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Magma Generation at a Large, Hyperactive Silicic Volcano (Taupo, New Zealand) Revealed by U-Th and U-Pb Systematics in Zircons |
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| Authors: | CHARLIER B L A; WILSON C J N; LOWENSTERN J B; BLAKE S; VAN CALSTEREN P W; DAVIDSON J P |
| Institution: | 1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF DURHAM, SOUTH ROAD, DURHAM DH1 3LE, UK 2 DEPARTMENT OF EARTH SCIENCES, THE OPEN UNIVERSITY, WALTON HALL, MILTON KEYNES MK7 6AA, UK 3 INSTITUTE OF GEOLOGICAL & NUCLEAR SCIENCES, PO BOX 30368, LOWER HUTT 6315, NEW ZEALAND 4 VOLCANO HAZARDS TEAM, US GEOLOGICAL SURVEY, MAILSTOP 910, 345 MIDDLEFIELD ROAD, MENLO PARK, CA 94025, USA |
| Abstract: | Young (< 65 ka) explosive silicic volcanism at Taupo volcano,New Zealand, has involved the development and evacuation ofseveral crustal magmatic systems. Up to and including the 26·5ka 530 km3 Oruanui eruption, magmatic systems were contemporaneousbut geographically separated. Subsequently they have been separatedin time and have vented from geographically overlapping areas.Single-crystal (secondary ionization mass spectrometry) andmultiple-crystal (thermal ionization mass spectrometry) zirconmodel-age data are presented from nine representative eruptiondeposits from 45 to 3·5 ka. Zircon yields vary by threeorders of magnitude, correlating with the degrees of zirconsaturation in the magmas, and influencing the spectra of modelages. Two adjacent magma systems active up to 26·5 kashow wholly contrasting model-age spectra. The smaller systemshows a simple unimodal distribution. The larger system, usingdata from three eruptions, shows bimodal model-age spectra.An older 100 ka peak is interpreted to represent zircons (antecrysts)derived from older silicic mush or plutonic rocks, and a youngerpeak to represent zircons (phenocrysts) that grew in the magmabody immediately prior to eruption. Post-26·5 ka magmabatches show contrasting age spectra, consistent with a mixtureof antecrysts, phenocrysts and, in two examples, xenocrystsfrom Quaternary plutonic and Mesozoic–Palaeozoic metasedimentaryrocks. The model-age spectra, coupled with zircon-dissolutionmodelling, highlight contrasts between short-term silicic magmageneration at Taupo, by bulk remobilization of crystal mushand assimilation of metasediment and/or silicic plutonic basementrocks, and the longer-term processes of fractionation from crustallycontaminated mafic melts. Contrasts between adjacent or successivemagma systems are attributed to differences in positions ofthe source and root zones within contrasting domains in thequartzo-feldspathic (<15 km deep) crust below the volcano. KEY WORDS: zircon; U-series dating; rhyolite; Taupo Volcanic Zone; Taupo volcano |
| Keywords: | : zircon U-series dating rhyolite Taupo Volcanic Zone Taupo volcano |
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