Regional aspects of cainozoic volcanology in the north island of New Zealand— crustal fusion produces intermediate magma

Abstract

Early Tertiary crustal melting produced a primary magma of intermediate acidity that underlay, and possibly still underlies in a partly molten condition, the Auckland Arc. Growth of the magma finally resulted in mid-Tertiary tensional collapse of the overlying crust inducing block faulting and widespread andesitic eruptions. There then followed pronounced differentiation within the westward-dipping magma chamber so that later Plio-Quaternary eruptions were mainly basaltic in the west and rhyolitic in the east. Such differentiation may also account for a western increase in aeromagnetic values. The New Zealand — Tonga Volcanic Zone is a zone of andesitic volcanism along which differentiation to produce large volumes of rhyolite has yet to take place. Interaction of this seismically and tectonically active zone with the southern end of the older Auckland Arc has been important in providing heat and also surface access for the acidic differentiate derived from the older arcuate zone.