Timescales and Evolution of the Ruapehu Magmatic System, Taupo Volcanic Zone, New Zealand

Abstract

Mt Ruapehu volcano of the Taupo Volcanic Zone, New Zealand, has been active for at least the last 250,000 years. The Ruapehu stratovolcano comprises four Formations, which from oldest to youngest are: Te Herenga Formation (~250-183 ka), Wahianoa Formation (147-119 ka), Mangawhero Formation (53-21 ka) and the Whakapapa Formation (<10 ka). The total compositional range of Ruapehu eruptives is from dacite to basaltic andesite, with the vast volume being andesite. This study combines plagioclase crystal size distribution analysis, textural and in situ geochemical data to determine the processes and timescales involved in generating and modifying Mt Ruapehu eruptives over the last 250,000 years. Crystal size distribution (CSD) analysis shows each Formation has two crystal-size populations, phenocrysts and microphenocrysts, that vary in character and size across the Formations and through time. From the phenocryst populations, magma residence times of centuries to millennia years were calculated. The microphenocryst residence times are interpreted to represent eruption-triggering magma-mixing events and these are calculated as days to weeks in duration. Previous studies show that Ruapehu eruptives become more felsic, potassium rich, isotopically evolved and variable with time. The Ruapehu magmatic system is conceived of as having a complex array of sub-edifice mid to shallow crustal dykes and sills that experience parallel yet sometimes isolated/independent evolutionary paths, becoming more interconnected with time. This study extends our knowledge of the evolving Ruapehu magmatic system. Earlier Formations have plagioclase crystals with core 87Sr/86Sr ratios around 0.7060 and rims ratios around 0.7055. Combined with the major element, trace element and textural data, this indicates that dominantly closed system processes and mafic recharge triggered eruptions. The more recent Formations display significant variation in core to rim 87Sr/86Sr ratios; anorthite compositions and crystal textures show no systematic differences. This is interpreted as a result of multiple mixing events and more open system processes. The chemical and textural data also indicate that each Formation may evolve early in a closed system state relative to the end of the previous Formation. For example, the older Whakapapa Formation sample has in situ 87Sr/86Sr ratios that are fairly consistent around 0.7063, whereas the more recent Whakapapa Formation sample and the Mangawhero Formation show variations of around 0.001. The results of this study highlight the utility of CSDs, textural and in situ chemical analysis in identifying crystal-size populations and using these to fine tune our models and knowledge of the evolution of magmatic systems