Occurrence of Sillimanite-Garnet-Biotite Gneisses and Their Significance in Metamorphic Zoning in the South Island, New Zealand

Abstract

Boulders of garnetiferous gneisses from Davis Creek, Matakitaki, and from Snark Creek and Mistake Gully, south of Hokitika, were found to contain sillimanite. The gneisses were probably derived from thin layers interbedded in coarse-grained biotitequartzofeldspathic gneiss near the Alpine Fault. They are of two types: ilmenitegraphite- garnet-biotite gneiss, and rutile-sillimanite-garnet-biotite gneiss. The latter is isochemically similar to New Zealand Alpine feldspathic greywacke or Alpine feldspathic argillite. Low oxygen-pressure conditions during metamorphism are suggested by the chemical composition of the gneisses, specially in the graphite-bearing gneiss. The garnet in the gneiss differs from that found in the Alpine Schists, and the biotite is probably of the Ryoke-Abukuma type, i.e., the low-pressure type of regional metamorphism. The sillimanite-garnet-biotite gneisses do not represent the highest grade of the Alpine and Otago Schists, but are formed by an independent type of metamorphism; they were probably formed under P-T conditions of 500°c ana 6 kb. It is concluded that the gneisses are a low-pressure intermediate metamorphic group to the west of the Alpine Fault, and the Alpine, Otago, and Marlborough Schists are a high-pressure intermediate group to the east; together they form paired metamorphic belts in a manner similar to that postulated elsewhere in the Circum-Pacific Region. © 1967 Taylor & Francis Group, LLC. All rights reserved.