Wilson, C. J. N. and Charlier, B. L. A.
Rapid rates of magma generation at contemporaneous magma systems, Taupo Volcano, New Zealand: insights from U-Th model-age spectra in zircons.
Journal of Petrology, 50(5) pp. 875–907.
New and/or enlarged datasets of U–Th disequilibrium model ages from secondary ionization mass spectrometry (SIMS) analyses of zircons in eight eruptive units from the area of Taupo volcano, New Zealand, highlight the behavioural contrasts of two closely adjacent, contemporaneous but independent magma chambers. One yielded closely similar crystal-poor (‘Oruanui-type’) rhyolites, sampled in three small precursor eruptions (Tihoi, ‘New plinian’, Okaia) from ∼45 to 30 ka, then the major 27 ka Oruanui eruption. Three of the four eruptions had vents within the modern Lake Taupo, whereas the fourth (‘New plinian’) was sourced ∼20 km NNE of the other vents, fed by lateral magma migration. Samples from all four eruptions share a common model-age peak at ∼95 ka of antecrystic zircons. However, three of the four differ in younger pre-eruptive model-age peaks that require their parental melt-dominant bodies to have been physically extracted independently from a common mush zone represented by the ∼95 ka peak. A sample from a fifth eruption (‘New phreatoplinian’, also at ∼45 ka) shares an older 80–100 ka peak but has numerous older grains and distinctly contrasting Sr-isotopic characteristics to the ‘Oruanui-type’ magmas. The 530 km3 Oruanui melt-dominant body was produced in at most ∼3000 years as shown by differences in zircon model-age spectra and average ages between it and the 30 ka Okaia eruption, despite their coincidence in vent locations. The second suite of eruptions at ∼47, 28 and 16 ka ejected moderately crystal-rich biotite rhyolites from a second source chamber, which vented over a 15 km wide area NE of Taupo (overlapping with Maroa volcano). This second chamber is inferred to have comparable horizontal dimensions to the vent spacing. The three biotite rhyolites show unimodal model-age spectra that peak at 30, 15–25 and 6 kyr prior to each eruption, respectively, and underwent single cycles of melt generation and eruption with no recycling of significantly older antecrysts or xenocrysts (< 1% equiline grains). Crystallization peaks defined by probability density function curves are not in phase between the two magma chambers and they had wholly independent thermal and chemical histories, despite their close geographical proximity. Post-Oruanui activity involved recycling of Oruanui-age zircons, but these crystals are xenocrystic, as the host melts show no lineage towards or mixing with the Oruanui compositions. Magma chambers at Taupo accumulated melt-dominant bodies as rapidly as > 5 m3/s (Oruanui) and effectively drained the mush of melt in doing so (Oruanui vs post-Oruanui activity), probably mediated by active rifting processes and tectonic disruption of the mush pile. Comparisons of ‘magma residence times’ and discussion of the growth histories of large silicic chambers represented by volcanic or plutonic rocks are self-limited by the uncertainties in the respective SIMS analyses. Growth times of Miocene and older plutons dated by SIMS U–Pb techniques are comparable with the 2 Myr lifetime of the whole Taupo Volcanic Zone, and the associated 1σ SIMS analytical uncertainties exceed the lifetime of a volcano such as Taupo. Subtle details that indicate the rapidity of magma accumulation and recycling of crystals in the young Taupo system cannot be discerned in most pre-300 ka silicic systems. Averaging of SIMS model-age data further obscures subtle details that would allow discrimination of newly crystallized versus recycled zircons. Discussions of volcano–plutonic relationships and accumulation rates for large silicic melt-dominant bodies cannot rely on age data in isolation, but require knowledge of the stratigraphic and compositional settings.
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