Information AboutMetamorphism |
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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''. 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 Rock s. 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. Migmatite s are rocks formed on this borderline. They present both melting and solid-state features. KINDS OF METAMORPHISM Regional metamorphism Regional or Barrovian metamorphism covers large areas of Continental Crust typically associated with mountain ranges, particulaly subduction zones or the roots of previously Eroded mountains. Conditions producing widespread regionally metamorphosed rocks occur during an Orogenic Event . The collision of two Continental Plate s or Island Arc s 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 temperature/pressure conditions throughout the orogenic terrane. Metamorphic facies Metamorphic Facies are recognizable terranes or zones with an Equilibrium Assemblage of key minerals that were in equilibrium under specific range of temperature and pressure during a metamorphic event. The facies are named after the metamorphic rock formed under those facies conditions from Basalt . Facies relationships were first described by Eskola (1920). Facies:
Metamorphic grades 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 :::: Biotite zone ::::::: Garnet zone :::::::::: Staurolite zone ::::::::::::: Kyanite zone :::::::::::::::: Sillimanite zone Contact metamorphism Contact metamorphism occurs typically around Intrusive Igneous Rock s 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 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 Basalt s 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. Impact metamorphism 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 Polymorph s Coesite and Stishovite ) and textures are characteristic of these conditions. Dynamic metamorphism Dynamic metamorphism is associated with major Fault planes. Metamorphism is localised adjacent to the fault plane and is caused by frictional heat generated by the fault movement. Cataclasis, crushing and grinding of rocks into angular fragments, occurs in dynamic metamorphic zones, giving cataclastic texture. The textures of dynamic metamorphic zones are dependant on the depth at which they were formed, as the confining pressure determines the deformation mechanisms which predominate. Within depths less than 5km, dynamic metamorphism is not often produced because the confining pressure is too low to produce frictional heat. Instead, a zone of Breccia or cataclasite is formed, with the rock milled and broken into random fragments. This generally forms a Mélange . At depth, the angular breccias transit into a ductile shear texture and into mylonite zones. Within the depth range of 5-10km pseudotachylyte is formed, as the confining pressure is enough to prevent brecciation and milling and thus energy is focused into discrete fault planes. The frictional heating in this case may melt the rock to form pseudotachylyte glass or mylonite, and adjacent to these zones, result in growth of new mineral assemblages. Within the depth range of 10-20km, deformation is governed by ductile deformation conditions and hence frictional heating is dispersed throughout Shear Zones , resulting in a weaker thermal imprint and distributed deformation. Here, deformation forms Mylonite , with dynamothermal metamorphism observed rarely as the growth of Porphyroblast s in mylonite zones. Overthrust ing may juxtapose hot lower crustal rocks against cooler mid and upper crust blocks, resulting in conductive heat transfer and localised contact metamorphism of the cooler blocks adjacent to the hotter blocks, and often retrograde metamorphism in the hotter blocks. The metamorphic assemblages in this case are diagnostic of the depth and temperature and the throw of the fault and can also be Dated to give an age of the thrusting. SEE ALSO REFERENCES Eskola P. 1920. ''The mineral facies of rocks.'' Norsk. Geol. Tidsskr., 6, 143-194. Winter J.D., 2001. ''An introduction to Igneous and Metamorphic Petrology''. Prentice-Hall Inc. , 695 pages. ISBN 0-13-240342-0. EXTERNAL LINKS
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