Pre-eruption thermal rejuvenation and stirring of a partly crystalline rhyolite pluton revealed by the Earthquake Flat Pyroclastics
deposits, New Zealand
CATHERINE MOLLOY
PHIL SHANE
IAN NAIRN
10.6084/m9.figshare.3454499.v1
https://geolsoc.figshare.com/articles/journal_contribution/Pre-eruption_thermal_rejuvenation_and_stirring_of_a_partly_crystalline_rhyolite_pluton_revealed_by_the_Earthquake_Flat_Pyroclastics___deposits_New_Zealand/3454499
<p>The Earthquake Flat Pyroclastics form a <em>c</em>. 10 km<sup>3</sup> rhyolite deposit erupted at <em>c</em>. 50 ka from the margin of Okataina Volcanic Centre, immediately following the caldera-forming eruption of the Rotoiti Pyroclastics
(<em>c</em>. 100 km<sup>3</sup>) from vents <em>c</em>. 20 km to the NE. Earthquake Flat Pyroclastics deposits display textural and compositional complexity on a crystal-scale
consistent with rejuvenation of a near-crystalline pluton in the upper crust. Quartz and plagioclase crystals are resorbed,
whereas hornblende and biotite are euhedral. Fe–Ti oxides indicate large variations in pre-eruption temperatures (702–805
°C). Differences of up to 70 °C within pumice lapilli show that crystals were chaotically juxtaposed during magma stirring
and evacuation. Chemical zoning within hornblende crystals is consistent with rimward increases of <em>c</em>. 50 °C. These features are consistent with a convective self-stirring process. Previous isotope studies demonstrate a long
(>100 ka) crystallization history for the magma. Resorption of crystals deep in the magma may have produced a Ca-, Fe- and
Mg-enriched rhyolite melt that allowed the growth of reverse-zoned hornblende. Microdiorite lithic fragments in the Earthquake
Flat Pyroclastics and Rotoiti deposits and a basaltic eruption that immediately preceded the Rotoiti eruption suggest that
mafic underplating beneath Okataina Volcanic Centre provided a major thermal and volatile pulse to drive the caldera eruptions.
</p>
2016-06-21 12:10:02
hornblende crystals
Previous isotope studies
New Zealand
rhyolite pluton
20 km
magma
plagioclase crystals
Pyroclastics deposits
chemical zoning
Pyroclastics form
Microdiorite lithic fragments
basaltic eruption
Rotoiti eruption
Rotoiti deposits
mafic underplating
10 km 3 rhyolite deposit
NE
100 km 3
Fe
Pyroclastics deposits display textural
50 ka
rimward increases
rejuvenation
crystallization history
pumice lapilli show
Okataina Volcanic Centre
vents c
caldera eruptions
Rotoiti Pyroclastics
Geology