Metamorphic rock formed at high pressure and low temperature, commonly above 5 kilobars (500 megapascals) and below 750°F (400°C). 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 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.
Blueschist metamorphic rocks are found almost exclusively in the young mountain belts of the circum-Pacific and Alpine-Himalayan chains. The rocks are usually metamorphosed oceanic sediments and basaltic oceanic crust. Previously continental rocks rarely exhibit blueschist metamorphism. The tectonic mechanism for blueschist metamorphism must move the rocks to depths of more than 6 to 12 mi (10 to 20 km) while maintaining relatively cool temperatures (390–750°F or 200–400°C). These temperatures are much cooler than for continental crust at those depths. For example, surface geothermal gradients of the order of 30°C per kilometer are common in continental crust and in thick sedimentary basins. In contrast, a steady-state surface gradient of about 44 to 58°F per mile (15 to 20°C per kilometer) would be required for typical blueschist metamorphism. Heat flow measurements above long-lived subduction zones, together with thermal models, suggest that the conditions of blueschist metamorphism exist today above subduction zones just landward of deep-sea trenches. This tectonic setting at the time of blueschist metamorphism is independently inferred for a number of metamorphic terranes.
What is not well understood is how the blueschist metamorphic rocks return to the surface; clearly the mechanism is not simple uplift and erosion of 12–18 mi (20–30 km) of the Earth's crust. Blueschist metamorphic rocks are usually in immediate fault contact with much less metamorphosed or unmetamorphosed sediments, indicating they have been tectonically displaced relative to their surroundings since metamorphism. See also Metamorphic rocks; Metamorphism.
