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Metamorphism can be defined as the mineralogical, chemical and crystallographic changes in a solid-state rock, i.e. without melting, in response to new conditions of pressure and/or temperature and/or introduction of fluids.
Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.
Limits of 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 some types of metamorphism can occur at extremely low pressures (see below).
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.
Kinds of metamorphism
Regional or Barrovian 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.
- Low T - low P : Zeolite
- Mod - high T - low P : Hornfels
- Mod P - low to high T: Blueschist - Greenschist - Amphibolite - Granulite
- High P - Mod - high T : Eclogite
Metamorphic grades are also classified by mineral assemblage based on the appearance of key minerals:
Low grade ------------------- Intermediate --------------------- High grade
- Greenschist ------------- Amphibolite ----------------------- Granulite
- Slate --- Phyllite ---- Schist --------- Gneiss ----------------------- >>>melt
- Chlorite zone
Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the hot intruding magma. 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 metasomatism. 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 carbonate the result is a skarn. Skarns can localize the deposition of metallic ore minerals and thus are of economic interest.
Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of variable composition. The difference in composition between existing rock and the invading fluid triggers a set of metamorphic and metasomatic reactions. The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating groundwater, or ocean water. Convective circulation of water in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. The patterns of this hydrothermal alteration is used as a guide in the search for deposits of valuable metal ores.
This kind of metamorphism occurs when either an extraterrestrial object (a meteorite for instance) collides with the Earth's surface or during an extremely violent volcanic eruption. Impact metamorphism is, therefore, characterized by ultrahigh pressure conditions and low temperature. The resulting minerals (such as SiO2 polymorphs coesite and stishovite ) and textures are characteristic of these conditions.
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