STRATIGRAPHIC, TEXTURAL, AND GEOCHEMICAL ANALYSIS OF A PYROCLASTIC DENSITY CURRENT DEPOSIT IN THE AKAROA VOLCANIC COMPLEX, NEW ZEALAND

Pyroclastic density current (PDC) deposits are classically associated with intermediate to silicic explosive volcanism, though associations with basaltic end members are increasingly common. Exposed on the outer flanks of the Akaroa Volcanic Complex (AVC) of Banks Peninsula, New Zealand, is a 30 metre thick, ashy, lithified deposit similar to PDC deposits. The late-Miocene AVC is primarily composed of basaltic to trachytic lava flows, intrusives, and localized scoria deposits. Here we present the first identification of a geochemically and stratigraphically distinct ignimbrite in the AVC.

This study investigates the stratigraphic, textural, geomechanical, and geochemical properties of the lithified deposit. The exposure is massively bedded and trachytic in composition, and contains at least four ignimbrite packages distinguished by variation in lithology, chemistry, and welding. All packages are composed of a crystal-rich groundmass, K-feldspar phenocrysts, and high-temperature alteration minerals. Three of the ignimbrite packages display Si-enrichment trends indicating upwards evolution. Degree of welding was interpreted using geomechanical (density, permeability, porosity) and textural (groundmass orientation, pumice clast collapse) properties. At least two packages were extensively welded by very high temperatures during emplacement, indicating a deeper or more volatile magma source.

The four ignimbrite packages represent different pulses of PDC eruption. They are interbedded with two airfall ash deposits that serve as eruptive time breaks and indicate at least two distinct eruption events. The deposit may represent co-eval glass-rich ashfall and crystal-rich ignimbrites with eruptive mechanisms involving either the segregation of a dilute turbulent flow into a PDC and vitric-enhanced turbulent cloud or the escape of glassy material during eruption column collapse.

This is the first recognition of a PDC deposit in the AVC, changing our understanding of the eruption dynamics, volcanic products, and magma sources associated with the AVC during its eruptive lifespan. The AVC could produce PDCs in addition to its typical basaltic and trachytic lava flows, which has implications for eruptive styles and hazard management in similar, modern lava-flow-dominated volcanic complexes.