An integrated model for the temporal evolution of andesites and rhyolites and crustal development in New Zealand's North Island

Abstract

The Taupo Volcanic Zone (TVZ) is the southern expression of the Tonga–Kermadec subduction system within the continental crust of New Zealand's North Island. It is a region of rapid extension and high heat flow containing one of the most productive, frequently active rhyolitic magmatic systems on Earth. Rhyolites make up >80% of the total erupted material with rhyolitic magmatism being prevalent from the earliest stages of development of the central TVZ. At the present day, the principal focus of andesitic magmatism is at the southern extremity in the Tongariro Volcanic Centre and, on the basis of the patterns of volcanism preserved in the geological record elsewhere in the TVZ, we predict that these volcanoes will eventually founder and be buried by later rhyolite as crustal extension propagates southwards. In the TVZ, andesitic magmatism precedes the development of large-scale rhyolitic systems. Glass inclusions and groundmasses in andesites and xenoliths from Ruapehu, which is the largest active volcano of the Tongariro Volcanic Centre, and Taranaki (Egmont Volcano), which lies to the west outside the TVZ, have major and trace element compositions that show similarities to those of TVZ rhyolites and dacites. Pyroxene and plagioclase crystals contained within xenoliths show morphological and compositional similarities to phenocrysts in the host andesites. TVZ rhyolites and andesites show overlapping Sr and Nd isotopic compositions. We argue that andesites are generated through the interaction of mantle-derived magmas with lower crustal melts and restites and differences between Ruapehu and Taranaki andesites reflect contrasting physical conditions during melting as well as subtle variations in the mantle and crustal source compositions. Magmas produced by these processes are complex mixtures of crustal melt, strongly fractionated mantle melt, restitic crystals, lithic lower crustal fragments (represented by xenoliths), and phenocrysts derived from mantle melts. In many cases, the crustal component appears to dominate and to have been further increased during crystal fractionation and assimilation of crustal melt at higher levels as magmas rose towards the surface. Rhyolitic magmatic systems in the TVZ evolve as crustal extension and thinning cause rising geothermal gradients so that crustal melting becomes progressively more significant. Earlier andesitic intrusives are remelted and recycled as the system reaches thermal maturity. Andesites and rhyolites in the TVZ are directly related and represent different aspects of a continuous, evolutionary spectrum of petrological processes by which new crust is formed, grows, and is recycled.