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Describe how regional metamorphism of shale produces a sequence of rocks and minerals.

Metamorphism is a process by which preexisting rocks can be altered in composition or structure, to form a new rock. Regional metamorphism occurs in areas of above average pressures and temperatures, such as subduction zones. Shale is a sedimentary rock, which has a varied chemical composition, and it contains platy clay minerals that can align to pressure (associated with regional metamorphism).

 

Low-grade metamorphism (low pressure and temperature) of shale forms slate. Distinguishable by platy cleavage, where mica crystals align perpendicularly to pressure as well as the index mineral Biotite.

 

Medium-grade metamorphism (medium pressure and temperature) of shale forms schist. Schist is distinguishable by the index minerals Kyanite and/or Garnet, it may also show slight banding of dark/light minerals.

 

High-grade metamorphism (high pressure and temperature) of shale forms Gneiss. Gneiss shows very distinct banding between light and dark minerals, which have separated due to different compositions and has the index mineral Sillimanite.

 

It is important to point out that index minerals show the minimums pressure and temperatures the shale must have experienced. A metamorphosed shale may contain Biotite and Garnet (two minerals formed at different P+T), this shows that metamorphosed shale must have at least experienced medium grade metamorphism.

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