AGE RELATIONSHIPS AND TECTONIC IMPLICATIONS OF LATE CENOZOIC BASALTICVOLCANISM IN NORTHLAND, NEW-ZEALAND

An episode of late Miocene - Recent essentially basaltic volcanism is the latest in a sequence of magmatic events recognised in the tectonic ally 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 die Ti Point and Stony Batter rocks being geochemically distinct. A consistent mode l for these observations is that the magmas originate from different l evels of a layered mantle source in which the upper part carries a geo chemical signature inherited from an earlier subduction event.