CYCLICITY OF EXPLOSIVE BASALTIC ERUPTIONS AT AN INTRAPLATE VOLCANO, AKAROA VOLCANIC COMPLEX, NEW ZEALAND

Where intraplate volcanism is dominated by effusive mafic eruptions, occasional explosive events can be difficult to forecast. The incised and eroded intraplate, Miocene Akaroa Volcanic Complex, New Zealand, reveals stacks of lava flows and less conspicuous intervening ash horizons. Previous research has focused on the vertical sequences of lava flows, identifying eruption cycles that record evolving magma batches from picrite to benmorite. This research investigates these cycles by characterising the ash horizons to reveal the nature, timing, and origin of explosive volcanism. Twenty ash horizons were described, sampled and geochemically analysed. The ashes are variably weathered and dominated by fine-to-medium grained glass with minor 1-2mm diameter lithics. From x-ray fluorescence (XRF) bulk-composition analysis, most samples lie in the lower picrite range on an alkalis vs. silica plot, with 3 hawaiite samples. Of significance is 10 samples having LOI values greater than 10%, silica oxide values less than 46.9%, and sodium+potassium oxide values less than 2.7%, suggesting significant post-depositional weathering compared to relatively unaltered lavas. To determine original magmatic compositions, we quantified ash cation mobility using LOI as a proxy for weathering and an extrapolation technique involving chemical index of alteration (CIA) values. The maximum Na+K value extrapolated was 5.7%, corresponding to Si values within 46.0–50.7%. There are uncertainties in quantifying weathering that affect the accuracy of our reported values; however, to plot within the linear magmatic trend for Akaroa volcanics, the CIA-corrected samples must lie at the more primitive picrite to hawaiite end. This suggests that explosive events at Akaroa systematically occurred at the beginning of eruption cycles. The composition and cyclicity of these ash horizons are consistent with a magmatic control on the volcanic processes, perhaps one involving volatilization accompanying magma recharge events, leading to over-pressurization and eruption. Further research would constrain our understanding of the mechanisms responsible for occasional explosive events at predominantly effusive, mafic volcanoes.