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.

The Fraser Complex: High‐grade metamorphic, igneous and mylonitic rocks in central Westland, New Zealand

“…Hattori (1967) and Grew (1977) identified kyanite-and sillimanite-bearing gneisses from the Maruia area, part of the Glenroy Metamorphic Complex of Tulloch & Braithwaite (1986), and Mason & Taylor (1987) described similar rocks from Rotomanu and the Ahaura River area. Mason & Taylor (1987) also described hornblende gneisses and a large number of granitoids within the gneiss terrains.…”

Gamet compositions in metamorphic rocks and granites of Westland, New Zealand

“…This concentrate was expected to contain garnets with more than 4.5% MgO, since high-MgO garnets occur in the Fraser Formation within the Mikonui watershed; their absence from the concentrate is ascribed to the relatively small area of outcrop of this formation relative to the Haast Schist. o < 1% (Hattori, 1967), these garnets were not found in the beach sands, presumably because they form only a very small percentage of the total garnet component. In order to obtain complete coverage, a search was made for granite-derived garnets.…”

“…The sillimanite-almandine association has been recorded by Hattori (1967) not only from the Hokitika River but also from the Glenroy River in southeast Nelson. Grew (1977) has confirmed the latter occurrence, and his map shows sillimanite occurrences near the Cascade River in south Westland, in Fiordland, and in Stewart Island.…”

Garnet-bearing sands from the east coast, North Island, New Zealand

“…Theit closest-known equivalents in New Zealand are the pyralspites in garnet. biotite gneisses south of Hokitika, which accordin~ to Hattori (1967) formed under low pressure, intermediate metamorphic conditions. However, garnets of this composition usually occur in gneisses with hi~h-grade mineral assemblage typical of the granulite facies of metamorphism.…”

“…;inally from a varied series of gneisses and associated irroeous and metamorphic rocks that were products of granulite facies metamorphism. Brown 1967Brown , 1969Hattori 1967;Mason 1962;Myashiro 1955;Pabst 1938;Reed 1958;Trager 1959;Williams 1934).…”