8.7.1: Metamorphosed Granitic Rocks

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The quartz, K-feldspar, and plagioclase that make up most granites and intermediate igneous rocks are stable at all grades of metamorphism. So, metamorphism of granites may not lead to significant mineralogical changes. However, many granites contain mafic minerals, most commonly biotite and hornblende. These minerals may dehydrate to produce new metamorphic minerals at medium and high grade. The table below lists the most common minerals in metamorphosed granites (also called metagranites). At the highest grade, metagranites become granulites, defined by the presence of orthopyroxene formed by dehydration of mafic minerals. Figure 8.31, earlier in this chapter, showed an example of a granitic granulite. Accessory minerals found in unmetamorphosed granites may also be present after metamorphism. At high grade, granitic rocks sometimes develop gneissic banding, even if mineralogy has not significantly changed.

Common Minerals in Metamorphosed Granitic Rocks
unmetamorphosed	low grade	high grade
Quartz SiO₂	biotite K(Fe,Mg)₃(AlSi₃O₁₀)(OH)₂	Fe-rich garnet (almandine) Fe₃Al₂Si₃O₁₂
K-feldspar KAlSi₃O₈	hornblende (complex amphibole)	orthopyroxene (Fe,Mg)SiO₃

8.64.jpg — Figure 8.64: Metagranite from western Norway.

The photo above (Figure 8.64) shows a metagranite from the Western Gneiss Region of Norway. During metamorphism, K-feldspar recrystallized to form very large pink crystals. Gray glassy quartz, white plagioclase, and black biotite are also present. This rock shows a significant amount of deformation, recorded by the deformed sheets of biotite. Note the presence of gneissic banding, most notably to the right of the marker pen.