- Geologists group metamorphic rocks by mineralogy, either by the presence of individual minerals or by the presence of particular mineral assemblages.
- Maps of regional metamorphic rocks may be divided into metamorphic zones based on the minerals present. Zones are commonly based on minerals observed in metamorphosed shales: chlorite, biotite, garnet, staurolite, kyanite, andalusite, and sillimanite.
- Metamorphic grade is term used in a general way to describe how much a rock has changed from its protolith. Rocks that have not changed much from their protolith are said to have experienced a low grade of metamorphism. Rocks that are significantly changed from their protolith are said to have achieved a high grade of metamorphism.
- Although pressure is also important for metamorphism, modern usage is that metamorphic grade refers to the relative temperature of metamorphism so that higher-grade metamorphic rocks been heated to higher temperatures than lower-grade metamorphic rocks.
- The first or last appearance of an index mineral or mineral association in a metamorphic complex may be marked on a map as a line (a surface in 3D) called an isograd. Isograds should represent mineral assembly changes, not changes in bulk composition of the metamorphic rock.
- The similarity in the pattern of metamorphic zones found in different metamorphic belts supports the conclusion that metamorphic rocks have approached chemical equilibrium during metamorphism.
- Petrologists group metamorphic rocks that have experienced similar conditions of metamorphism into metamorphic facies based on mineral assemblages in rocks with basalt as a protolith.
- Metamorphic belts may be grouped into facies series that typify the conditions and tectonic setting that caused the metamorphism.
3.5 Summary
