Spatiotemporal Evolution of the Dunedin Volcano, New Zealand: Implications for Development of Long-Lived Intraplate Volcanoes

Intraplate volcanism is widespread globally but remains enigmatic. Magmatism can manifest as dispersed, small-volume monogenetic volcanic fields, as complex, large-volume polygenetic volcanoes, or in combination. Departures from sensu stricto monogenetic or polygenetic volcanism result in complexities such as polymagmatic monogenetic volcanoes or nested/clustered monogenetic volcanism developing a central edifice.

Reconstructing the evolution of volcanic systems from their inception is often hampered by burial of early deposits by subsequent eruptions, especially challenging for long-lived complex systems. The composite Dunedin Volcano (DV) (South Island, New Zealand), understood to have been constructed from 16-11 Ma, is the largest, polygenetic centre within the dominantly monogenetic Dunedin Volcanic Group, where dispersed eruptions occurred for 9 Myr prior to the onset of focused activity at DV. The current edifice is erosionally dissected, locally exposing the earliest eruptive deposits, allowing insight to the initial stages of volcanic activity.

Here we present a synthesis of new field mapping, geochronological (40Ar/39Ar and U-Pb zircon dating), geochemical and paleomagnetic data for DV, focused on a transect extending from the oldest eruptive site currently identified (Otapahi; 16.0 Ma) to the 'centre' of the volcano (Port Chalmers), proximal to the youngest remaining volcanic deposits.

Our dates from the transect have not replicated the 16.0 Ma age and suggest a younger onset of eruptions; no newly dated rocks are older than 14.4 Ma. In contrast, recalculated published dates shift to maintain an older onset (16.3 Ma). Further dating is underway to resolve this large, 2 Myr, inconsistency. We have produced a new map, showing a wide compositional variety of complex, primitive to evolved lavas, pyroclastic deposits and subvolcanic intrusions erupted and emplaced, some simultaneously, from migrating centres rather than a single central conduit. Petrographic evidence reveals deeper mantle-sourced rapidly ascending magma tapped pre-existing evolved crustal magma storage/mush prior to eruption. Further investigation will reveal how long-lived intraplate volcanoes grow, focusing on the birth of a complex central volcanic system and its relation to a wider enclosing volcanic field.