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Volcanology of the basaltic lava succession within the Auckland pit of the Bombay Quarry, Bombay Volcanic Complex, South Auckland Volcanic Field

The South Auckland Volcanic Field (SAVF), which was active 1.59 – 0.51 million years ago and comprises around 82 volcanic centres, represents a complete history of a monogenetic field preserved in the geological record. The Auckland pit in the Bombay Quarry was recently exposed, revealing an infilled palaeovalley of volcanic and sedimentary deposits possibly associated with the nearby Bombay Volcanic Complex. A set of vertical drill cores from across the quarry were available for this study. The stratigraphy of the volcanic and sedimentary deposits and the facies architecture were examined and described from the drill cores available, and a set of stratigraphic logs were produced. Volcanic and sedimentary units identified were: basement Waitemata and Tauranga group sediments, three individual ponded basalt lavas with intercalated scoria and Quaternary alluvium and/or Kauroa ash deposits. Facies identified include: moderately vesicular basalt (A.1), vesicular basalt with vesicle trails (A.2), non-vesicular basalt (A.3), poorly vesicular basalt (A.4), scoria deposit (B), scoriaceous basalt (C) and intercalated silt/clay (D). Petrographic characteristics were analysed by optical microscopy, which show that all three basalt lavas have minerals comprising: olivine and clinopyroxene phenocrysts, and a groundmass of plagioclase, opaques and mafic minerals, however, the proportions of each mineral vary between samples. Olivine, clinopyroxene and plagioclase elemental compositions for each of the three basalt units were determined by electron microprobe analysis and revealed that the middle basalt had relatively lower proportion of Mg- and Ca-rich minerals compared to the upper and lower basalts. Furthermore, mineral compositions were consistent with the broad group B rock type of the SAVF lavas. Bulk-rock geochemical characteristics were analysed by X-ray fluorescence spectrometry where the basalt samples were classified as basanites and ne-hawaiites. The lower and upper basalts have a relatively wide range of major and trace element compositions; whereas, the middle basalt has less variation. The three basalt lava flows represent pahoehoe and/or transitional lava flows, which occurred during magmatic eruptions separated by periods of volcanic quiescence represented by Quaternary alluvium and/or Kauroa ash deposits. The magma source beneath the Bombay area reveals that it consists of dominantly a garnet-bearing peridotite source where only group B type lavas were erupted over time. This process indicates a polygenetic-like eruption history within a monogenetic field, which may be an ideal analogue for understanding the future of shield volcanism in the South Auckland and Auckland Volcanic Field.
Type of thesis
Kapasi, A. (2016). Volcanology of the basaltic lava succession within the Auckland pit of the Bombay Quarry, Bombay Volcanic Complex, South Auckland Volcanic Field (Thesis, Master of Science (Research) (MSc(Research))). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/11010
University of Waikato
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