Dynamics of Crystal-rich Ignimbrites: Investigation of the Whakamaru Supervolcano System, New Zealand

Large caldera-forming ignimbrite eruptions represent a significant hazard and yet their eruptive dynamics and underlying magma chamber processes are poorly understood. Here preliminary results are presented from a new crystal-specific investigation of the ~340 ka crystal-rich Whakamaru group ignimbrites from the Taupo Volcanic Zone, representing products of the largest known eruption in New Zealand history (>1000km3 magma). Whole-rock geochemistry indicates that juvenile components within the five identified ignimbrite units - Whakamaru, Manunui, Rangitaiki, Te Whaiti, and Paeroa - are of rhyolitic composition and form discrete compositional groupings. Mineral and glass chemistry data, obtained by electron microprobe, provide further constraints on these compositional groupings of magmatic bodies. Petrologically, pumice from all units is similar, being crystal-rich and characterised by crystal assemblages of quartz, plagioclase, ilmenite and magnetite, with varying ferromagnesian populations (amphibole, orthopyroxene, biotite) and trace zircon and apatite. Plagioclase crystals are characterised by normal zoning (calcium-rich cores and sodium-rich rims) and display strong oscillatory zoning under polarised light, with abundant fluid inclusions. Mineral chemistry of Rangitaiki ignimbrite pumice is distinct from Whakamaru pumice, being characterised by high-An plagioclase, common augite phenocrysts, higher temperatures, and petrographic and geochemical features suggestive of magma mixing processes. Geothermometry of co- existing oxides indicate temperatures of ~800°C for the parental magmas feeding the large ignimbrite units.