CHEMICAL ANALYSIS ON VOLCANICLASTIC DEPOSITS FROM THE TARANAKI BASIN, NEW ZEALAND

The Taranaki Basin, located off the west coast of North Island, New Zealand, contains a variety of middle to late Miocene lithofacies, including deep water mud, volcaniclastic sediment, and sandy turbidites. Due to continental slope instability volcanic deposits, believed to have originated from the contemporaneous andesitic volcanoes, were reworked and deposited as volcaniclastic turbidites of the Mohakatino Formation in the western Taranaki Basin. Since the Miocene, sediment has covered the volcanic edifices located offshore to the west as the northern Taranaki Basin experienced uplift exposing the sediment. A chemical analysis of minerals within four volcaniclastic deposits, collected from a vertical succession of the Mohakatino Formation near Awakino, was conducted to understand the petrogenesis of the covered volcanic massifs in the Taranaki Basin. Primary volcanic minerals in the volcaniclastic deposit are plagioclase and amphibole. Secondary minerals include calcite and clay minerals. Small amounts of chlorapatite from an ambiguous source were also observed. Initial results show small variations of plagioclase and amphibole compositions across all four samples. The average size of plagioclase phenocrysts are 0.7mm. Plagioclase composition ranges from 45% to 93% An. The magnesium number (Mg/(Mg+Fe+Mn))for amphiboles range from 0.62 to 0.82, and amphibole phenocrysts are an average size of 0.8mm. Both plagioclase and amphibole phenocrysts are subhedral with fractured or broken edges. Continued analysis will compare the mineral compositions of the volcaniclastic deposits to the composition of minerals from core samples of the Miocene andesite volcanoes, in attempt to achieve a greater understanding of the petrogenesis of the now completely buried volcanic edifices.