Petrology

BREAKSEA ORTHOGNEISS

Exposures of omphacite granulite are heterogeneous on a cm-scale with extensive dioritic gneiss plagioclase-rich domains that comprise ~30-50 % of the rock mass, (Fig. I). These alternate with 2-3 mm podiform gabbroic gneiss domains rich in omphacite and garnet that form 70-50 % of the rock mass, (Fig. I). In most cases, both layers form elongate ellipsoid stringers that pinch and swell to define a penetrative S1 fabric. Often coaxial, post-S1 symplectites are characterised by the pseudomorphing and recrystalistaion of omphacite by sodic diopside-plagioclase. A low angle S2 amphibolite facies foliation attenuates the relic gneissic fabric within the Breaksea Orthogneiss and its contained eclogite. These, often localised, amphibolite shear zones and secondary S2 amphibolite fabrics are obserevd to be coplanar to the RISZ.
The omphacite granulite contains a series of dark green pods to lenses and semi-continuous layers to dykes of eclogite and orthopyroxene eclogite (Fig. II & III). Eclogite exposures occur as cm- to several m-scale boudins and thin, semi-continuous layers or dykes that are interpreted to have intruded the Breaksea protolith and/or have been emplaced syn-intrusively as dykes and sills. The S1 gneissic foliation is prominent within the eclogite bodies and is defined by garnet and pyroxene-rich domains, that are commonly coplanar with the S1 within the omphacite granulite host (Fig. I). Localised S2 retrograde amphibolite overprint and fabric attenuation is also observed within the eclogite bodies.

Fig. I

Fig. II

Fig. III

Omphacite granulite contains concordant alternating domains (3-4mm thickness) of gabbroic to dioritic gneiss rich in omphacite, garnet and rutile together with plagioclase, quartz, garnet, antiperthite and kyanite, respectively (Fig. I & II). Granoblastic omphacite is the dominant mineral in the mafic domains with minor subhedral rutile, ilmenite and magnetite, quartz, and apatite. coarse-grained garnet is also common and occurs as clusters towards the mafic domain margins.
Felsic domains mostly comprise granoblastic plagioclase and antiperthite, with minor quartz, garnet, rutile, ilmenite-magnetite, apatite, and tabulate kyanite. Coarse-grained garnet is less abundant in the felsic domain; it is generally euhedral and occurs as large isolated grains or aggregates of small grains. Garnet may be poikilitic in both mafic and felsic domains, with quartz, plagioclase, omphacite, rutile, ilmenite and magnetite and apatite inclusions. Rutile exsolution lamellae within the cores of garnet are also commonly observed.
Post-S1 symplectites on garnet consist of hornblende or biotite with plagioclase, quartz and ilmenite in mafic domains (Fig. IV), whereas in the felsic domains such symplectites are armored by orthopyroxene. Omphacite displays pseudomorphing by sodic-diopside and plagioclase. D2 amphibolitisation induces severe recrystallisation with omphacite consumed to form skeletal to spongy amphibolite symplectites or coarse-grained amphibole and clinozoisite textures (Fig. IV, V). Felsic domains are reduced to interlobate plagioclase, antiperthite and quartz with or without clinozoisite rods. Fine-grained, dynamically recrystallised tracks of seriate plagioclase and quatrz are also common.
The entrained eclogite is defined by a polygonal groundmass consisting of omphacite, garnet and rutile. Minor fine-grained quartz and apatite occur at crystal grain junctions. Orthopyroxene eclogite is characterised by coarse anhedral orthopyroxene surrounded by coarse-grained garnet and magnetite-and-ilmenite intergrowths set within a matrix dominated by granoblastic omphacite (Fig. II & III). Rutile is occasionally found in association with ilmenite and garnet, along with quartz and apatite. Omphacite occurs as elongate stringers of polygonal grains that surround clusters of garnet and orthopyroxene (Fig. III).
Post-S1 decompression symplectites and S2 amphibolitisation of peak eclogite asemblages are indistinguishable from those observed in the omphacite granulite.

Width of displayed microphotography is 3.5mm.

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