Complex arrangement of early pyroclastic and intrusive rock records onset of intraplate Dunedin Volcano

Little is understood about the birth of composite volcanoes because products of early eruptive activity are buried by products of later eruptions. The oldest rocks of the Dunedin Volcano (active 16.0–11.0 Ma) are exposed at Otapahi (Allans Beach) and allow us to study the inception of a long-lived composite volcano. Seven rock units are visible at Otapahi, investigated with fieldwork and detailed componentry. A bedded deposit of phonolitic pumice clasts is vertically cross-cut by at least three diatreme deposits, as well as a compositionally diverse array of dikes emplaced penecontemporanously with the diatremes. The complicated cross cutting relationships among diatremes and different generations of dikes allow inference of the relative timing of events. Componentry revealed lithic clasts from the underlying Cenozoic sedimentary sequence and basement schist in the oldest diatreme, thus indicating it quarried to basement depths. A lack of coherent cognate igneous clasts in the oldest diatreme is also consistent with the earliest eruption in the area having formed the ~60 m thick bedded pumice deposit. Younger diatremes lack any identified lithic fragments from underlying strata but contain abundant basaltic clasts with compositions matching those of dikes injected penecontemporaneously. The increase of igneous clasts in the younger diatreme deposits, together with the decreasing number of cross-cutting dikes records a chaotic succession of diatreme formation and repeated dike intrusion. This interplay of exogeneous with endogenous processes built an early submarine eruptive centre, and initiated development of a long-lived intraplate composite volcano with activity that persisted across the next 5 million years.