Journal of Petrology, Volume 40, Issue 1: January 1999.

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

R. C. PRICE¹, R. B. STEWART², J. D. WOODHEAD³ AND I. E. M. SMITH⁴

¹SCHOOL OF SCIENCE AND TECHNOLOGY, THE UNIVERSITY OF WAIKATO, PRIVATE BAG 3105, HAMILTON, NEW ZEALAND
²DEPARTMENT OF SOIL SCIENCE, MASSEY UNIVERSITY, PALMERSTON NORTH, NEW ZEALAND
³SCHOOL OF EARTH SCIENCES, UNIVERSITY OF MELBOURNE, PARKVILLE 3052, VIC., AUSTRALIA
⁴DEPARTMENT OF GEOLOGY, UNIVERSITY OF AUCKLAND, AUCKLAND, NEW ZEALAND

Egmont Volcano (Mt Taranaki) is located 140 km west of the Taupo Volcanic Zone (TVZ), the principal locus of volcanic activity in the North Island of New Zealand, and is one of four closely associated Quaternary andesitic volcanoes in Taranaki province. Taranaki eruptives are enriched in K and other large ion lithophile elements compared with their counterparts at Ruapehu in the southern TVZ, with the youngest Egmont andesites being the most K rich. Egmont andesites are invariably fractionated but isotopic information indicates that, unlike those at Ruapehu, they have not extensively assimilated enriched crust. Ti/Zr, Ba/La, Ce/Pb, and K/Rb ratios indicate that a more depleted mantle wedge and compositionally different slab-derived fluids were involved in the generation of Taranaki primary magmas. Magmas parental to Egmont eruptives were relatively undersaturated, hydrous, high-Mg basalts generated by low degrees of partial melting in a depleted mantle wedge fluxed by deep slab fluids. Fractionation of these magmas at the base of the crust produced basaltic andesite and extensive ultramafic cumulates. Plagioclase fractionation was suppressed by high a_H2O. Rising geothermal gradients eventually resulted in partial anatexis of amphibolitic underplated crust, and interaction of basaltic andesites with these melts led to progressively more K-rich compositions.

Keywords: cross-arc variation;high-K andesite;subduction magmatism

Pages 167-197

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