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Abstract
Karioi, in western North Island, New Zealand, is the northwesternmost volcano of the Alexandra Volcanic Lineament. Active from c. 2.48 to 2.28 ± 0.07 Ma (unmodified chronology), its products comprise the Karioi Formation. That formation is here subdivided into the basal Te Toto Member (new) of basaltic lavas and breccias erupted from small central‐vent volcanoes; the middle Whaanga Member (new) of shield‐building basalts and basaltic andesites erupted from fissures; and the uppermost Wairake Member (new) of cone‐building basalt, basaltic andesite, and andesite lavas, tuffs, vent breccias, dikes, and valley‐filling lavas and laharic deposits on the surface of the Whaanga shield. Wairake units were erupted from at least two distinct central vents. Karioi is surrounded by small monogenetic volcanoes of the Okete Formation that erupted basanitic, alkali olivine basaltic, and hawaiitic magmas and formed scoria cones, lavas, and tuff rings. Okete Formation is subdivided into units that predate Karioi volcano (Pauaeke Member, new), and those that postdate Karioi volcano i Marumaruaitu Member, new). Pauaeke volcanism occurred between 2.58 ± 0.06 and 2.48 Ma, and Marumaruaitu volcanism between 2.37 ± 0.08 and 1.90 ± 0.07 Ma. Early Okete and Karioi units erupted from vents close to sea level, onto a gently westward‐sloping, mostly subaerial plain. This plain had been veneered by beds referred to as Ohuka Carbonaceous Sandstone. The Karioi area has experienced little or no net uplift during the late Pliocene and Pleistocene, as opposed to elsewhere along the west coast. Its stability may have been facilitated by fault‐bounded detachment of the Karioi block and loading of the crust by the volcano itself.