Abstracts Earth and Environmental Sciences

Petrographic and geochemical characterisation of the Breaksea Orthogneiss, Fiordland, New Zealand

by J. W. Watton

Characterisation of the Breaksea Orthogneiss shows four distinct lithologies : Breaksea Omphacite Granulite (BOG) is the dominant country rock; Breaksee Eclogite (BE); layered garnetite-pyroxenite and hornblende occur as inclusionary rafts within the main body. The BOG shows chemical characteristics similar to the Western Fiordland Orthogneiss, froma source at a depth below that at which plagioclase is stable. Hornblende peridotite samples is chemically analagous to the hornblende peridotite of the Hawes Head Ultramafics unit found at Charles Sound, Fiordland. Preservation of primary igneous textures and crystallisation of garnet + Tschermak's diopside is observed in the layered garnetite-pyroxenite rock. retrograde thermal re-equiilibration results in a phase transition from Tschermak's diopside to diopside, and in the presence of Na, diopside to omphacite. Re-equiilibration is observed on a mineral scale and proposed on an outcrop scale. Outcrop scale re-equilibration causing Na to diffuse from the BOG into inclusionary rafts facilitates the diopside-omphacite change in Na poor rocks, accounting for the garnet-omphacite BE unit and omphacite present in the garnetite unit. Further evidence of retrograde metamorphic activity can be observed in simplectite development in the BOG, BE and hornblende peridotite.