Age relationships and tectonic implications of late Cenozoic basaltic volcanism in Northland, New Zealand

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Abstract
An episode of late Miocene ‐ Recent essentially basaltic volcanism is the latest in a sequence of magmatic events recognised in the tectonically complex geological development of the Northland Peninsula. New K‐Ar dates together with an extensive collection of new major and trace element chemical analyses prompt a reassessment of the significance of these late Cenozoic basalts. The main time/ space groupings recognised are Tertiary volcanics in the Kaikohe ‐ Bay of Islands, Puhipuhi, Ti Point, and Stony Batter areas and Quaternary basalts in the Kaikohe ‐ Bay of Islands and Whangarei areas and at Tara. Basalts in the Kaikohe ‐ Bay of Islands area are transitional to alkalic in character, while those in the south are transitional to tholeiitic, with the Ti Point and Stony Batter rocks being geochemically distinct. A consistent model for these observations is that the magmas originate from different levels of a layered mantle source in which the upper part carries a geochemical signature inherited from an earlier subduction event.