HR: 11:35h
AN: V42C-06    [Abstracts]
TI: The 280 ka Matahina Eruption, Okataina Volcanic Centre, New Zealand: A Protracted Rhyolite Magma Assembly by Fractional Crystallization, Melt Extraction, Rhyolite-Rhyolite Mixing, and Magmatic Stratification
AU: * Deering, C D
EM: cdd21@student.canterbury.ac.nz
AF: University of Canterbury, Private Bag 4800, Christchurch, 8140, New Zealand
AU: Cole, J W
EM: jim.cole@canterbury.ac.nz
AF: University of Canterbury, Private Bag 4800, Christchurch, 8140, New Zealand
AB: The 280 ka Matahina eruption from the Taupo Volcanic Zone (TVZ), New Zealand, produced voluminous pyroclastic deposits representing >220 km³ of andesitic to rhyolitic magma and the climax of magmatic/volcanic activity from ca. 340 ka to 280 ka within the Okataina Volcanic Centre. The Murupara subgroup of eruptions preceding this large, caldera-forming event is used to document the progressive growth of a large volume magma body in the mid- to upper-crust over 10³ or 10⁴ years. The dominant magma composition of the caldera-forming eruption was rhyodacite/rhyolite (70.8 to 78.1 wt % SiO₂), with a subordinate volume of andesitic to rhyolitic (58.5 to 77.6 wt % SiO₂) juvenile pyroclastics erupted following the caldera collapse. Phenocrysts (plagioclase+quartz+opx±amph+Fe-Ti oxides) from the pumice clasts define two distinct compositional populations that evolved independent of one another following rhyolite melt extraction from a fractionating andesitic parent magma body. We interpret these phenocryst populations to represent two phase assemblages that evolved in a slightly stratified rhyolitic magma chamber overlying a less-evolved basal mush layer. Oxide geothermometry and phase equilibria suggest a 740- 783^°C magma storage temperature at 7-10 km depth. Most of the bulk-rock, melt, and phenocryst compositional variations are consistent with fractional crystallization (FC) of an andesitic parent magma body. However, trace element variation in pumice clasts, distinct rhyolitic glass major element compositions, and plagioclase disequilibrium accompanied by complex zoning are not consistent with closed-system FC alone, but are consistent with both FC and mixing in characterizing the magmatic diversity. In addition, the application of Polytopic Vector Analysis (PVA), a multivariate statistical treatment of the bulk-rock geochemistry, provides a mixing solution with a robust platform for accurately defining the timing and nature of the mixing event. Hence, the subtle geochemical disparities that occur between the main erupted rhyolitic magma and post-caldera collapse andesite-rhyolite assemblage specifically demonstrate mixing with a compositionally distinct rhyolite that must have followed the extraction of the evolved melt from the host crystal-mush as the magma system evolved. Mingled textures in the least-evolved, crystal-rich (35-40 % crystal content), pumice clasts also indicate a mafic input immediately preceding the eruption; probably acting as a thermal trigger for the main caldera-forming eruptive episode. Therefore, similar to some more recent large, rhyolitic eruptions from the TVZ (50 ka Rotoiti and Earthquake Flat eruptions, OVC; 26.5 ka Oruanui eruption, Taupo Volcano), the Matahina rhyodacite/rhyolite records a protracted, polygenetic, multi-stage magmatic and eruptive history, punctuated by the main caldera-forming eruption.
DE: 1036 Magma chamber processes (3618)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 8404 Volcanoclastic deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting