Orthopyroxene - omphacite- and garnet - omphacite-bearing magmatic assemblages, Breaksea Orthogneiss, New Zealand: Oxidation state controlled by high-P oxide fractionation

Abstract

The Breaksea Orthogneiss comprises a monzodioritic host partially recrystallised to omphacite-garnet-plagioclase-rutile granulite at 850 °C and 1.8 GPa, with metre to decametre-scale, cognate inclusions ranging from ultramafic through gabbroic to monzodioritic composition. Coarsely layered garnetite and diopsidic clinopyroxenite cumulate preserves igneous textures, whereas garnet-omphacite cumulate shows a partial metamorphic overprint to eclogite. Garnet and omphacite in undeformed to weakly deformed rocks have similar major and rare earth element characteristics reflecting their common igneous origin, pointing to a lack of metamorphic recrystallisation. Inclusions of omphacite-orthopyroxene-plagioclase-ulvöspinel orthogneiss have whole-rock compositions almost identical to the host monzodiorite. Reaction zones developed along contacts between the orthopyroxene-bearing inclusions and host contain metamorphic garnet that is microstructurally and chemically distinct from igneous garnet. The presence of orthopyroxene is interpreted to reflect redox distinctions: early, oxidised magma crystallised orthopyroxene and ulvöspinel at high-P (~ 1.8 GPa), garnet crystallisation having been suppressed. Progressive fractionation of oxygen into early formed phases (ulvöspinel, magnetite, orthopyroxene, ferric iron-rich omphacite and rare garnet) drove the magma to less oxidising conditions, resulting in the more common igneous assemblage of garnet, omphacite and rutile in the main host.