MAGMA MINGLING IN AN ANDESITE PYROCLASTIC FLOW OF THE POURAHU MEMBER,RUAPEHU VOLCANO, NEW-ZEALAND

We describe a magma mingling episode from Ruapehu volcano between two andesite magmas, one very much minor in volume relative to the other. The event acted to trigger eruption of the andesitic Pourahu pyroclast ic flow which is preserved in a thick sequence of tephras and laharic deposits in the southeastern ring plain of the volcano. The predominan t andesite is pale brown coloured and porphyritic containing phenocrys ts of plagioclase-clinopyroxene-orthopyroxene-Fe-Ti oxides. Rare clast s of a darker andesite are different texturally, less vesicular, and c ontain distinctive microphenocrysts of plagioclase and quench olivine. Equally rare clasts, of streaky pumice consisting of interbanded 'dar k' and 'light' andesite attest to mingling between these two andesite components. Chemical analyses of discrete clasts demonstrate that the Pourahu pyroclastic flow andesites span much of the compositional spec trum of Ruapehu andesites. This observation demonstrates heterogeneity in the products of a relatively small eruption. The darker clast anal yses and those from associated distal fall deposits lie within the fie lds defined by the dominant light coloured clasts. Phenocryst and micr ophenocryst geothermometry suggest slightly higher temperatures in the dark component. However, glasses from groundmass and phenocryst inclu sions in the same specimen may differ considerably, leading us to conc lude that many phenocrysts are in fact xenocrystic and were incorporat ed in the melts as they migrated towards the surface. We prefer a mode l in which a small volume of hot andesite magma injects a vent-feeding magma chamber, triggering vesiculation and eruption. We infer that th e process of magma withdrawal extended downward into the magma body ca using the dark component to intermingle with the lighter (dominant) co mponent, 'sucking' more dark magma into the chamber. Our observations are entirely consistent with the existence of a plexus of small, possi bly interlinked magma chambers beneath Ruapehu.