Chronology and Evolution of Caldera-forming and Post-caldera Magma Systems at Okataina Volcano, New Zealand from Zircon U–Th Model-age Spectra

摘要

U–Th disequilibrium model-age data are presented for zircons from four young eruptive units from Okataina volcano, New Zealand. These data highlight contrasts in the time-scales over which eruptible silicic magma bodies are generated and accumulated below a highly active rhyolite volcano prior to and following the ∼61 ka caldera-forming Rotoiti eruption (80 – 120 km3 magma). The Rotoiti event was followed by ≥12 explosive eruptions of the Mangaone Subgroup between ∼45 and 30 ka. A change in eruptive styles between ∼30 and 25 ka brackets the major Oruanui eruption of Taupo volcano ∼80 km away; subsequently eight rhyolitic, lava-dominated and two basaltic explosive eruptions occurred between 25 ka and 1886 ce. We present (238U–230Th) zircon model-age data determined by secondary-ion mass spectrometry (SIMS) and thermal ionization mass spectrometry (TIMS) from the Rotoiti, Ngamotu (Unit B) (∼45 ka), Mangaone (Unit I) (33 ka) and Rotorua (15·4 ka) eruptive units. We couple these data with published and new compositional information to trace magma crystallization and storage patterns. Population density curves of SIMS model ages from zircons in the two pumice types (biotite-free and biotite-bearing) from the Rotoiti eruption differ. Zircons from the former yield a model-age peak coincident with eruption age, whereas zircons from the latter show a peak at 70–90 ka and little variation in TIMS model-age values from different crystal size fractions. Concentration weighted means of model ages from the two pumices, however, are the same within 1 SD error, and their Sr isotopic values overlap at 2 SD precision, suggesting that they are genetically linked. Model-age spectra for the Ngamotu, Mangaone and Rotorua pumices are complex, indicating recycling of crystals from multiple older populations that largely pre-date the 61 ka caldera-forming eruption. Superimposed on the older age spectra are variably developed younger pre-eruptive suites of ages reflecting varying amounts of crystallization prior to each eruption. A lack of commonality in these younger peaks in the post-caldera eruptive rocks, along with compositional and isotopic differences between (and sometimes within) the eruptive units, collectively precludes their origin from a single melt-bearing mush system. Okataina contrasts with Taupo, where comparable-age eruption deposits have simpler age spectra, consistent with larger-scale crystallization cycles and thermal events in the magma chambers there. When compared with other caldera-related silicic systems for which suitable data are available, Okataina differs in lacking a simple pre-eruptive history prior to its caldera-forming event and having a complex, non-coherent magmatic history of post-caldera eruptions.