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Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.
The temperature lower limit of metamorphism is considered to be between 100-150°C, to exclude diagenetic changes, due to compaction, which result in sedimentary rocks. There is no agreement as for a pressure lower limit. Some workers argue that changes in atmospheric pressures are not metamorphic. But, see below, some types of metamorphism can occur at extremely low pressures.
The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The temperature interval is between 700-900°C, with pressures that depend on the composition of the rock. Migmatites are rocks formed on this borderline. They present both melting and solid-state features.
Regional metamorphism covers large areas of continental crust typically associated with mountain ranges or the roots of previously eroded mountains. Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism. These orogenic mountains are later eroded exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects. The techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or zones of varying temperature/pressure gradients and conditions throughout the orogenic terrane.
Metamorphic facies are recognizable terranes or zones with an assemblage of key minerals that were in equilibrium under specific range of temperature and pressure during a metamorphic event.
Facies:
Metamorphic grades are also classified by mineral assemblage based on the appearance of key minerals:
Contact metamorphism occurs typically around intrusive igneous rockIgneous rocks are formed when molten rock ( magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive ( volcanic) rocks. This magma can be derived from either the Eas, as a result of the temperature increase caused by the hot intruding magmaThis article is about the type of rock. For magmas in mathematics, see Magma (algebra). For the experimental jazz band, see Magma (band). For the computer algebra system, see Magma computer algebra system. For the EDA software company, see Magma Design Au. The area surrounding the igneous rock where the contact metamorphism effects are present is called the metamorphic aureole. As expected, the contact metamorphic effects are greater adjacent to the intrusive rock and fade away toward the exterior of the aureole. Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. Extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatismMetasomatism is a geologic process where metamorphism causes an alteration in a mineral or rock mass that involves a chemical change of the substance with the addition of material, as when chrysolite (olivine) is converted to to serpentine basically by th. Rocks formed by contact metamorphism do not present signs of strong deformation and are often fine grained. Contact metamorphic rocks are usually known as hornfels . If the intruded rock is rich in carbonateCarbonate is an anion with a charge of -2 and an empirical formula of CO2-. An aqueous solution of carbon dioxide contains a minute amount of HCO, called carbonic acid, which dissociates to form hydrogen ions and carbonate ions. It would be a fairly stron the result is a skarnSkarn is a fine grained metamorphic rock that is usually variably colored green or red, occasionally grey, black, brown or white. It usually forms by thermal metamorphism and metasomatism in the contact zone of magmatic intrusions like granites with carbo. Skarns can localize the deposition of metallic oreAn ore is a mineral deposit containing a metal or other valuable resource in economically viable concentrations. Usually, it is used in the context of a mineral deposit from which it is economical to extract its metallic component. Ores are mined. Ore bod minerals and thus are of economic interest.
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