HR: 1340h
AN: V33C-1515    [Abstracts]
TI: Pre-eruption Thermal Rejuvenation and Stirring of a Partly Crystalline Rhyolite Pluton Revealed by the Earthquake Flat Pyroclastics Deposits, New Zealand
AU: * Shane, P
EM: pa.shane@auckland.ac.nz
AF: University of Auckland, Geology Program, Private Bag 92019, Auckland, 1142, New Zealand
AU: Molloy, C
EM: c.molloy@auckland.ac.nz
AF: University of Auckland, Geology Program, Private Bag 92019, Auckland, 1142, New Zealand
AU: Nairn, I
EM: i.a.nairn@xtra.co.nz
AF: GNS Science, Wairakei Research Centre, Taupo, 2000, New Zealand
AB: The Earthquake Flat Pyroclastics (EFP) form a 10 km3 rhyolite deposit erupted at 50 ka from the margin of Okataina Volcanic Centre (OVC), immediately following the caldera-forming eruption (100 km3) of the Rotoiti Pyroclastics from vents 20 km to northeast. The EFP deposits display textural and compositional complexity on a crystal-scale consistent with remelting and stirring of a near-crystalline pluton located in the upper crust. Quartz and plagioclase crystals are extensively resorbed, while hornblende and biotite are euhedral. Pre-eruption temperatures estimated from Fe-Ti oxides cover a wide range (702 - 805°C). Differences of up to 70°C within individual pumice lapilli show that crystals were chaotically juxtaposed during magma stirring and evacuation. Al- and Ti- zoning reflects thermally-controlled atomic substitutions within hornblende crystals, with rim-ward increases in temperature of about 50°C estimated from plagioclase-hornblende geothermometry. The EFP deposits contain no evidence for widespread magma-mixing with a mafic intrusion that could have acted as a heat source; this absence implies a convective self-stirring process that was likely driven by mafic underplating. Extensive resorption of crystals deep in the EFP magma may have produced a Ca, Fe and Mg-enriched rhyolite melt that percolated upward allowing the growth of reverse zoned hornblende. Similar textural rejuvenation features are also found in late ejecta of the Rotoiti Pyroclastics, suggesting that the Rotoiti-EFP eruptions disrupted a semi-continuous, partly crystalline pluton that extended at mid-crustal depths across the OVC. Development of this pluton coincided with a period of relative volcanic quiescence lasting >100 kyrs and is consistent with previous isotope studies that demonstrate a long crystallisation history for the magmas. The rejuvenation of the EFP magma, together with microdiorite lithics in the EFP and Rotoiti deposits and a small basaltic eruption that immediately preceded the Rotoiti eruption, suggest that extensive mafic underplating provided a major thermal and volatile pulse to drive these caldera eruptions.
DE: 3618 Magma chamber processes (1036)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting