Abstract
The alkaline intraplate Dunedin Volcano (DV) (South Island, New Zealand), constructed between 16-11 Ma from compositionally diverse lavas, pyroclastic deposits and subvolcanic intrusions, linked to numerous vents, preserves the record of a complex eruptive history. A unique and significant deposit is the Port Chalmers Breccia (PCB); a phonolitic, vent-filling unit exposed in a series of NW-SE trending diatremes. The massive, poorly sorted, matrix supported PCB contains minimal identifiable juvenile material and comprises an abundance of lithic fragments (up to 1.6 m). The lithic clasts, derived from varying depths, include a wide range of volcanic and subvolcanic fragments, plus pre-volcanic sedimentary rocks and basement schist. Questions surround the PCB's emplacement. Originally understood to have occurred early within DV activity, the predominance of lithic phonolite clasts within the PCB given the scarcity of phonolite during early activity, shifted understanding towards a later timing of emplacement. Field evidence dictates the PCB cannot have reached temperatures high enough to weld the deposit, so by determining paleotemperatures for a variety of lithic clasts we can reconstruct the thermal state of the diatreme during emplacement. Here we present results from a paleomagnetic investigation of the PCB. Orientated samples collected from several lithic clast sizes and types (volcanic, plutonic, schist), will be stepwise thermally demagnetised at 25 degrees C increments in a ASC Scientific TD-48 single-chamber oven and magnetic moment measurements made on a 2G Enterprises pass-through long core superconducting magnetometer. Pilot alternating field demagnetisation data of matrix reveal a complex collection of multiple components including a strong horizontal viscous magnetisation. Rock magnetic analyses conducted on a Princeton Instruments Vibrating Samples Magnetometer indicate the primary magnetic mineral is magnetite. Whilst elucidating the PCB occurrence, this work will serve to further overall understanding of diatremes, a volcanic landform for which the emplacement is inherently difficult to study given its subsurface nature.