Under the influence of pressure, high temperature, removal or introduction of substances into rocks - sedimentary, magmatic, metamorphic, any - after their formation, processes of change occur, and this is metamorphism. Such processes can be divided into two broad groups: local metamorphism and deep. The latter is also called regional, and the former - local metamorphism. It depends on the scale of the process.
Local metamorphism
Local metamorphism is too large a category, and it is also subdivided into hydrothermal metamorphism, that is, low and medium temperature, contact and autometamorphism. The latter is the process of change in igneous rocks after solidification or hardening, when they are affected by residual solutions, which are the product of the same magma and circulate in the rock. Examples of such metamorphism are the serpentinization of dolomites, ultramafic rocks and basic rocks, and the chloritization of diabases. The next type is characterizedalready by its name.
Contact metamorphism occurs at the boundaries of host rocks and molten magma, when temperatures, fluids (inert gases, boron, water) coming from magma act. A halo or zone of contact impacts can be from two to five kilometers from the solidified magma. These rocks of metamorphism often exhibit metasomatism, where one rock or mineral is replaced by another. For example, contact skarns, hornfelses. The hydrothermal process of metamorphism occurs when rocks are altered due to aqueous thermal solutions that are released through the solidification and crystallization of an eruption. Here, too, the processes of metasomatism are of great importance.
Regional metamorphism
Regional metamorphism occurs over large areas where the earth's crust is mobile and is submerged under the influence of tectonic processes in large areas to a depth. This results in particularly high pressures and high temperatures. Regional metamorphism transforms simple limestones and dolomites into marbles, and granites, diorites, syenites into granite gneisses, amphibolites, and schists. This is due to the fact that at medium and great depths such temperatures and pressure indicators that the stone softens, melts and flows again.
Rocks of metamorphism of this type are distinguished by their orientation: when massive textures flow, they become striped, linear, shale, gneissic, and all landmarks are given relative to the direction of flow. Small depths do not allow this. Because the metamorphism of rocks shows uscrushed, shale, clay or frayed rocks. If altered rocks can be associated with some lines, we can speak of local near-fault dislocation metamorphism (dynamometamorphism). The rocks formed by this process are called mylonites, shales, kakirites, cataclasites, breccias. Igneous rocks that have gone through all stages of metamorphism are called orthorocks (these are orthoschists, orthogneisses, and so on). If the rocks of metamorphism are sedimentary, they are called para-rocks (these are paraschists or paragneisses, and so on).
Metamorphism facies
Under certain thermodynamic conditions of the course of metamorphism, groups of rocks are distinguished, where mineral associations correspond to these conditions - temperature (T), total pressure (Рtotal), partial pressure of water (P H2O).
Types of metamorphism include five main fasciae:
1. Green slates. This fascia occurs at a temperature below two hundred and fifty degrees and the pressure is also not too high - up to 0.3 kilobars. It is characterized by biotite, chloride, albite (acid plagioclases), sericite (fine-flake muscovite) and the like. Usually this fascia is superimposed on sedimentary rocks.
2. Epidote-amphibolite fascia is obtained with a temperature of up to four hundred degrees and a pressure of up to a kilobar. Here, amphiboles (often actinolite), epidote, oligoclase, biotite, muscovite, and the like are stable. This fascia can also be seen in sedimentary rocks.
3. Amphibolite fascia is found on any typerocks - both igneous, and sedimentary, and metamorphic (that is, these fasciae have already been subject to metamorphism - epidote-amphibolic or greenschist fascia). Here, the metamorphic process takes place at temperatures up to seven hundred degrees Celsius, and the pressure rises to three kilobars. This fascia is characterized by such minerals as plagioclase (andesine), hornblende, almandine (garnet), diopside and others.
4. Granulite fascia flows at a temperature of over a thousand degrees with a pressure of up to five kilobars. Minerals that do not contain hydroxyl (OH) crystallize here. For example, enstatite, hypersthene, pyrope (magnesian garnet), labrador and others.
