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