Geochemical and volcanological aspects of rhyolite eruptions from Taupo Volcanic Zone, New Zealand

Abstract

The Taupo Volcanic Zone (TVZ) of New Zealand is the most frequently active rhyolitic zone on Earth, with more than 50 rhyolitic eruptions from its active volcanic centres, Okataina (OVC) and Taupo (TVC), in the last 50 kyr. The millennial-scale frequency of eruptions in the TVZ implies that the magmatic systems may not be analogous to those documented at other rhyolitic centres in continental settings which generally have eruption frequencies that are orders of magnitude greater. This study was undertaken to further understanding of the migration, storage, and eruption of rhyolitic magmas in the OVC. The fractionated silicic melts in OVC appear to be derived from crystal-rich mush zones, similar to other calderas (e.g., Long Valley). In the OVC, these melts subsequently migrate into the upper crust (mainly 5-7 km depth) and crystallise, as indicated by phenocryst and glass equilibrium. Geochemical and mineralogical heterogeneity in the OVC deposits, often on a clast-scale, suggests that the eruptions commonly tap several homogeneous, compositionally distinct magma batches. Temporal fractional crystallisation trends cannot link the magmas indicating that they are not derived from a single magma body and instead are derived individually from the mush zone/s. The occurrence of mingled clasts, with bimodal glass and disequilibrium mineral compositions, demonstrate that magmas are often in contact. It appears that magmas at OVC commonly accumulate and assemble in the upper crust. The mixing and possibly hybridisation of magmas complicates the use of zircon chronology to estimate magma residence times. At OVC, compositionally distinct magmas erupt from vents spread along eruptive fissures >10 km long. Some of the vents tap a single magma while others tap several, suggesting they are fed by a series of dikes that vary in length. Alternation between effusive and explosive activity indicates wide variation in ascent rates and/or conduit permeability during eruption. Several magmas are often ejected during eruption episodes concurrently suggesting that the magmas are triggered into activity by a common event, such as an intrusion or a seismic disturbance. Triggering by mafic intrusion is common during some events at OVC. Triggering by silicic intrusion is also evident. During the Rotorua eruption a stagnated cool, crystal-rich rhyolite magma was reactivated by the intrusion of hotter, aphanitic rhyolite. Such processes are rarely reported in the literature. Both OVC and TVC experienced contemporaneous changes in eruption style, mineralogy, and geochemistry following caldera-forming events at ~50 and 26.5 ka, suggesting the tectonic regime fundamentally changed at these times. After caldera-forming events, a more open magmatic system develops, with eruptions tapping less evolved (<71 wt % SiO2) and hotter (>820°C) magmas from greater depths (~16 km).