5. Eclogite fascia passes at the highest temperatures - more than one and a half thousand degrees, and the pressure can be more than thirty kilobars. Pyrope (garnet), plagioclase, omphacite (green pyroxene) are stable here.
Other fasciae
A variety of regional metamorphism is ultrametamorphism, when rocks are completely or partially melted. If partially - this is anatexis, if completely - this is palingenesis. Migmatization is also distinguished - a rather complex process in which rocks are formed in layers, where igneous rocks alternate with relict, that is, the source material. Granitization is a widespread process, where the end product is a variety of granitoids. This is, as it were, a special case of the general process of granite formation. Here we need the introduction of potassium, sodium, silicon and the removal of calcium, magnesium, iron with the most active alkalis, water andcarbon dioxide.
Diaphthoresis or regressive metamorphism is also widespread. Associations of minerals formed at high pressures and temperatures are replaced by their low-temperature fasciae. When amphibolite fascia is superimposed on granulite fascia, and greenschist and epidote-amphibolite fascia and so on, diaphtoresis occurs. It is in the process of metamorphism that deposits of graphite, iron, alumina, and the like appear, and the concentrations of copper, gold, and polymetals are redistributed.
Processes and Factors
The processes of change and regeneration of rocks occur in very long periods of time, they are measured in hundreds of millions of years. But even not too intense, significant factors of metamorphism lead to truly gigantic changes. The main factors are, as already mentioned, pressures and temperatures that act simultaneously with different intensities. Sometimes one factor or another prevails sharply. Pressure can also act on rocks in different ways. It can be comprehensive (hydrostatic) and directed unilaterally. An increase in temperature increases chemical activity, all reactions are accelerated by the interaction of solutions and minerals, which leads to their recrystallization. Thus begins the process of metamorphism. Red-hot magma penetrates into the earth's crust, puts pressure on rocks, heats them up and brings with it a lot of substances in liquid and vapor state, and all this facilitates reactions with host rocks.
Types of metamorphism are diverse, just as diverse are the consequences of these processes. ATIn any case, the old minerals are transformed and new ones are formed. At high temperatures, this is called hydrometamorphism. A rapid and sharp increase in the temperature of the earth's crust occurs when magma rises and intrudes into it, or it may be the result of submersion of entire blocks (large areas) of the earth's crust during tectonic processes to great depths. There is an insignificant melting of the rock, which nevertheless causes the ores and rocks to change the chemical and mineral composition and physical properties, sometimes even the shape of the mineral deposits changes. For example, hematite and magnetite are formed from iron hydroxides, quartz from opal, coal metamorphism occurs - graphite is obtained, and limestone suddenly recrystallizes into marble. These transformations take place, albeit for a long time, but always in a miraculous way, which gives mankind deposits of minerals.
Hydrothermal processes
When there is a process of metamorphism, not only high pressures and temperatures affect its characteristics. A huge role is assigned to hydrothermal processes, where both juvenile waters released from cooling magmas and surface (vandose) waters are involved. The most typical minerals thus appear in metamorphosed rocks: pyroxenes, amphiboles, garnets, epidote, chlorites, micas, corundum, graphite, serpentine, hematite, talc, asbestos, kaolinite. It happens that certain minerals predominate, there are so many of them that even the names reflect the magnitude of the content: pyroxene gneisses, amphibole gneisses, biotiteslates and the like.
All processes of mineral formation - both magmatic, and pegmatite, and metamorphisms - can be characterized as a phenomenon of paragenesis, that is, the joint presence of minerals in nature, which is due to the commonality of their formation process and similar conditions - both physicochemical and geological. Paragenesis shows the sequence of phases of crystallization. First - magmatic melt, then pegmatite remnants and hydrothermal emanations, or these are sediments in aqueous solutions. When magma comes into contact with the underlying rocks, it changes them, but it changes itself. And if changes occur in the composition of the intrusive rock, they are called endocontact changes, and if the host rocks change, they are called exocontact changes. The rocks that have undergone metamorphism constitute a zone or halo of changes, the nature of which depends on the composition of the magma, as well as on the properties and composition of the host rocks. The greater the discrepancy in composition, the more intense the metamorphism.
