Abstract
Intraplate volcanoes are enigmatic; their occurrence within plate interiors, removed from the influence of plate boundary processes, warrants investigation into the mechanisms and controls on their development and evolution. Further complexity arises as the spectrum of intraplate volcanoes can range from monogenetic to polygenetic, including composite volcanoes. Reconstructing the onset of volcanism and the growth of these composite volcanic edifice(s) poses challenges, as earlier deposits are often obscured by products from subsequent eruptions, originating from multiple shifting vents. The Dunedin Volcano (South Island, New Zealand) is a Miocene intraplate composite volcano, characterised by a vast compositional array of lavas, pyroclastic deposits, and intrusions, emplaced through repeated submarine to subaerial eruptions, from numerous vents. It represents the largest, polygenetic, centre within the wider, dominantly monogenetic, Waipiata Volcanic Field (collectively the Dunedin Volcanic Group). The erosional state of the Dunedin Volcano, deeply incised by the Otago Harbour, provides a window into the inner architecture of the volcano. The current and most exhaustive construction model for the Dunedin Volcano, however, has not been rigorously updated since the 1960s. This thesis aims to elucidate the life of the Dunedin Volcano and to gain insight into long-lived intraplate composite volcanoes. Various methods are employed here to achieve this: high-resolution field mapping and remote sensing, sampling and petrography, radioisotopic dating (40Ar/39Ar and U-Pb), geochemistry (whole-rock and mineral), with paleomagnetic (thermal demagnetisation) and rock magnetic (alternating field demagnetisation, hysteresis, isothermal remnant magnetisation, and temperature dependent susceptibility) analyses.
The volcanism that produced the Dunedin Volcanic Group progressed from the formation of dispersed monogenetic volcanoes within the Waipiata Volcanic Field over ~10 Myr, to the onset of spatially focused eruptive activity that produced the composite polygenetic Dunedin Volcano from 14.9 ± 0.2 Ma onwards. The Dunedin Volcano grew through three eruptive phases defined by pulses of activity (Early, Mid, and Late volcanism), governed by fractional crystallisation, a plethora of magma recharge and mixing events, and the formation of a crystal mush within the shallow plumbing system that modulated successive volcanism. Early volcanism, from 14.9 to 14.1 Ma, is characterised by the progressive geochemical evolution of predominantly silica-saturated eruptive products. Mid volcanism, from 13.8 to 12.4 Ma, produced the most evolved deposits, encompassing the transition from silica-saturated to silica-undersaturated eruptive products. Late volcanism, from 12.1 to 11.3 Ma, is marked by the progressive devolution of predominantly silica-undersaturated eruptive products, which became increasingly primitive. Unique tuff breccias central to the composite volcano, emplaced through explosive diatreme-forming eruptions, in some cases record subsequent hydrothermal activity during the closing stages of volcanism. Volcanic activity at the Dunedin Volcano lasted for ~3.6 Myr, culminating by 11.3 ± 0.4 Ma, with dispersed eruptions persisting for another ~2.4 Myr.
The composite Dunedin Volcano represents a long-lived, low-volume endmember within the spectrum of volcano types, the sudden inception of which marked a significant change within the volcanic system, caused by both internal (i.e., increased magma supply) and external (i.e., tectonic stresses) factors. This new understanding into the occurrence of a complex polygenetic volcano within an outlying monogenetic volcanic field may be applicable to intraplate volcanoes worldwide.