The Waiteariki Ignimbrite: Eruption of a Large-Volume, Monotonous Intermediate Ignimbrite at the Dawn of the Taupō Volcanic Zone, New Zealand

Abstract Monotonous intermediate eruptions typically eject large volumes of pyroclastic material generally thought to represent material from the underlying magma mush systems. Their geological occurrence represents an upper end-member in terms of eruption size and styles, providing key information to understand the construction of large magma reservoirs. Here, we constrain dispersal and geochemical aspects of the ~2.1 Ma Waiteariki supereruption that erupted ~870 ± 270 km3 dense rock equivalent (DRE) crystal-rich (~37%), rhyodacite (~71 wt. % SiO2) magma from the newly defined Omanawa Caldera of the Tauranga Volcanic Centre (TgaVC), Aotearoa New Zealand. The Omanawa caldera is identified here using ignimbrite thickness variations, textural features and the presence of numerous silicic lava domes. Our geologically constrained vent location correlates with a prominent gravity anomaly situated at the southern end of an asymmetrical rifted graben underlying the northern Mamaku Plateau. The monotonous Waiteariki Ignimbrite is characterized by: (1) a mineral assemblage comprising plagioclase, hornblende, orthopyroxene and quartz, (2) restricted ranges in whole-rock geochemistry, and (3) absence of both compositional gradients through stratigraphy and a precursory Plinian fallout phase. In contrast, changes in fiamme mineralogy, large ranges in mineral chemistry and subtle variations in glass compositions provide evidence for open system magma processes and a highly heterogeneous and vertically extensive mush-source zone. 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 Taupō Volcanic Zone, one of the most productive silicic systems on Earth. This places the Waiteariki Ignimbrite within an important transitional period between the older Coromandel Volcanic Zone and the currently active Taupō Volcanic Zone.