Sequence
Contact transformations are more pronounced in acid intrusions rich in volatile ingredients. The host rocks can be arranged in the following sequence (as the degree of metamorphism decreases): clays and shales, limestones and dolomites (carbonate rocks), then igneous rocks, volcanic tuffs and tuffaceous rocks, sandstones, siliceous rocks. Contact metamorphism increases with increasing porosity and fissuring of the rock, since gases and vapors circulate easily in them.
And always,absolutely in all cases, the thickness of the contact zone is directly proportional to the size of the intrusive body, and the angle is inversely proportional where the contact surface forms a horizontal plane. The width of contact halos is usually several hundred meters, sometimes up to five kilometers, in very rare cases even more. The thickness of the exocontact zone is much greater than the thickness of the endocontact zone. The processes of metamorphism in the metal formation of the exocontact zone are much more diverse. The endocontact rock is fine-grained, quite often porphyritic, and contains more non-ferrous metals. In the exocontact, the intensity of metamorphism decreases quite sharply, moving away from the intrusion.
Subspecies of contact metamorphism
Let's take a closer look at contact metamorphism and its varieties - thermal and metasomatic metamorphism. Normal - thermal, it occurs at a fairly low pressure and high temperature, there is no significant influx of new substances from an already cooling intrusion. The rock recrystallizes, sometimes new minerals are formed, but there is no significant change in the chemical composition. Clay shales smoothly pass into hornfelses, and limestones into marbles. Minerals are rarely formed during thermal metamorphism, except for the occasional deposits of graphite and apatite.
Metasomatic metamorphism is clearly visible at contacts with intrusive bodies, but its manifestations are often recorded in those areas where regional metamorphism developed. Such manifestationsquite often can be associated with mineral deposits. It can be mica, radioactive elements and the like. In these cases, the replacement of minerals took place, which proceeded with the obligatory participation of liquid and gas solutions and was accompanied by changes in the chemical composition.
Dislocation and impact metamorphism
There are a lot of synonyms for dislocation metamorphism, so if kinetic, dynamic, cataclastic metamorphism or dynamometamorphism are mentioned, we are talking about the same thing, which means the mineral structural transformation of the rock when tectonic forces acted on it in zones of purely discontinuous disturbances during mountain folding and without any participation of magma. The main factors here are hydrostatic pressure and simply stress (one-sided pressure). According to the magnitude and ratio of these pressures, dislocation metamorphism recrystallizes the rock completely or partially, but completely, or the rocks are crushed, destroyed, and also recrystallize. The output is a variety of shales, mylonites, cataclasites.
Impact or impact metamorphism occurs through a powerful meteoritic shock wave. This is the only natural process where these types of metamorphism can be observed. The main characteristic is the instantaneous appearance, huge peak pressure, temperature above one and a half thousand degrees. Then high-pressure phases set in for a number of compounds - ringwoodite, diamond, stishovite, coesite. Rocks and minerals are crushed,their crystal lattices are destroyed, diaplectic minerals and glasses appear, all rocks melt.
Metamorphism values
In a deep study of metamorphic rocks, in addition to the main types of changes listed above, some other meanings of this concept are often used. This, for example, is prograde (or progressive) metamorphism, which proceeds with the active participation of endogenous processes and preserves the solid state of the rock without dissolution or melting. Accompanied by the appearance of higher-temperature associations of minerals at the place of existence of low-temperature ones, parallel structures appear, recrystallization and release of carbon dioxide and water from minerals.
Regressive metamorphism (or retrograde, or monodiaphthoresis) is also taken into account. In this case, mineral transformations are caused by the adaptation of metamorphic rocks and magmatic rocks to new conditions at lower stages of metamorphism, which led to the appearance of low-temperature minerals in place of high-temperature ones. They were formed during previous processes of metamorphism. Selective metamorphism is a selective process, changes occur selectively, only in certain parts of the sequence. Here, the heterogeneity of the chemical composition, features of the structure or texture, and the like.