Rift-Related Transition from Andesite to Rhyolite Volcanism in the Taupo Volcanic Zone (New Zealand) Controlled by Crystal–melt Dynamics in Mush Zones with Variable Mineral Assemblages

Abstract

The Taupo Volcanic Zone (TVZ), located in the North Island of New Zealand, represents part of a magmatic arc that is at present undergoing active extension. Around 0·9 Myr ago, an acceleration in rifting was followed by a progressive transition in the composition of volcanic products (until 0·7 Ma) from typical arc-type andesite into overwhelmingly large, caldera-forming rhyolitic eruptions with subordinate basalt and dacite in the Central TVZ. Despite an obvious compositional gap in the erupted products in the Central TVZ within the last 0·7 Myr (little to no erupted products with SiO2 contents between 55 and 65 wt %), phenocryst minerals (plagioclase, amphibole, pyroxene) show an uninterrupted compositional record that suggests crystallization from a continuum of melt compositions. Coupled with radiogenic isotope evidence, the whole-rock and mineral chemistry data are consistent with magmatic differentiation controlled by crystal fractionation of primary mantle-derived magmas accompanied by some assimilation of local wall-rocks. In the Southern TVZ and in the early part of the Central TVZ, magmatic differentiation was dominated by the lower crustal evolution of relatively dry ( 1wt % H2O) arc basalts, crystallizing a pyroxene^plagioclase-dominated assemblage. However, the conditions of crystallization in the lower crust appear to have changed within the last million years in the Central TVZ, with amphibole and oxides appearing earlier in the crystallization sequence. In this framework and using numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of magma reservoirs, we show that melts extracted from crystal mushes within an optimal ‘extraction window’ ( 50 and 80 vol. % crystals) match those erupted at the surface. Lower crustal mushes fed by basalt with 1wt % H2O (dominated by a pyroxene^plagioclase assemblage) release andesitic melts at the extraction window. These melts then erupt at the surface to form the observed andesitic part of the arc.With a slightly higher water content ( 2 wt %) in the basalt, the melt composition at the extraction window from lower crustal mushes is dacitic rather than andesitic. Although some dacitic melts will reach the surface, most will be trapped in the upper crust and crystallize to form a silicic mush. Extraction of the interstitial liquid after450% crystallization from this upper crustal reservoir produces the large volumes of rhyolitic magma erupted over the past 0·7 Myr (44000 km from ignimbrite-forming eruptions).