Chemical and mineralogical characterization of pyroclastic deposits from the ca. 1 Ma Kidnappers and Rocky Hill eruptions, Taupo Volcanic Zone, New Zealand

Schipper, C. Ian. (Christopher Ian)

The Kidnappers deposits and Rocky Hill lgnimbrite are the products of two large caldera-forming rhyolite eruptions from Mangakino Caldera, Taupo Volcanic Zone, New Zealand. The eruptions followed each other in short succession at ea. 1 Ma, and tapped a common, largely homogeneous magma chamber. The magmas tapped by each eruption eoevolved by a process of plagioclase-dominated crystal fractionation. The Kidnappers deposits include a widespread phreatoplinian fall deposit overlain by a voluminous, unwelded ignimbrite. The Kidnappers Ignimbrite contains white pumices < 40 cm, with 70-90% parallel-aligned, tubular vesicles. Phenocrysts account for 6% of solid material (e.g., crystals+ glass), in the assemblage: plagioclase (74%) +hornblende (8%) + Fe-Ti oxides (7%) + orthopyroxene (5%) + biotite (5%) +quartz (1 %) ±zircon± apatite. Whole-pumice SiOz content ranges from 71-75 wt.%. Plagioclase composition ranges An31 -A1146, hornblende is predominantly ferri-magnesiohornblende, orthopyroxene is in the hypersthene (E1146Fss2 Woz) - ferrohypersthene (EnszFS4s Wo3) range, magnetite has compositions Xutv=30-39%, and ilmenite has composition Xilm=89-93%. Fe-Ti oxide geothermometry/ geobarometry yields temperature estimates of745-855 °C, and log f02 estimates of -12.02 to -15.40. The Kidnappers fall deposits show increasing crystal content and increasingly evolved glass compositions from stratigraphic bottom to top. The moderately welded Rocky Hill lgnimbrite contains glomeroporphyritic pumices < 10 cm, with 60-80% tubular vesicles. Phenocrysts account for 12% of solid material, in the assemblage: plagioclase (80%) +quartz (6%) +hornblende (6%) + Fe-Ti oxides ( 4%) + orthopyroxene (3%) + biotite (2%) ± zircon± apatite. Whole-pumice Si02 content ranges from 69-76%. Plagioclase composition ranges Anz7-A1143, hornblende is predominantly ferri-magnesiohornblende with minor ferritschermakite, orthopyroxene is in the hypersthene (E1145Fs54 Wo1) - ferrohypersthene (EnssFs43 Woz) range, magnetite has compositions Xutv=29-33%, and ilmenite has composition Xilm=89-94%. Fe-Ti oxide geothermometry/ geobarometry yields temperature estimates of755-827 °C, and log f02 estimates of -13.20 to -15.25. The Rocky Hill lgnimbrite contains a single population of glass, which has statistically different chemistry from glass in the Kidnappers lgnimbrite and fall deposits. The Kidnappers and Rocky Hill magmas eo-evolved by a process of plagioclasedominated convective fractionation in a largely homogeneous, low aspect ratio (height:width) magma chamber at Mangakino Caldera. The opening of a narrow central vent into the chamber marked the onset of volcanic activity at ea. 1 Ma, and led to apparent reverse zonation of glass evolution through the Kidnappers falL deposits.

xi, 183, [34] leaves : ill. ; 30 cm.

2004Schipper

Abstract Only