Weak chemical and mineralogical zonation in the Kaingaroa Ignimbrite, Taupo Volcanic Zone, New Zealand

The 0.23 Ma Kaingaroa Ignimbrite is a composite, multiple flow-unit ignimbrite erupted from the Reporoa Caldera, Taupo Volcanic Zone, New Zealand. Reporoa Caldera evolved from a single vent during the initial stages of the Kaingaroa eruption, to a trapdoor caldera with the opening up of fractures along the eastern margin and asymmetric eruption of early phases of the ignimbrite. Finally, plate collapse occurred during the later stages of the eruption. Kaingaroa Ignimbrite pumice clasts range in composition from dacite to rhyolite. Five pumice types have been identified based largely on the geochemical variation of Rb and Sr and ferromagnesian mineral composition. Pumice types A, B, C, and D exhibit variations in mineralogy, trace element chemistry, and isotopic composition consistent with derivation from a weakly zoned magma chamber. A subordinate dacitic juvenile component (type E) appears to be unrelated to the Kaingaroa pumices by crystal fractionation or assimilation, and is interpreted as a distinct magma batch that was incorporated into the eruption. The asymmetrical removal of magma from the chamber during caldera collapse is inferred to have resulted in a greater degree of drawdown along the eastern margin of the chamber, as suggested by the current spatial distribution of post-caldera rhyolites.