Most Ruapehu lavas and those of related vents (Taupo Volcanic Zone, New Zealand) are calc-alkaline, medium-K basic and acid andesites, though minor volumes of basalt and dacite occur. Nearly all are porphyritic with phenocrysts of plagioclase, augite, olivine (mainly in basalts and basic andesites), orthopyroxene (mainly in acid andesites and dacites), and titanomagnetite (chrome spinel in basic lavas). The lavas have been subdivided into six groups, each petrographically, geochemically and isotopically distinct: Type 1 plagioclase-pyroxene phyric lavas dominate, and range from basalt to dacite. Least squares mass balance calculations indicate that these lavas were probably generated from low-alumina basalt by combined crystal fractionation (15–55 per cent) and crustal assimilation (1–30 per cent). Xenolith studies indicate that the assimilant is most likely to be a partial melt of gneiss, originally Torlesse terrane greywacke. Crystal accumulation occurs to a minor extent in Type 1 lavas and becomes important in Type 2 (plagioclase-phyric) and Type 3 (pyroxene-phyric) lavas. Type 4 lavas are rare and of unknown origin, though they may be similar to rare hornblende-bearing andesites from nearby Maungakatote volcano. Type 5 lavas are clinopyroxene-olivine-phyric andesites which were probably generated from a primitive basalt by crystal fractionation without crustal assimilation. Type 6 lavas show strong evidence of disequilibrium and were probably generated by mixing Type 5 basalt with Type 1 dacite in proportions of between 60:40 and 50:50. The assertion that the assimilant involved in contamination of most Ruapehu andesites is a partial melt of basement greywacke is a significant departure from previously published theories and has important implications for trace element and isotopic modelling.
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Graham et al. (1987) studied this question.