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Blueschist is a rock that forms by the metamorphism of sodium-rich basic rocks at high pressures and low temperatures, approximately corresponding to a depth of 15 to 30 kilometers and 200 to 400 degrees Celsius (cool by metamorphic standards).
This rock is considered a schist because all traces of original structure in the rock have been wiped out along with the original minerals, and strongly layered fabric has been imposed. The texture of blueschist is medium-grained, being non-foliated to moderately-foliated. The blue color of the rock comes from the presence of the mineral glaucophane. However, not all blueschist rocks are blue. Because of this, petrologists prefer to talk about the glaucophane-schist facies. Specimines usually display many different shades. Glaucophane and lawsonite are the major blue species in blueschist, but jadeite, epidote, and quartz are also common.
The conditions for blueschist formation are prevalent in subduction zones, like the northwest coast of North America. Here, basaltic oceanic crust and sediments are being subducted beneath the North American plate and are being kneaded by changing tectonic motions while sodium-rich fluids circulate through the rocks. Basically, the rocks are quickly shoved to an extreme depth and pressure and returned to the surface before they have time to heat up. This is what happens in subduction areas, and bluschist is common along the long linear zones that mark ancient plate subduction boundaries. In fact, the term blueschist metamorphism is often used to describe this form of mineral change that occurs under conditions of extremely high pressures and relatively low temperatures.
In 1962, Edgar Bailey of the U.S. Geological Survey introduced the concept of "blueschist" into the world of metamorphic geology. His carefully constructed definition established the pressure and temperature conditions which produce this type of metamorphism.
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