Stratigraphy and geochemistry of the Turoa area, with implications for andesite petrogenesis at Mt Ruapehu, Taupo Volcanic Zone, New Zealand

Abstract

Detailed mapping and sampling on Turoa skifield and surrounding areas on Mt Ruapehu has enabled identification of individual flow packets that represent small scale eruptive events during the major cone-building episodes previously identified on Ruapehu by Graham and Hackett. The area is dominated by plagioclase-pyroxene-phyric andesitic lavas of Mangawhero Formation, which are petrographically and geochemically typical of post-120 ka Ruapehu lavas. Although the lavas are from a relatively small area of the volcano, geochemical and isotopic compositions show a range in variation similar to that observed for the entire volcano. Examination of geochemical variations between individual flow packets and also between sequential lava flows indicates complex processes of assimilation and influx of 'new', variably evolved and fractionated magmas into high level magma chambers. These chambers are most likely heterogeneous, and individual eruptions may also sample compositionally distinct regions of the same chamber. It is probable that a complex plumbing system exists beneath Ruapehu with at least two levels of magma storage, evolution, and crustal interaction. Consequently, attempts to numerically quantify such a complex system using models such as assimilation-fractional crystallisation processes are inherently oversimplifications.