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

Timothy Chapman*, Geoffrey L. Clarke, Nathan R. Daczko, Sandra Piazolo, Adrianna Rajkumar

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

16 Citations (Scopus)

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.

Original languageEnglish
Pages (from-to)1-16
Number of pages16
JournalLithos
Volume216-217
DOIs
Publication statusPublished - 1 Feb 2015

Keywords

  • Omphacite-garnet granulite
  • Orthopyroxene eclogite
  • Omphacite-orthopyroxene granulite
  • REE
  • Igneous omphacite
  • EBSD microstructure

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