Identifying Regional Metamorphic Rocks

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Foliated rocks produced by regional metamorphism

These rocks have all been affected by pressure to some degree during regional metamorphism. Any platy minerals will have taken on a preferred alignment known as foliation. The most common platy mineral is clay, so rocks described below all have shale as the parent rock.

Slate

Parent rock of slate is shale. Shale is composed of clay minerals and fine quartz particles. Because clay minerals are rich in aluminium, so are the metamorphic minerals in slate. This is minly composed of clay minerals and mica. Shale is fine grained and shows slaty cleavage. Traces of original bedding may still be preserved as relict bedding.

Low grade regional

Schist

The parent rock of schist is shale. Schist is produced by higher temperatures and pressure than those producing slate. It is medium grained and crystalline. Although it can occur in a variety of colours, it always has a shiny appearance where the surfaces of muscovite and biotite mica crystals are visible. Schist is typically composed of mica and garnet. The garnets often form large crystals called porphyroblasts. The mica crystals are all aligned at right angles to the maximum pressureb, forming the texture schistosity.

Medium grade regional.

Gneiss

The parent rock of gneiss is shale. Gneiss is formed by the highest temperatures and pressures during regional metamorphism. It is a coarse graied, crystalline rock with gneissose banding. Gneiss is typically composed of quartz and feldspar in the light nads and biotite mica in the dark bands.

High grade regional.

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