Geochemical fingerprinting of basaltic tephra deposits in the Auckland Volcanic Field

Abstract

Small volume (<2 km³) basaltic volcanoes have been active throughout the late Quaternary in the Auckland Volcanic Field (AVF) in northern New Zealand. The main pyroclastic products of these centres are phreatomagmatic surges containing abundant accidental ejecta and magmatic fall deposits of limited aerial extent. Previously they had received no attention from a tephrostratigraphic perspective. Despite extensive weathering, many of these deposits contain basaltic glass that can be used to geochemically fingerprint the volcanic source and individual eruptive events. The glasses are predominantly basanites with SiO₂ contents in the range 42–50 wt%. Many individual emplacement units are compositionally homogeneous on the basis of electron microprobe analysis (SiO₂ ± 0.5 wt%), and can be distinguished on the basis of TiO₂, CaO, K₂O, and P₂O₅ contents. Individual tephra beds can easily be characterised, however volcanoes are more compositionally diverse. Although the volcanoes are considered monogenetic, they display a wide range of different eruptive styles, and their pyroclastic deposits can display a compositional range (SiO₂ 45–50 wt%) within short stratigraphic sequences that show no evidence of hiatus. This increases the difficulty in matching distal tephra deposits to their source. Glass compositional data from 18 volcanoes show no evidence of spatial or temporal trends within the AVF.

Keywords:

• tephra
• tephrochronology
• Auckland Volcanic Field
• glass chemistry