Obducted ophiolites of North Island, New Zealand: Origin, age, emplacement and tectonic implications for Tertiary and Quaternary volcanicity

Abstract

Igrieous massifs in Northland and EastCape contain disrupted ophiolite sequences (pillow lavas, sheeted dikes, noncumulate and cumulate gabbros) with low-grade metamorphic assemblages. Sediments associated with pillow lavas, or as cappings on individual massifs, have fossils ranging in age from Early Cretaceous to Early Miocene. KAr ages extend from 102 to 20 Ma; an isochron age at 33.6 ± 2.1 Ma reflects argon homogenisation by plate boundary deformation at the ophiolite sites prior to obduction of the disrupted massifs during the Late Oligocene-Early Miocene. The compatibility of the fossil and radiometric ages, and the diversity of rock types (tholeiitic to alkali basalt and alkaline syenite), indicate long-continued igneous activity at localised seamounts along a nonconvergent plate boundary (transform zone) which was situated off the northeast coast of North Island and extended at least from near North Cape to East Cape. Obduction was from northeast to southwest by gravity sliding of the dismembered seamounts upon an autochthonous substrate of Permo-Jurassic greywackes and Cretaceous-Oligocene sediments. The obductive event also marked the start of midTertiary calcalkaline volcanicity in North Island. Obductive overriding of the transform zone during the latest Oligocene-earliest Miocene produced andesites along the Northland east coast, but Early Miocene-Quaternary calcalkaline belts were oriented northeast-southwest with a decreasing age for volcanism towards the Taupo Volcanic Zone. Regional tensional rifts in the latest Pliocene or Quaternary followed northwest-southeast trends.

Upper Tertiary and Quaternary volcanism and subduction zone regression, North Island, New Zealand

“…In a discussion of North Island volcanism, Kamp (1984) favours oblique subduction from NE to SW beneath the North Island, with initial emplacement in the more northern regions. He quotes few data for K-Ar ages and KzO-SiOz indices, and therefore has different values from those given in this paper, and makes no reference to the East Belt-West Belt divisions originally recognised by Thompson (1965) and Stipp (1968); similarly, his account does not consider concepts of underplating or subduction regression and associated extensional tectonics as previously described by Brothers and Delaloye (1982) and Brothers (1983a, b, c). A variety of Miocene-Holocene tectonic settings (rift, intraplate, back arc) have been inferred by Hodder (1984) from the geochemistry of the whole volcanic province.…”

“…comm., 1985). The remaining ages at Brothers and Delaloye (1982, fig. 7) as indicating an older, separate and tectonically-distinct high-pressure calcalkaline arc containing garnet andesites and initially generated during Late Oligocene obduction of the Northland ophiolite massifs.…”

“…That is, Upper T erti ar y volcanism commenced in Northl an d in the Ea rly M iocen e, and th e m agm a-gen erating sou rce zone shifted southea stward , bu t th rou ghout th is m igrat ion it m aintai ned a N E-SW or ientation , with NW dip , that has persisted in to th e modern su bd uction surface passi ng under the North Islan d from the vicinity of th e Hikurangi Trench. One explanation for migration of the Benioff zone suggested by Brothers and Delaloye (1982) and summarised by Brothers (1984) is that, prior to the Early Miocene, the North Island was underplated by a shallow, low-angle, rapid subduction system which originated from the Hikurangi Trench and penetrated northwest to underlie Northland, but was not accompanied by volcanism. At its northwesterly limit, below mid-Northland, the Benioff surface plunged sufficiently deeply to generate the first of the NE-SW calc-alkaline volcanic arcs in the Early Miocene (21 Ma), and subsequently it retreated southeastward from that position throughout the Late Tertiary and Quaternary to its present configuration across the centre of the North Island.…”

“…Regional Tertiary volcanism commenced in the North Island in the Early Miocene, immediately after an obductive event which had emplaced allochthonous ophiolite massifs along the eastern coasts of Northland and East Cape (Brothers and Delaloye, 1982;Brothers, 1983a). The earliest volcanics were erupted in the northern part of the North Island as basalt-andesite-dacite suites, with some associated intermediate plutonic phases.…”

Identity of igneous bodies beneath the continental shelf of west Northland, New Zealand

“…of a dolerite sill in nearby Waimamaku driUhole. As argued by Hayward (1979a), the sill is within the displaced Northland Allochthon and part of the obducted Tangihua Volcanics of paleontologically determined Cretaceous age (Hayward 1983), that have a wide range of unreliable Cretaceous-Neogene KAr age determinations (Brothers & Delaloye 1982). The offshore igneous bodies do not have the northem distribution, shape, or size that would be expected if they were blocks of Tangihua Volcanics obducted in the mid Cenozoic.…”