Concept
The temperature and pressure at which the mineralogical composition of a rock equilibrated can vary laterally through a metamorphic terrane. The two parameters together determine the metamorphic grade. The difference in grade between two spots is called the metamorphic gradient. Planes that connect points with the same metamorphic grade are called isograds. The secant lines of isograds with the surface form lines on a geologic map.
Changes in mineralogical composition in a terrane reflect differences in metamorphic grade of the rocks. Minerals that are characteristic for a certain metamorphic grade are called index minerals. The first or last appearance of an index mineral (the place where a metamorphic reaction is observed) forms an easily recognizable isograd. A metamorphic zone is the region between two such easily recognizable isograds. Often they are named for the most characteristic index mineral of the zone.
Whether a certain index mineral occurs is also dependent on the composition of the rock itself. Many index minerals have complicated chemical compositions. If not all necessary elements are abundant, the mineral will not grow. When mapping the metamorphic grade of a terrane, a geologist has to take the lithology of the rock in account. Lithologies are mainly dependent on the protolith, the original rock before metamorphism. The main lithologies are ultramafic, mafic, felsic (or quartzo-feldspatic), pelitic and calcareous. In all of these (and other) lithologies different combinations of minerals occur at a certain grade. The metamorphic zones in these lithologies can also be different.
Read more about this topic: Metamorphic Zone
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