The Eruptive History of Rangitoto Volcano: Geochemical and Lithological Variation in a 148 m Core

Abstract

Rangitoto Volcano is currently understood to be a relatively young (~500 year old) basaltic shield volcano in the Auckland Volcanic Field (AVF). Although eruptions from terrestrial basaltic fields are typically monogenetic, Rangitoto Volcano has evidence for two distinct compositions – an early alkalic basalt and subsequent subalkalic basalt – that resulted from separate mantle melting events at different sources and are associated with tephra deposits that are separated by a ~50 year time gap. The longevity of Rangitoto has recently been questioned, with tephra layers from nearby Lake Pupuke suggesting multiple phases of activity over a long time span. From a hazards perspective, it is important to understand the uncharacteristic behaviour of Rangitoto as it may have implications for the nature of future volcanic eruptions within the field. This study analysed the volcanic sequence from a 148 m drill core to characterise and petrogenetically interpret a continuous sequence of compositional and lithological variation through Rangitoto Volcano. Stratigraphic data revealed approximately 10 m of submarine pyroclastic ash and lapilli underlying 128 m of lava flows. The core did not encounter alkalic basalt material, however, of the subalkalic basalt lava flows in the core, four compositional groupings are described. Sr-Nd-Pb isotopic analyses revealed there was no significant difference in source composition, however partial melt modelling showed there were at least two different degrees of melting within the core data – a 3 – 5% partial melt and a 5 – 7% partial melt, with magma mixing between the two, followed by the eruption of an olivine-clinopyroxene fractionated group. Radiocarbon ages from within the pyroclastic sequence provide ages of up to 6864 Cal. years BP., introducing the possibility of far greater longevity at Rangitoto than previously believed. The discovery of compositional variation created by a range of petrogenesis processes within the subalkalic basalt suite at Rangitoto infers there may be a dynamic melting environment within the AVF, creating pulses of magma over long periods of time. The findings of this study have far-reaching hazard implications for the Auckland region and add to our understanding of polymagmatic activity in monogenetic volcanic fields.