Retrograde metamorphism

Metamorphism is the change of minerals or geologic texture (distinct arrangement of minerals) in pre-existing rocks (protoliths), without the protolith melting into liquid magma (a solid-state change). The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering and/or diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis (max. 200°C), and melting (~850°C).

Three types of metamorphism exist: contact, dynamic, and regional. Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.

Metamorphic rocks can change without melting. Heat causes atomic bonds to break, and the atoms move and form new bonds with other atoms, creating new minerals with different chemical components or crystalline structures (neocrystallization), or enabling recrystallization. When pressure is applied, somewhat flattened grains that orient in the same direction have a more stable configuration.

The temperature lower limit on what is considered to be a metamorphic process is generally considered to be 100 – 200 °C; this excludes diagenetic changes due to compaction and the formation of sedimentary rocks. There is no agreement on a pressure lower limit for metamorphism; some argue that changes in barometric pressure are not metamorphic, but some types of metamorphism can occur at extremely low pressures.

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