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regional metamorphism[′rēj·ən·əl ‚med·ə′mȯr‚fiz·əm]
the changes that occur in rock acted upon by abyssal transmagmatic solutions (fluids), oriented (unilateral) and hydrostatic (all-encompassing) pressure, and temperature.
Regional metamorphism, involving profound transformations in the structure and mineral composition of rocks over vast areas, is related to the development of folding and orogeny. Unilateral pressure produces slaty and gneiss textures in meta-morphic rocks. Hydrostatic pressure is determined by depth, and as the pressure increases it causes metamorphic reactions among minerals that reduce the rock volume. Depth facies of metamorphic rocks are identified with respect to hydrostatic pressure. These facies make it possible to judge the depth of erosion of regionally metamorphosed structures (folded belts, blocks, and shields). Depending on the temperature, three levels of regional metamorphism may be distinguished: high, medium, and low.
The products of regional metamorphism (amphibolites, phyllites, gneisses, and migmatites) crop out at the earth’s surface within ancient shields and crystalline blocks. At great depths, regional metamorphism is usually uniform (the same degree of metamorphism is maintained over large areas). At shallower depths, where different degrees of metamorphism are observed, regional metamorphism is not uniform. A consecutive decrease in the degree of metamorphism can be traced in anticlinoria, granite-gneiss domes, and other geological structures where there is a zonal distribution of regional metamorphic products that differ mineralogically and structurally (zonal metamorphism). As the scope of metamorphic manifestations diminishes, regional metamorphism gives way to local metamorphism, which is controlled by such local structures as contacts with intrusive masses (contact metamorphism) and fractures (fracture metamorphism).
REFERENCEMarakushev, A. A. Petrologiia metamorficheskikh gornykh porod. Moscow, 1973.
A. A. MARAKUSHEV