Metamorphosed tonalite

Metamorphosed tonalite; partly migmatitic and pervasively foliated with gneissic fabric clearly defined by alignment of biotite-hornblende. Quartz-plagioclase intergrown in granoblastic texture. Magnetite is the opaque species, although minor pyrite and rare chalcopyrite are present. The plagioclase is weakly sericite altered.

This sample has suffered essentially complete recrystallisation and weak ductile deformation in response to a regional metamorphic event in the amphibolite facies. This produced the weakly foliated metamorphic assemblage of plagioclase + hornblende + minor biotite + quartz + opaques (including magnetite). The metamorphic assemblage, including patchy but non-banded mineral distribution, allows interpretation of the protolith as an intermediate metaluminous moderately oxidised igneous rock (eg quartz diorite). Felsic and ferromagnesian patches on the scale of many millimetres are interpreted as recrystallised and weakly elongated felsic and ferromagnesian primary grain sites. Preservation of accessory small euhedral zircon crystals of possible igneous origin supports this interpretation.

Weakly foliated hornblende-biotite quartzo-feldspathic gneiss (meta-?quartz diorite) In thin section, this sample displays an inequigranular granoblastic metamorphic texture with weak foliation and patchy mineral distribution. Plagioclase is abundant, forming equant anhedral grains mostly ~0.4-0.8 mm in size. They display well-developed combined simple and multiple twinning, and no compositional zoning (ie fully equilibrated plagioclase). Their relatively uniform size, their sharp grain contacts, and 120° triple-grain junctions suggest they have formed by equilibrium recrystallisation of precursor plagioclase. Dominance of plagioclase in larger areas many millimetres in size possibly reflects sites of primary plagioclase grains that are recrystallised but not destroyed. Hornblende is the other principal mineral. It forms anhedral grains of ragged shape, ranging widely in size from ~0.2 mm up to ~3 mm. Larger grains are aligned in the trace of the weak foliation. Pleochroism from pale beige brown, through green, to olive green confirms it has a genuine Al- and Ti-rich hornblendic composition. Biotite occurs in significant amount as well-shaped small flakes ~0.2-0.8 mm long. They are most abundant in and near the hornblende grains, where their weak preferred orientation contributes to the weak metamorphic foliation. Pleochroism from reddish brown to straw yellow suggests they have a moderately oxidised composition. Quartz occurs in minor amount as anhedral grains distributed sparsely through the rock. They range up to ~1 mm in size, and larger grains tend to be weakly elongated in the trace of the metamorphic foliation. They display weak shadowy strain extinction effects. Opaques are present in minor amount as small anhedral grains closely associated with the ferromagnesian grains (hornblende, biotite). The response of the hand sample suggests that at least some is magnetite. Zircon crystals occur in trace amount as euhedral blocky prisms, some with thin growth zoning. These crystal forms suggest it is a relict primary igneous phase.