fundamental changes in the texture, structure, and mineral and chemical composition of rocks in the earth’s crust and mantle owing to the effect of abyssal fluids (volatile constituents), temperature, and pressure. The term “metamorphic rocks” was introduced in 1883 by the English geologist C. Lyell.
Metamorphism occurs in the crystalline (solid or plastic) state without melting of the rocks (it does not include the near-surface processes of compaction, cementation, and diagenesis of sediments or weathering); metamorphism is always associated with tectonic dislocation (folding, abyssal fractures) and sometimes also with the upwelling of magmatic masses. Dislocations penetrating into the abyssal zones of the earth stimulate the formation of ascending streams of fluids and a rise in temperature, which leads to the development of magmatism, metamorphism, and the formation of endogenic deposits. All of these phenomena are genetically related, reflecting the ascending migration of matter in the course of evolution of the earth’s crust.
The factors in metamorphism that determine the mineral composition of metamorphic rocks are temperature (T), litho-static pressure (Ps) which is determined by the depth at which the metamorphism develops, and sometimes the partial pressures or chemical potentials of the gases that are contained in fluids, including H2O, H2, CO2, CO, CH4, H2S, C12, and F2. The regions of stability for the chief minerals in metamorphic rocks (metamorphic facies) are identified in relation to these factors (primarily T, Ps, and PH2o), which is the basis for dividing all metamorphic rocks and for studying the degree of metamorphism. Unilateral pressure (stress) is not a factor in metamorphism because it does not lead to the formation of new minerals. At the same time, however, it influences the textures of metamorphic rocks, increases rock permeability to fluids, and has a catalytic effect on metamorphic reactions.
Metamorphism involving change in the content of volatile components (H2O, CO2, O2) only is arbitrarily called isochemical; metamorphism involving change in the content of other components (K2O, Na2O, CaO) is called allochemical; when there are intensive local alterations in the chemical composition of rocks and part of the components change to a completely mobile state the metamorphism is called metasomatism. In the series of processes of isochemical metamorphism, allochemical metamorphism, and metasomatism, the degree of alteration in the chemical composition of the initial rocks is greater in each process than in the preceding one.
Metamorphism may affect rocks over enormous areas (regional metamorphism) or manifest itself locally, being confined to contacts with igneous rocks (contact metamorphism) or to fractures (fracture metamorphism).
In the history of geosynclinal development a distinction is made between early (“pregranitic”) sodic metamorphism (the formation of spilites, albite-chlorite slate, glaucophane slate, eclogites) and metamorphism associated with the development of plagiogranites (plagiomigmatites, plagiogneisses, albite mica slates) or normal potassium granites (migmatites, gneisses, mica slates, phyllites). The sodic nature of metamorphism in early geosynclinal development changes in the course of evolution of metamorphic zones in the direction of a greater role for potassium in the metamorphosing solutions. In depth zones metamorphism is often combined with areas of regional development of granitoid magmatism.
Metamorphism that occurs in the course of increasing temperatures is called progressive. It is accompanied by the loss of volatile components (dehydration, decarbonatization) by the initial rocks. The opposite processes against a background of decreasing temperature are termed regressive metamorphism. Repeated regressive metamorphism is called diaphthoresis.