Petrography, mineralogy and chemistry of Titiraupenga volcano, North Island, New Zealand

Titiraupenga is an early Pleistocene volcano sited approximately 20 km northwest of Lake Taupo, North Island, New Zealand, at the junction of the Taupo Volcanic Zone and a line of andesite volcanoes extending from Ohakune in the south to Waiheke Island in the north. Exposures on the volcano are restricted to a central plug (type 1 andesite) and rare exposures of flows on the lower slopes (type. 2 andesite). Both type 1 and type 2 andesites are porphyritic with phenocrysts of orthopyroxene, augite, plagioclase, and scarce olivine in a groundmass of plagioclase, pyroxene, and iron OXIde. Pigeonite occurs only in the groundmass and as reaction rims to orthopyroxene phenocrysts in type 1 andesites. Chemically, most samples are low‐Si andesites, and variation is probably due to varying degrees of pyroxene accumulation within the magma reservoir. Type 2 andesites are inhomogeneous and probably result from mixing of magmas from different parts of the reservoir. Aphyric lenses (type 3 andesite) within type 2 andesites are thought to represent a wholly liquid portion of the reservoir which was incorporated in the type 2 magma during eruption. Slight differences in isotopic composition of type 3 magma may indicate some crustal contamination.