Metamorphites at Wilmot Pass, Central Fiordland, New Zealand.

Gibson, George McCartney.

Two distinct petrographic provinces are recognised in central Fiordland. A Doubtful Sound Province in which metasediments of miogeosynclinal origin predominate and a western Manapouri Province which largely comprises gneisses of inferred eugeosynclinal provenance. The provinces are juxtaposed along a major fault zone. Synmetamorphic granitoids characterise both regions but gabbroids and their metamorphosed equivalents (Mt George Metabasites) have only been observed in the Doubtful Sound Province; post-tectonic granitoids are recognised only in the Western Manapouri Province. A model is proposed whereby the two provinces were juxtaposed during the Tuhua Orogeny.
The Doubtful Sound Province has experienced multiphase deformation and metamorphism. First generation folds are uncommon, isoclinal, and developed during kyanite grade regional metamorphism. They are almost everywhere strongly overprinted by north-south trending second generation structures. Metamorphic reconstitution also accompanied second generation folding and is most pronounced within the environs of the Mt George Metabasites. A thermal aureole is recognised in which sillimanite and sillimanite-K feldspar zone assemblages have developed. Hornsfelsic rocks are absent and emplacement of the metabasites is inferred to have occurred whilst the country rocks were still undergoing regional metamorphism. Assemblages within the metabasites and country rocks are generally cofacial, although locally metamorphism under more anhydrous conditions has produced granulite and hornblende-granulite facies rocks within the former. A third generation of folding producing broad, open folds was accompanied by minor recrystallisation under greenschist-lower amphibolite facies conditions. Fold axes are dominantly north--south.
The Doubtful Sound Province is everywhere affected by faulting. Two periods of faulting are recognised: an early period of thrust faulting which followed or occurred late during second generation folding, and a subsequent period of high-angle, predominantly reverse faulting. Thrust faulting was directed from west to east and carried lower crustal rocks (the Black Giants Complex) into the Doubtful Sound Province. The complex comprises an incomplete sequence.of layered intrusive rocks that have been metamorphosed under extremely high pressures. They constitute the deepest structural level rocks yet reported in Fiordland.

315 p., 37 leaves of plates ; 30 cm. + + 3 maps in pocket.

1979Gibson

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