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

Detailed mapping and sampling on Turoa ski-field and surrounding areas on M t Ruapehu has enabled identification of individual flow packets that repres ent small scale eruptive events during the major cone-building episodes pre viously identified on Ruapehu by Graham & 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 lava s. Although the lavas are from a relatively small area of the volcano, geoc hemical and isotopic compositions show a range in variation similar to that observed for the entire volcano. Examination of geochemical variations bet ween individual flow packets and also between sequential lava flows indicat es 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 composi tionally distinct regions of the same chamber. It is probable that a comple x plumbing system exists beneath Ruapehu with at least two levels of magma storage, evolution, and crustal interaction. Consequently, attempts to nume rically quantify such a complex system using models such as assimilation-fr actional crystallisation processes are inherently oversimplifications.