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 Taupo Volcanic Z one (TVZ), the principal locus of volcanic activity zn the North Island of New Zealand, and is one of four closely, associated Quaternary andesitic vo lcanoes in Taranaki province. Taranaki eruptives are enriched in K and othe r large ron lithophile elements compared with their counterparts at Ruapehu in the southern TVZ with the youngest Egmont andesites being the most K ri ch. Egmont andesites are invariably fractionated bat isotopic information i ndicates that, unlike those at Ruapehu, they have not extensively assimilat ed enriched crust. Ti/Zr, Ba/La, Ce/Pb, and K/Rb ratios indicate that a mor e depleted mantle wedge and compositionally different slab-derived fluids w ere involved in the generation of Taranaki primary magmas. Magmas parental to Egmont eruptives were relatively undersaturated, hydrous, high-Mg basalt s generated by low degrees of partial melting in a depleted mantle wedge fl uxed by deep slab fluids. Fractionation of these magmas at the base of the crust produced basaltic andesite and extensive ultramafic cumulates. Plagio clase fractionation was suppressed by high a(H2O) Rising geothermal gradien ts eventually resulted in partial anatexis of amphibolitic underplated crus t. and interaction of basaltic andesites with these melts led to progressiv ely more K-rich compositions.