Trends in rhyolite geochemistry, mineralogy, and magma storage during the last 50 kyr at Okataina and Taupo volcanic centres, Taupo Volcanic Zone, New Zealand

摘要

The Taupo Volcanic Zone of New Zealand is the most frequently active rhyolitic zone on Earth. Since a major caldera-forming eruption episode at ～50 ka, >50 rhyolitic eruption episodes have occurred at Okataina Volcanic Centre (OVC) and Taupo Volcanic Centre (TVC). These two active calderas provide an opportunity to examine contemporaneous magmatic processes at high temporal and spatial resolution. Temporal trends indicate OVC intra-caldera eruption episodes tapped progressively more evolved (～71-77 wt.% SiO_2), cooler (940-730℃), and possibly shallower (～400-150 MPa) magmas. At TVC, pre-26.5 ka activity was relatively infrequent, and eruption episodes were from shallow (～100 MPa), cool (～750℃) magmas, containing hydrous mineral phases. Following the caldera-forming 26.5 ka Oruanui episode, initial eruption episodes were dacitic and were small volume hot magmas, which were probably from deep (> 400 MPa) chambers. At 12 ka, rhyolitic activity at TVC re-commenced, and eruption episodes have tapped progressively hotter (～790-850℃) magmas, which appear to be from deeper (～140-300 MPa) chambers. At both OVC and TVC volcanoes, the least evolved magmas were erupted following the caldera-forming episodes. Equilibrium between glass and phenocrysts in OVC and TVC rhyolites suggests that crystallisation occurs shortly prior to eruption. Low crystal contents (～5-15%), and the lack of pre-eruptive gradients suggests rapid convection and/or short crustal residence times. Melts are extracted from greater depths, as indicated by high whole-rock temperatures, before ponding in shallow storage chambers. The distinct pressure, temperature, and oxygen fugacity of each magma erupted, and the lack of temporal (fractionation) trends, suggests that the magmas are not derived from a single common magmatic system at their respective centres. OVC magmas are more oxidised than those from TVC, at any given temperature, suggesting the source areas are fundamentally different. Volumetrically subordinate pumice clasts in some OVC ejecta display mingled glasses and disequilibrium crystal populations resulting from the intrusion and mingling of separate rhyolite magmas prior to eruption. At OVC, some crystal-rich stagnating magmas have become reactivated by new intrusions or engulfed into larger magma bodies, and some eruption episodes were primed and triggered by mafic intrusion.