Metamorphic Facies

an ensemble of metamorphic rocks of varying composition that correspond to specific conditions of formation relative to the primary factors of metamorphism, namely, temperature, geostatic pressure, and the partial pressures of the volatile components in the fluids involved in metamorphic reactions among minerals (seeMETAMORPHISM). Metamorphic facies are usually classified according to the names of the main types of basic rocks that are stable in each facies (see Figure 1). Examples include the greenschist and glaucophane schist facies (low temperature, intermediate and high pressures), epidote-amphibolite and amphibolite facies (intermediate temperature, intermediate and high pressures), granulite and eclogite facies (high temperature and pressure), and sanidinite and pyroxene-hornfels facies (very high temperature and very low pressure). Metamorphic facies are also designated according to the names of metamorphic minerals and the mineral sequences that are typical of the corresponding regions of thermodynamic stability. Examples include the garnet-cordierite, hypersthene-sillimanite, staurolite, andalusite, sillimanite, and kyanite facies.

Figure 1. Metamorphic facies of rocks (after Marakushev, 1973): (1) slates, porphyries; (2) phyllites, calcite and chlorite schists, spilites, zeolitic rocks; (3) phyllites, glaucophane and chlorite schists containing jadeite and lawsonite; (4) two-mica schists and gneisses, actinolite schists, epidote amphibolites; (5) two-mica almandine schists, gneisses, gedrites, amphibolites, glaucophane schists, eclogites; (6) cordierite-biotite and pyroxene-amphibolite hornfelses; (7) biotite-sillimanite and andalusite gneisses, pyroxene amphibolites; (8) garnet (almandine-pyrope) gneisses, sillimanite gneisses, hyper-sthene gneisses, pyrozene amphibolites, eclogites; (9) almandine-pyrope and hypersthene-kyanite gneisses, kyanite eclogites; (10) cordierite-andalusite and pyroxene-plagioclase hornfelses; (11) hyper-sthene-cordierite gneisses, two-pyroxene schists; (12) almandine-pyrope and hyperstene-cordierite gneisses, two-pyroxene schists; (13) almandine-pyrope and hypersthene-sillimanite gneisses, eclogites; (14) sanidinites, larnite and spurite rocks, buchites

Depending on the types of mobile belts and the stages in the development of the belts, metamorphism occurs under conditions corresponding to different metamorphic facies. The relatively low-temperature metamorphism of the greenschist glaucophane schist, or zeolite facies is characteristic of the early preorogenic stages in the development of geosynclines. In the later orogenic stages, rocks undergo high-temperature metamorphism, primarily of the amphibolite and granulite facies. The high-temperature metamorphism is associated with the processes by which granitoids are formed in mobile belts. The pyroxene-hornfels and sanidinite facies are limited to contacts with plutonic rock bodies that are intruded in the postorogenic stages in the development of mobile belts or into cratonic structures.

In 1966, a map of the metamorphic facies of the USSR on a scale of 1:7,500,000 was published in the USSR. It was compiled under the direction of V. S. Sobolev at the Siberian Division of the Academy of Sciences of the USSR.