NEWLY RECOGNIZED FISSURE-FED ERUPTION DEPOSITS ON THE FLANK OF THE ANCIENT BANKS PENINSULA VOLCANIC COMPLEX IN NEW ZEALAND

Banks Peninsula in the South Island of New Zealand is an ancient Miocene volcanic complex comprised mostly of basaltic lavas and less abundant pyroclastic deposits. Previous work on Banks Peninsula has been focused on geochemistry and petrology with little research on eruption styles and mechanisms. Here we present new outcrop-scale stratigraphy and structural measurements that reveal intermixed layers of variably welded scoria deposits, and lava exposed along a NW-SE trending ridge on one flank of the volcanic complex. The deposits can be divided into four different types with characteristics that reflect different eruption styles. Type one deposits consist of scoria clasts ranging from 2 to 20 cm in a fine-grained matrix. Type two deposits contain weakly agglutinated lapilli to bomb sized pyroclasts dominated by clastic textures. Type three deposits contain moderately agglutinated bomb sized pyroclasts in which welding is dominant. Type 4.a. deposits are densely welded material with minimal to no clastic textures; type 4.b. deposits also consist of densely welded material but contain patches of high vesicularity (~70%). From these descriptions we believe the deposits are a result of eruptive styles ranging from Hawaiian style fire fountaining to more energetic Strombolian eruptions. Further, these deposits share a very similar NW-SE strike and in places are observed juxtaposed against a similar oriented sub-vertical dyke. Our field observations and measurements closely resemble those of well-documented eruptions at active basaltic volcanoes in Japan, Iceland, Hawaii and Italy where fissure eruptions have built proximal spatter ramparts. However, what is rarely to never observed at these active volcanoes is the feeder dyke. The welded nature of the proximal ramparts in our study, and the exposed feeder dyke could provide more insight into fissure eruption dynamics and mechanisms. We propose the necessity for further work on the flanks of Banks Peninsula.