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eclogite facies[′ek·lə‚jīt ‚fā·shēz]
conditions of thermodynamic stability of eclogites, defined with respect to the principal factors of meta-morphism: pressure, temperature, and partial pressures of volatile components in fluids. Eclogites correspond to conditions of high pressure attained in the earth’s crust (almandine eclogites, 7–15 kbar) or in the mantle (pyrope eclogites, more than 15 kbar).
In terms of temperature, a distinction is made between medium-temperature eclogites, which are part of alpine-type glaucophane metamorphic zones (500°–600°C), and high-temperature eclogites, which form associations with gneisses and granulites (600°–900°C). The pyrope eclogites that form inclusions in kimberlites and alkali basaltoids have even higher temperatures (above 900°C).
A number of scientists, such as A. A. Marakushev and L. L. Perchuk, believe that the metamorphism leading to the formation of eclogites originates under reduction conditions with the participation of fluids with low partial pressures of water and carbon dioxide. Under such conditions, the processes of hydration and carbonatization of minerals are relatively suppressed, which gives anhydrous minerals (omphacite, garnet) stability in both high-and medium-temperature facies.
REFERENCEFatsii regional’nogo metamorfizma vysokikh davlenii. Moscow, 1974.
A. A. MARAKUSHEV