Geology and Petrology of Ruapehu Volcano and Related Vents

Abstract

Ruapehu Volcano is an active, multiple-vent, andesite composite volcano at the southern terminus of the Taupo Volcanic Zone, central North Island, New Zealand. The present-day volume of Ruapehu is estimated at 110 km3, and construction of the massif probably occurred during the past 0.5 m.y. Geologic mapping and stratigraphic studies have led to the recognition of four periods of cone construction, each occurring over 104-105 year time intervals. On the basis of lithologic/petrographic differences, and conspicuous unconformities which separate the deposits of each cone- building period, four new formations are defined, comprises the Ruapehu Group. Te Herenga formation (new formation name) comprises the oldest deposits of Ruapehu (upper lavas ca. 0.23 Ma) and is exposed as planeze surfaces and arêtes on N and NW Ruapehu. The formation includes lava flows, tuff breccias, and small intrusive bodies surrounded by zones of hydrothermal alteration. There is little petrographic and compositional diversity; most lavas are porphyritic titanomagnetite- augite- hypersthene- plagioclase basic andesites. Wahiance Formation (new formation name) is younger than Te Herenga Fm,. but of unknown age. It is well exposed on SE Ruapehu, and comprises mostly lava flows and tuff breccias. The lavas comprise acid and basic andesites. Mangewhero Formation (new formation name) is well exposed everywhere except SE Ruapehu, and the upper lavas and pyroclastics (ca. 0.02 Ma) form the present high peeks and main cone of Ruapehu. The lavas are petrographically and geochemically diverse, ranging from basalt to decite in bulk composition. Some of the lower lavas are olivine-beering andesites of hybrid orgin. Whakapapa Formation (new formation name; ca 15,000 years to present) comprises conspicuously young lava flows, tuff breccias, airfall pyroclastics and minor pyroclastic flows of acid- and basic andesite. The deposits of these post-glacial summit and flank eruptions are subdivided into the lwikau, Rangataua, Tama and Crater Lake Members. ‘Related vents’ produced Heuhungatahi Andesite Fm. (> 0.5 Ma?), and Holocene deposits of basalt and basic andesite at isolated, monogenetic centres comprising Ohakune Andesite Fm., Pukeonake Andesite Fm., and Waimarino Basalt Fm. (new formation name). Most Ruapehu lavas are medium-K acid and basic andesites (mean of 144 bulk rock analyses is 57.8 wt % SiO2), but rare basalt and minor decite are present. Nearly all lavas are porphyritic in plagioclase, augite and hypersthene ± olivine, with titanomagnetite micro- phenocrysts, and contain abundant metamorphic and igneous rock inclusions. Petrography, mineral chemistry and bulk rock chemistry indicate fractional crystallization series from parental basalts (52-53 % SiO2, Q-normative, low-alumina) to medium-K basic- and acid andesites (58-59 % SiO2). Early fractionating minerals are olivine and clinopyroxene with minor chrome spinel and plagioclase, followed by plagioclase, orthopyroxene, clinopyroxene and minor titanomagnetite in later stages of differentiation. Thus, basalt differentiation to produce andesites involves ‘POAM-type’ (Gill, 1981) fractional crystallization. Three second-order differentiation processes operate concurrently with frational crystallization: (1) Crystal accumulation involves addition of co-genetic plutonic rock fragments and crystals derived from them. These inclusions are common and few rocks represent liquid compositions. (2) Magma mixing involves mingling of magmas in repeatedly- occupied conduits. End members are as diverse as basalt and decite, yielding petrogaphically and chemically distinctive high-Mg andesites of the upper cone complex and parasitic centres. (3) Selective crustal assimilation is suggested by partially fused metamorphic inclusions, positive correlation of 87Sr/86Sr with SiO2, and failure of simple ‘POAM’ fractionation to explain decites (63-65 % SiO2). Petrogenesis of Ruapehu andesites takes place under open-system condition, involving production of parental Q-normative basalts in the mantle wedge, concurrent fractional crystallization and crustal contamination, entrainment of co-genetic plutonic rocks, and mixing of magmas in common conduits.