TY - JOUR
T1 - Orthopyroxene-omphacite- and garnet-omphacite-bearing magmatic assemblages, Breaksea Orthogneiss, New Zealand
T2 - Oxidation state controlled by high-P oxide fractionation
AU - Chapman, Timothy
AU - Clarke, Geoffrey L.
AU - Daczko, Nathan R.
AU - Piazolo, Sandra
AU - Rajkumar, Adrianna
PY - 2015/2/1
Y1 - 2015/2/1
N2 - 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.
AB - 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.
KW - Omphacite-garnet granulite
KW - Orthopyroxene eclogite
KW - Omphacite-orthopyroxene granulite
KW - REE
KW - Igneous omphacite
KW - EBSD microstructure
UR - http://www.scopus.com/inward/record.url?scp=84919785171&partnerID=8YFLogxK
UR - http://purl.org/au-research/grants/arc/DP120102060
UR - http://purl.org/au-research/grants/arc/FT110100070
U2 - 10.1016/j.lithos.2014.11.019
DO - 10.1016/j.lithos.2014.11.019
M3 - Article
AN - SCOPUS:84919785171
SN - 0024-4937
VL - 216-217
SP - 1
EP - 16
JO - Lithos
JF - Lithos
ER -