PHYSICAL VOLCANOLOGY OF DISSECTED PARASITIC CONES OF THE MIOCENE AKAROA VOLCANIC COMPLEX, BANKS PENINSULA, NEW ZEALAND

Few flank vents have previously been identified on Akaroa Volcano, an extinct, Miocene, strato-shield volcanic complex, and this work extends the record of parasitic vents located around its flanks. The dissection of the cones due to coastal erosion allows for detailed temporal and spatial analysis of flank activity.

In recent mapping five parasitic cones have been identified in Pa Bay, Eastern Banks Peninsula. Eastern Pa Bay hosts two parasitic eruptive centers (NE and SE) while western Pa Bay, the focus of this study, has a complex sequence that is differentiated from the eastern deposits by textural and compositional differences, and identification of different sources (dyke feeders). Deposits in western Pa Bay and Pa Island display a range of small-scale explosive eruption styles, including Hawaiian, Strombolian and phreatomagmatic, sourced from both point sources and eruptive fissures. Deposits are classed into four lithofacies: 1) Variably Welded and Flattened Scoria; 2) Non-Welded and Non-Flattened Scoria; 3) Phreatomagmatic; 4) Intrusives. In NW Pa Bay, a thin sequence of SE dipping phreatomagmatic beds indicates an older, emergent, eroded cone. This sequence is unconformably overlain by variably welded and flattened scoria deposits, dips within this unit radiate away from a source west of Pa Island. A westward trending dike exposed on Pa Island is a possible source for this cone. Differentiated from this point source, are the somewhat linearly NW dipping deposits of the SW Pa Bay shore platform. The non-radiating bedding of this platform suggests fissure-fed eruptions from a linear vent in the middle of the bay. Similar textures and a conformable contact between the NW and SW cones implies paired eruptions from a similar magmatic source.