|Volcanic activity in the Tauranga Volcanic Center (TgaVC) occurred between 2.95 to 1.9 Ma, some of the eruptions were explosive and led to the distribution of ignimbrites of varying volumes across the western Bay of Plenty Region (Tauranga Basin), North Island. The Papamoa Formation forms part of the landscape in this area. This study was aimed at determining the volcanic history and processes involved in the distribution of the Papamoa Formation within the eastern Tauranga Basin.
Stratigraphic, petrographic and geochemical investigations were undertaken in the field and from samples within the study area. Field observations of the ignimbrites involved stratigraphic logging and lateral relationships, component measurements and lithological descriptions. Ignimbrite petrography was undertaken by optical microscopy. To determine the geochemical characteristics of the source magmas for the ignimbrites, the following analyses were conducted: electron microprobe analysis (EMPA) on glass shards extracted from the ignimbrite matrices, glass from pumice, and on minerals in the pumice, x-ray fluorescence spectroscopy (XRF) on whole pumice clasts and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on the same glass samples used for EMPA.
Five individual eruptions that led to the distribution of five ignimbrites have been identified within the ignimbrite succession of the Papamoa Formation: an early unnamed ignimbrite, followed by the Welcome Bay, Wharo, Arateka, and Otawera ignimbrites. The beige-brown, non-welded, pumice-rich, Welcome Bay Ignimbrite (2.4 Ma) is the most voluminous in the study area and also has an underlying pumice fall deposit. The dark brown to black, pumice-/fiamme-rich Wharo Ignimbrite (2.26 Ma, M. Prentice unpublished data, 2021) is the only moderate to densely welded ignimbrite. Both the Welcome Bay and Otawera ignimbrite contained black (andesitic-dacitic) and white (dacitic) pumice. The beige, non-welded Otawera Ignimbrite (2.21 Ma) is the least voluminous ignimbrite in the study area and comprises only white pumice. The beige-yellow brown Arateka ignimbrite is comprises of both the black and white pumice clasts. The unnamed Unit A ignimbrite, at the base of the succession has only one pumice population – grey pumice, but it is poorly exposed. All the ignimbrites are generally lithic poor. Petrographic analysis found that the main minerals occurring in the ignimbrites were: plagioclase, pyroxenes, hornblende and opaques. The main mineral in the black pumice was plagioclase and pyroxene. The white pumice, however, is mainly composed of plagioclase and hornblende. In terms of rock textures, the Welcome Bay Ignimbrite, Arateka Ignimbrite, Otawera Ignimbrite and Unit A-Unknown Ignimbrite had either a porphyritic or vitrophyric or a combination of both textures. The
Wharo Ignimbrite possessed a porphyritic and/or eutaxitic texture. Geochemical data showed that although the glass shards from each ignimbrite has a unique chemical composition, they were all rhyolitic. However, in terms of the two pumice types, the white pumice was dacitic while the black pumice was andesitic.
The presence of two pumice types (black and white) in the Welcome Bay, Wharo and Arateka Ignimbrites, supports the idea that the magma from which the ignimbrites were sourced, had a mixed andesitic to dacitic composition. The distribution and internal stratigraphy of the Papamoa Formation record a history of multiple ignimbrite-forming eruptions that were sourced locally, likely south of the study area; at least one ignimbrite (Welcome Bay Ignimbrite) was preceded by a sustained Plinian eruption column.