Correlation of basaltic tephra from Mt Wellington volcano: Implications for the penultimate eruption from the Auckland Volcanic Field

Mt Wellington volcano in the Auckland Volcanic Field is a basaltic complex comprising several vents, tuff rings, cones and lava flows. Interstitial glass in shard and lapilli groundmass displays considerable variability (∼41–48 wt% SiO2, anhydrous), but within-sample variation is relatively narrow (mostly <2–3 wt% SiO2) allowing two broad compositional affinities to be recognised that relate to the eruptive sequence. The tuff ring contains lower SiO2 glasses, whereas the main cinder cone comprises higher SiO2 glasses. Fe–Ti oxides also display a moderate compositional range (Ulvospinel, Usp = ∼40–55), although within-sample variation is generally narrower (∼5 Usp) allowing tuff ring deposits (Usp < 47) to be distinguished from those of the cinder cone (Usp > ∼50). Compositional outliers are evident amongst the Fe–Ti oxide crystals in many samples, and probably result from slow post-depositional cooling and/or variable oxidation conditions during syn-eruption recycling at the vent. Although abundant microlites restrict the deposits that can be geochemically fingerprinted, randomly selected analyses of glass shards show only limited variation within samples (±1 wt% SiO2), suggesting that variable microlite formation had little effect on the residue melt. The larger compositional differences such as that between the tuff ring and cinder cone also do not appear to be related to microlite formation because both deposits display a similar range in microlite content. Correlations between the edifice and distal deposits in maar sediments provide insights to the eruption episode. They demonstrate that the main cinder cone was a late-stage event, there was no significant hiatus (months to decades) within the entire sequence, and that the cinder cone was responsible for the wide dispersal of ejecta in the region. By placing the Mt Wellington event within a stratigraphic framework of well-dated tephra from other volcanoes, it is possible to evaluate radiocarbon chronologies and constrain the eruption age to ∼10,000 BP.