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In regional metamorphism conditions can range from
low to high pressures and temperatures that occur over a large area.
This will produce different zones or rocks that have characteristic
minerals. The minerals are like a pressure gauge and thermometer and
record the history of the conditions under which the rocks formed.
Finding these minerals in metamorphic rocks are
clues to the temperature and pressure. For example, chlorite, muscovite,
and biotite (all micas) are common in low grade metamorphism
which is low temperatures (200EC) and pressure. (Remember water boils at
100EC). Intermediate grade metamorphisms usually contains the minerals garnet
and staurolite. High grade metamorphism (800EC) usually produces kyanite
and sillimanite.
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A. Kyanite, B. Sillimanite
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