Glass-bearing plutonic fragments from ignimbrites of the Okataina caldera complex, Taupo volcanic zone, New Zealand; remnants of a partially molten intrusion associated with preceding eruptions

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doi: 10.1016/S0377-0273(98)00039-0
Authors:Burt, R. M.; Brown, S. J. A.; Cole, J. W.; Shelley, D.; Waight, T. E.
Author Affiliations:Primary:
University of Canterbury, Department of Geological Sciences, Christchurch, New Zealand
University of Western Australia, Australia
La Trobe University, Australia
Volume Title:Journal of Volcanology and Geothermal Research
Source:Journal of Volcanology and Geothermal Research, 84(3-4), p.209-237. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0377-0273
Publication Date:1998
Note:In English. 69 refs.; illus., incl. 4 tables, sketch map
Summary:Glass-bearing plutonic fragments occur as rare accessory lithics within the ca. 64 ka Rotoiti and Earthquake Flat ignimbrites that were erupted from Okataina caldera complex, Taupo Volcanic Zone, New Zealand. Granitoid lithic fragments are only found in the Rotoiti ignimbrite and fall into two groups. Group 1 granitoids have textures consistent with a period of slow cooling followed by rapid quenching, and were excavated by the Rotoiti eruption from a single incompletely solidified magma body. Although isotopic ratios for the Group 1 granitoids are similar to the host ignimbrite, they are not cognate, having different chemistry, mineralogy, mineral chemistry and crystallisation history. It is more likely that they represent fragments of a separate incompletely solidified magma chamber that was intercepted by the erupting Rotoiti ignimbrite magma. Low LILE and high HFSE abundances favour a comagmatic link with the ca. 0.28 Ma Matahina ignimbrite and it is suggested they are derived from an isolated cupola of the Matahina magma chamber that remained at depth (between 3.5 and 5 kbar pressure) after eruption of the Matahina ignimbrite. Migration toward the surface probably accompanied development of the Rotoiti magma system in the upper crust. Most geochemical variation in Group 1 granitoids is related to the abundance of biotite, the concentration of which is controlled by differential shear. REE abundance is controlled by light REE-enriched accessory minerals preferentially included within biotite. Although Eun remains constant in the Group 1 granitoids, Eu/Eu* varies systematically with (La/Yb)n and is controlled by variations in Sm and Gd rather than in Eu. Group 2 granitoid fragments have a wide range of composition, comparable to many Okataina rhyolites, including those found as lithic fragments in the Rotoiti ignimbrite. Rare microdiorite fragments occur in both Rotoiti and Earthquake Flat ignimbrites and typically contain vesicular interstitial glass indicating that they were incompletely solidified prior to eruption. Those from the Rotoiti ignimbrite are comparable to the (>64 ka) Matahi basaltic tephra and probably represent part of the same magmatic event which generated the Matahi tephra. Abstract Copyright (1998) Elsevier, B.V.
Subjects:Calderas; Eruptions; Geochemistry; Igneous rocks; Ignimbrite; Intrusions; Magma chambers; Magmas; Major elements; Mineral assemblages; Mineral composition; Petrography; Phenocrysts; Plutons; Pyroclastics; Rhyolites; Secondary minerals; Trace elements; Variations; Volcanic features; Volcanic rocks; Volcanism; Volcanoes; Australasia; New Zealand; North Island; Taupo volcanic zone; Okataina Caldera
Coordinates:S390000 S380000 E1770000 E1760000
Abstract Numbers:99M/2195
Record ID:1999002905
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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