Suppe, John Department of Geological Sciences, Princeton University, Princeton, New Jersey.
Last reviewed:March 2021
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- Mineral assemblages and physical conditions
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Regional metamorphism with the highest pressures and lowest temperatures, commonly above 500 megapascals (5 kilobars) and below 400°C (750°F). Metamorphic rocks of the relatively uncommon blueschist facies contain assemblages of minerals that record these high pressures and low temperatures. The name “blueschist” derives from the fact that at this metamorphic grade, rocks of ordinary basaltic composition are often bluish (Fig. 1) because they contain the sodium-bearing blue amphiboles glaucophane or crossite rather than the calcium-bearing green or black amphiboles actinolite or hornblende, which are developed in the more common greenschist- or amphibolite-facies metamorphism. This difference in amphiboles in metamorphosed basalts led J. P. Smith in 1906 to conclude that glaucophane-bearing metamorphosed basalts had experienced different temperature and pressure conditions from those of other metamorphic rocks. In 1939 P. Eskola proposed a glaucophane-schist facies of regional metamorphism. More recently the term “blueschist facies” has been preferred because glaucophane is not always developed in many important rock types, for example, graywacke sandstones, which may contain other minerals that are characteristic of high pressure and low temperature such as aragonite, lawsonite, and jadeite + quartz. “Blueschist facies” is more generally applicable than “glaucophane-schist facies.” See also: Glaucophane
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