8.3.6: Gneiss

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At higher grades, metamorphic rocks may develop compositional layering because different minerals concentrate in layers of contrasting colors. We call such rocks gneisses. The defining characteristics of most gneisses, such as the gneisses seen in Figure 8.29 and Figure 8.30, are that the rocks are medium- to coarse-grained and contain alternating layers of light and dark-colored minerals that give the rock foliation called gneissic banding. The banding in the garnet gneiss (Figure 8.30) is not particularly well-developed but is present.

8.29.png — Figure 8.29: A biotite-quartz gneiss. The specimen is 6.8 cm across.

8.30.png — Figure 8.30: A garnet gneiss. The garnets are up to 5 mm across.

Gneisses, the highest temperature-pressure kinds of foliated metamorphic rock, typify many regions that have undergone high-temperature metamorphism. Gneissic banding most commonly forms in response to directed stress. Sometimes, however, layering may form solely due to chemical processes that concentrate different minerals in different layers. The felsic light-colored layers typically contain quartz and feldspars, and the more mafic darker layers typically contain biotite, hornblende, or pyroxene. Accessory minerals such as garnet are common.

8.31.png — Figure 8.31: Deformed granitic gneiss

Sometimes gneissic banding is deformed, as seen in Figure 8.31. This gneiss, from the Czech Republic, contains pink K-feldspar rich layers alternating with darker layers that contain biotite. Metamorphism produced parallel layers of contrasting mineralogy (and color) and subsequent deformation caused the layers to become deformed. Figure 8.5 shows another example of a deformed gneiss.

Gneisses are often named based on their protoliths, and petrologists use the general terms orthogneiss for gneisses derived from igneous rocks, and paragneiss for gneisses derived from sedimentary rocks. More specific names abound – for example, pelitic gneisses form by metamorphism of originally clay-rich sedimentary rocks, granitic gneisses (such as the one shown in Figure 8.31) form by metamorphism of granites, and mafic gneisses form by metamorphism of mafic igneous rocks. Sometimes key minerals are often included in rock names. For example, a garnet gneiss is a gneiss that contains conspicuous garnet crystals.

Some gneisses do not display well-defined dark- and light-colored banding but still maintain less distinct foliation. For example, the foliation in kyanite gneiss may come from alignment of light-colored kyanite crystals in an otherwise quartz- and muscovite-rich rock. An augen gneiss, such as the gneiss shown in Figure 8.32, contains large feldspar crystals – “eyes” (augen is German for eyes) – stretched in one direction. The gneiss in this photo is oriented so the stretch direction (and, thus, the foliation) is horizontal.