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Journal of Petrology | Volume 40 | Number 1 | Pages 167-197 | 1999
© Oxford University Press 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

1 School of Science and Technology, the University of Waikato Private Bag 3105, Hamilton, NEW ZEALAND
2 Department of Soil Science, Massey University Palmerston North, New Zealand
3 School of Earth Sciences, University of Melbourne Parkville 3052, Vic., AUSTRALIA
4 Department of Geology, University of Auckland Auckland, NEW ZEALAND

Received May 27, 1997; Revised typescript accepted June 1, 1998


  Abstract

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 on of four closely associated Quaternary andesitic volcanoes in Taranaki province. Taranak eruptives are enriched in K and other large ion lithophile elements compared with thei counterparts at Ruapehu in the southern TVZ, with the youngest Egmont andesites being th most K rich. Egmont andesites are invariably fractionated but isotopic information indicate 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 aH2o. 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.

KEY WORDS: cross-arc variation; high-K andesite; subduction magmatism

* Corresponding author. Telephone: 64 7 838 4520. Fax: 64 7 838 4218. e-mail: rprice{at}waikato.ac.nz


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