Structure, petrology, and genesis of Cenozoic volcanism, Taupo Volcanic Zone, New Zealand—a review

Abstract

The Taupo Volcanic Zone extends for approximately 300 km north-eastwards across the central North Island, New Zealand. Within the zone are five volcanic centres: Tongariro, which includes over 95% andesites; Taupo, Maroa, and Okataina, rhyolitic centres, each probably associated with multiple caldera collapse; and Rotorua, a simple collapse structure, sited on the western side of Taupo Volcanic Zone. The lavas of the Taupo Volcanic Zone are all calc-alkaline. Two types of basalt occur; a low-alumina basalt within the Tongariro Volcanic Centre and a high-alumina basalt in the Rotorua-Taupo area. Andesites can be divided into “normal” (57–63% Si0₂) and low-Si (53–57% Si0₂) types. Pyroxene low-Si andesites probably represent cumulate fractions of the “normal” andesite, but olivine low-Si andesite may be derived from a separate younger magma. Two types of dacite can also be distinguished. Bay of Plenty lavas show a complete gradation from dacite to andesite, and lavas from the Rotorua-Taupo region show a complete gradation from dacite to rhyolite. Rhyolites and ignimbrites which form the bulk of the volcanic rocks of Taupo Volcanic Zone (12 000km³–97-8%) are chemically distinctive in having Na₂O>K₂O. Strontium isotopic ratios of all lava types are similar with basalts having lowest ratios (av. 0.7043) and the remainder between 0.7052 and 0.7060. Marginal facies greywacke and argillites of the surrounding ranges are comparable (av. 0.7053). The Taupo Volcanic Zone is considered to represent a tensional graben structure or marginal basin caused by a change from a NW-trending arc to a NNE-trending structure. The change in orientation probably began c. 6 m.y. ago, but rhyolitic volcanism only commenced from the zone c. 1 m.y. ago, and olivine-bearing andesite eruptions associated with the NNE arc commenced about 50 000 years B.P. The location of the andesites and vent alignments suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island-arc andesites, such as those of Tonga, in particular by having higher contents of alkali elements, light REE, and Sr and Pb isotopes. This suggests some crustal contamination, and it is suggested that small amounts of Cenozoic sediments from the accretionary prism on the eastern side of the North Island may be subducted with oceanic crust. initially to produce amphibolite, and below 100 km, phlogopite eclogite. “Andesitic” magma is most likely derived from partial melting of amphibolite at 80100 Km, followed by a multi-stage fractionation en route to the surface. High-alumina basalts are thought to be derived by partial melting of peridotite in the mantle wedge overlying the subduction zone, locations of the vents controlled largely by faults within the crust. Rhyolites and ignimbrites were probably derived from partial melting of Mesozoic greywacke and argillite under the Taupo Volcanic Zone, the necessary heat and water coming from dehydration of amphibolite within the subduction zone of the earlier NW-trending arc. Release of the magma was allowed by the formation of the tensional graben. Bay of Plenty dacites probably resulted from mixing of andesitic magma with small amounts of rhyolitic magma, but those from the Rotorua-Taupo area were more likely formed by a higher degree of partial melting of the Mesozoic greywacke/argillite basement.