Metamorphism

Isochemical metamorphism is distinguished - in which the chemical composition of the rock changes insignificantly, and not an isochemical metamorphism ( metasomatism ), which is characterized by a noticeable change in the chemical composition of the rock as a result of fluid transfer of the components.

By the size of the ranges of metamorphic rocks, their structural position and the reasons for metamorphism are distinguished:

• Regional metamorphism , which affects significant volumes of the earth's crust, and spread over large areas.
• Ultrahigh Pressure Metamorphism
• Contact metamorphism (local) is associated with magmatic intrusions and comes from the heat of cooling magma.
• Dynamometamorphism occurs in fault zones, associated with significant deformation of rocks.
• Impact metamorphism (shock) occurs when a meteorite hits a planet’s surface.
• Auto-metamorphism

The main factors of metamorphism are temperature, pressure and fluid.

Temperature is the most important factor in metamorphism, affecting the processes of crystal formation and determining the composition of mineral associations. Metamorphic transformations of rocks occur in the temperature range of 250–1100 ° C. It is at this boundary, in connection with a sharp increase in the rates of chemical reactions, that the line between diagenesis and metamorphism is drawn.

Pressure is a factor affecting the volume of rocks, their density, melting point, as well as reservoir properties. There are two types of it - geostatic (comprehensive) and lateral (one-sided) .

Fluids are the volatile components of metamorphic systems. This is primarily water and carbon dioxide. Less commonly, oxygen, hydrogen, hydrocarbons, halogen compounds, and some others can play a role. In the presence of fluid, the stability region of many phases (especially those containing these volatile components) changes. In their presence, rock melting begins at much lower temperatures.

Metamorphic rocks are very diverse. As rock-forming minerals, more than 20 minerals are established in them. Rocks of similar composition, but formed under various thermodynamic conditions, can have completely different mineral composition. The first researchers of metamorphic complexes found that it is possible to distinguish several characteristic, widespread associations that formed under different thermodynamic conditions. The first division of metamorphic rocks according to the thermodynamic conditions of formation was made by Escola. In the rocks of basaltic composition, he identified green schists , epidote rocks, amphibolites , granulites and eclogites . Subsequent studies have shown the consistency and content of such a division.

Subsequently, an intensive experimental study of mineral reactions began, and the efforts of many researchers compiled a metamorphism facies diagram — the PT diagram, which shows the semi-stability of individual minerals and mineral associations. The facies diagram has become one of the main tools for analyzing metamorphic complexes. Geologists, having determined the mineral composition of the rock, correlated it with any facies, and, using the appearance and disappearance of minerals, made maps of isograds - lines of equal temperatures. In a practically modern version, the metamorphism facies scheme was published by a group of scientists led by V. S. Sobolev in the Siberian Branch of the USSR Academy of Sciences.

Protolite - a substance dissolved in water that can act as an acid or base .

• Turner, J.F. Evolution of Metamorphic Rocks . M .: ed. "Foreign Literature", 1951
• Eliseev N.A. Metamorphism . M .: publishing house "Nedra", 1963.
• The nature of metamorphism . A collection of articles edited by Pitcher U.S. and Flynn G.W. M.: Mir Publishing House. Series "Earth Sciences", 1967.
• Winkler G. Genesis of metamorphic rocks . Per. with him. M .: publishing house "Nedra", 1969.
• Facies of metamorphism . M .: publishing house "Nedra", 1970.
• Winkler G. Petrology of metamorphic rocks . M .: publishing house "Mir", 1973.

• Azonality
• Metamorphic rocks
• Coalification

• Levinson-Lessing F. Yu. Metamorphism // Brockhaus and Efron Encyclopedic Dictionary : in 86 volumes (82 volumes and 4 additional). - SPb. , 1890-1907.