of the TgaVC is regarded here to represent the first silicic system of the TVZ. Geothermobarometry indicate crystallisation and storage occurred within the mid- to shallow crust (c. 3.5 to 6 km depth) between 50-160 MPa and 740-900 °C indicating that magmas have broadly resided under near-identical, shallow conditions over the lifespan of the TVZ (~2.5 Ma). Dominated by high-silica, crystal-poor rhyolitic magmas, the continued eruption of homogenous, crystal-rich magmas throughout the TgaVC is unique within the context of the TVZ and it is inferred that active crustal rifting enabled continual tapping of the crystal-mush reservoir which culminated in a deep-seated failure of the magma reservoir, followed by rapid decompression and large-scale evacuation of the Omanawa magmatic system.
