Volcanology and geochemistry of the Kaingaroa Ignimbrite, Taupo Volcanic Zone, New Zealand.

Abstract

The 0.23 Ma Kaingaroa Ignimbrite is a composite multiple flow-unit ignimbrite erupted from Reporoa Caldera, Taupo Volcanic Zone (TVZ), New Zealand. The Kaingaroa Ignimbrite has a complex internal stratigraphy with a complex basal tephra sequence of intercalated fall, surge and flow deposits, and three ignimbrite units, with strikingly proximal to medial facies variation. Proximal facies deposits are dominated by coarse lithic breccias up to 45m thick which are interpreted as co-ignimbrite lag breccias. These lag breccias are-some of the thickest so far documented. Welding and thickness variations in the extensive Old Waiotapu Rd (OWR; kg1) and Webb ignimbrite unit (WIU; kg2) suggests gradual thickening away from source, interpreted to represent ponding in a shallow alluvial lowland or basin. A detailed lithic componentry study indicates changes in lithic diversity and abundance between stratigraphic units which mark changes in vent conditions, increasing depth of lithic provenance and hence inferred fragmentation level. Lithic fragments reveal aspects of the sub-caldera geology, which is dominated by an andesitic volcano with leuco-gabbroic subvolcanic roots, intercalated welded ignimbrites, rare low-grade metasedimentary basement and meta-rhyolites. Gabbros and meta-rhyolites suggest complex metasomatic and fumarolic processes adjacent to the Kaingaroa magma system. The presence of tourmaline-bearing meta-rhyolites and meta-ignimbrites and tourmalinite is the first documented occurrence of tourmaline and tourmalinite in TVZ. Four pumice types are defined on pumice chemistry and mineralogy. These pumices are interpreted to represent samples of a weakly continuously zoned magma chamber (70-75% SiO2), which was progressively tapped during the eruption. Trace element and rare earth element systematics are consistent with an origin of type A magma from a type D parent by minor fractionation of plagioclase, zircon, and trace contents of Fe/Ti oxides and orthopyroxene. An additional hornblende-, 2-pyroxene-phyric dacite pumice/bleb (69% SiO2) was sampled from the Tokiaminga sub-unit, but is mineralogically and compositionally different from Kaingaroa pumices. Post-caldera rhyolites are mineralogically and chemically variable, with broad similarities to Kaingaroa pumices. The Kaingaroa magma components show reverse isotopic zonation i.e. decreasing 87Sr/86Sr and increasing 143Nd/144Nd with differentiation, suggesting syn-eruptive mingling and evisceration of the multiple magma batches occurred during the climactic caldera collapse phase. The Kaingaroa Ignimbrite has been mis-correlated by previous workers with the Matahina, Mamaku, and Rangatira Point ignimbrites, and three new units described in this thesis; Kawerau ignimbrite, Wheao sheet, and the welded ignimbrite of Wairakei drill holes. It is clear that ignimbrite correlation is difficult in TVZ because of the poor exposure and the limited stratigraphic sections that document multiple units. The Kawerau ignimbrite remains an enigma, largely because of the anomalously high Zr, Hf and Zn contents, suggestive of a relationship to 'alkaline' rhyolites, and the presence of unusual magnesium poor manganoan fayalite of vapour-phase origin. Identification of these units and other intermediate size ignimbrite in the stratigraphic interval between Whakamaru-group, and Mamaku ignimbrites requires further careful documentation, but suggests a temporal clustering of ignimbrites sourced from throughout TVZ.