Garnet-bearing quartzofeldspathic gneiss.

Garnet-bearing quartzofeldspathic gneiss. The rock is dominated by quartz-K-feldspar and lesser plagioclase that forms an equigranular, granoblastic texture within which are brown biotite laths that define a gneissic foliation. The foliation also contains elongate quartz grains that likely represent formerly more extensive migmatitic segregations that are now deformed into parallelism with the biotite-defined fabric. The garnets are mostly euhedral and <250mm in diameter and contain inclusions of quartz, biotite and in places opaque (likely magnetite). The biotite occurs as small laths <200mm in length and is light brown in colour. Opaque mineral occurs as small crystals within the quartzofeldspathic matrix and is dominantly magnetite. The rock may have been originally sedimentary in origin.

This sample is considered to have initially crystallised as a granodioritic intrusive igneous rock, but has recrystallised under high P-T conditions of the amphibolite facies, producing the new recrystallised foliated metamorphic assemblage of plagioclase + K-feldspar + quartz + biotite + minor garnet (with trace sillimanite inclusions) + opaques. Trace igneous zircon appears to have survived. The presence of biotite and garnet as the metamorphic ferromagnesian phases suggests that the protolith was peraluminous in composition. The metamorphic structure is defined by aligned biotite flakes and subparallel indistinct felsic bands composed of coarser-grained quartz + K-feldspar + plagioclase. It remains uncertain whether the felsic bands represent metamorphic segregation bands (ie localised migration of mobile components into the foliation plane during metamorphism) or whether they represent magmatic segregation bands which formed at the time of granitoid crystallisation.

Foliated garnet-biotite(-sillimanite) meta-quartzo-feldspathic rock (meta-?granodiorite) In thin section, this sample displays a foliated granoblastic metamorphic texture. Plagioclase is abundant, forming equant anhedral grains mostly ~0.4-0.8 mm in size. They display combined simple and multiple twinning, but little or no compositional zoning. Larger plagioclase grains occur in indistinct bands associated with minor thin discontinuous quartz-rich vein-like lenticles. K-feldspar occurs in significant amount as anhedral grains similar in size to plagioclase. Larger grains up to ~1 mm in size are concentrated with the thin quartz-rich vein-like lenticles. All of the K-feldspar displays the tartan twinning typical of microcline, and K-feldspar triple-grain junctions display 120° interfacial angles appropriate for equilibrium recrystallisation origin. Quartz forms small to large anhedral grains. Smaller grains ~0.2-0.4 mm in size lie at contacts of plagioclase and K-feldspar throughout much of the rock. Minor larger grains range up to ~2-4 mm long and with high axial ratio ~5-10, and are elongated in the trace of metamorphic foliation with tendency to be concentrated in thin discontinuous vein-like bodies. All of the quartz grains display mild shadowy strain extinction (ie mild deformation effects). Biotite occurs in moderate amount as small uniformly sized well-shaped plates ~0.2-0.4 mm long. They are distributed more-or-less uniformly throughout the rock, but are notably less abundant in the coarser-grained indistinct felsic bands. Their preferred orientation subparallel to the felsic bands contributes to the metamorphic foliation, and their pleochroism from tan brown to straw yellow suggests it has a moderately oxidised composition. Garnet is present in minor amount as irregularly scattered equant grains and subhedral crystals ~0.4-1.5 mm in size. They display the typical high relief, very pale off-pink colour, lack of cleavages but microcracked, and perfect isotropism of this mineral. Larger grains are aligned in the trace of the foliation. Sillimanite occurs in one larger garnet grain as numerous small acicular colourless inclusions aligned in the foliation, and also as trace small prisms in some smaller garnet grains. Opaques occur in minor amount as small grains sparsely scattered through the rock in sharp contact with other minerals. Opaque inclusions also occur in some garnet grains. Zircon is observed as uncommon small grains mostly ~50-100 μm in size, displaying equant squat prismatic shapes, high relief and high birefringence. Some occur discretely, and some occur as inclusions in garnet and biotite grains.