Petrogenesis of High-K Arc Magmas: Evidence from Egmont Volcano, North Island, New Zealand

Egmont Volcano (Mt Taranaki) is located 140 km west of the INTRODUCTION Taupo Volcanic Zone (TVZ), the principal locus of volcanic activity Egmont Volcano is a 2518 m high stratovolcano (Mt in the North Island of New Zealand, and is one of four closely Taranaki) resting on an extensive ring plain of volcanic associated Quaternary andesitic volcanoes in Taranaki province. debris in Taranaki province, western North Island, New Taranaki eruptives are enriched in K and other large ion lithophile Zealand. It is the youngest and largest of four Taranaki elements compared with their counterparts at Ruapehu in the southern volcanoes (Fig. 1), which together form a NW–SE trendTVZ, with the youngest Egmont andesites being the most K rich. ing lineament along which activity has migrated southEgmont andesites are invariably fractionated but isotopic information eastward with time (Neall et al., 1986). The Taranaki indicates that, unlike those at Ruapehu, they have not extensively volcanoes lie 140 km to the west of the Taupo Volcanic assimilated enriched crust. Ti/Zr, Ba/La, Ce/Pb, and K/Rb Zone (TVZ), the principal locus of subduction-related ratios indicate that a more depleted mantle wedge and compositionally magmatism in the North Island. different slab-derived fluids were involved in the generation of There are significant petrological contrasts between Taranaki primary magmas. Magmas parental to Egmont eruptives Egmont Volcano and the contemporaneous andesitic were relatively undersaturated, hydrous, high-Mg basalts generated stratovolcanoes of the southern TVZ, collectively referred by low degrees of partial melting in a depleted mantle wedge fluxed to as the Tongariro Volcanic Centre (TVC). For example, by deep slab fluids. Fractionation of these magmas at the base of the Egmont andesites are commonly hornblende bearing crust produced basaltic andesite and extensive ultramafic cumulates. whereas those of the TVC are generally clinopyroxene Plagioclase fractionation was suppressed by high aH2O. Rising and orthopyroxene types. Egmont eruptives are also geothermal gradients eventually resulted in partial anatexis of relatively potassic (Neall et al., 1986; Price et al., 1992; amphibolitic underplated crust, and interaction of basaltic andesites Stewart et al., 1996); they are high-K andesites according to the classification of Gill (1981). with these melts led to progressively more K-rich compositions. Both Taranaki and TVC are active volcanic systems associated with the Tonga–Kermadec–New Zealand convergent plate margin. On the basis of isotopic data, Price et al. (1992) suggested that both volcanic centres lie above mantle wedge with broadly similar chemical characteristics. The volcanic systems are, however, located on either side of a major terrane boundary and they