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METAMORPHIC ROCKS

METAMORPHIC ROCKS. Metamorphism. The transformation of pre-existing rock into texturally or minerologically distinct new rock as a result of high temperature and pressure or both, but without melting of rock. So,the rocks formed as a result of metamorphism are known as METAMORPHIC ROCKS.

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METAMORPHIC ROCKS

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  1. METAMORPHICROCKS

  2. Metamorphism • The transformation of pre-existing rock into texturally or minerologically distinct new rock as a result of high temperature and pressure or both, but without melting of rock. • So,the rocks formed as a result of metamorphism are known as METAMORPHIC ROCKS. • Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. • Mineralogical, chemical and crystallographic changes can occur during this process. • Marble,Slate,Phyllite,Gneiss are well known metamorphic rocks.

  3. Sedimentary rock 0 km Metamorphic rock Igneous Sediment rock 10 km Metamor-phism occurs between about 10 and 50 km of depth ~200ºC Sedimentary rock Metamorphism Increasing depth and temperature 50 km Melting ~800ºC These rocks don’t melt

  4. Factors Controlling Characteristics of Metamorphic Rocks Composition Of the Parent Rock • The mineral content of metamorphic rock is controlled by the chemical composition of the parent rocks. • For example, marble is indicative of parent rock composed of Caco3. • Slate is result of metamorphism of Shale. • Similarly, Granite Gneiss shows metamorphism of Granite.

  5. Factors Controlling Characteristics of Metamorphic Rocks TEMPERATURE What is temperature range at the site of metamorphism. Each mineral has its own melting and boiling point. Pressure at the site is also very important. Generally acidic rocks have low temp. range with compare to basic rock.

  6. Factors Controlling Characteristics of Metamorphic Rocks PRESSURE Earth pressure is called litho static pressure. Earth pressure is generally, confining, but may be differential. Under effect of pressure, rock grains get closer and eliminate pore space. Trapped water is released, and may play role in metamorphism.

  7. Origin of pressure in metamorphism

  8. Factors Controlling Characteristics of Metamorphic Rocks TIME The effect of time is very hard to comment. Laboratory calculation have resulted million of years for note-able effect of metamorphism, e-g acidic rocks have growth rate 1.4 millimeter per one million year. Others effects like temp. pressure, location of metamorphism also effect rate of metamorphism.

  9. TYPES OF METAMORPHISM CONTACT METMORPHISM Contact metamorphism /Barrovian zone occurs adjacent to pluton, when magma intrudes relatively cool country rocks. The zone of contact metamorphism is called AUREOLE. Dikes/Sills generally have small aureoles with minimal metamorphism whereas large ultramafic intrusions can have significantly thick and well-developed contact metamorphism. Meteorite can also cause metamorphism,called Impact metamorphism.

  10. REGIONAL METAMOPHISM Also known as dynamo thermal metamorphism is caused at very large scale and at greater depth about 5 km deep. It covers large areas of continental crust typically associated with mountain ranges, particularly subduction zones or the roots of previously eroded mountains.

  11. The temp range is about 3000c to 8000c. Depending on pressure and temp conditions, a parent rock may recrystallized into different metamorphic rocks, e-g BASALT (low P,T) GREEN SCHIST BASALT (high P,T) AMPHIBOLE SCHIST Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane.

  12. PROGRESSIVE METAMORPHISM Change of metamorphic rocks with progressive change of PRESSURE,TEMPERATURE. At higher pressure and temperature, we have greater metamorphism effects, and as we away get away from the site of metamorphism progressively, the effect of metamorphism decreases. E-g Shale Slate Phyllite Schist Gneiss

  13. Progressive regional metamorphism: from low grade (slate); to high grade (gneiss)

  14. METASOMATISM Meatsomatism takes place, when very hot water is introduced in the process of metamorphism. In this process, different ions are exchanged. Water can hold different ions like K +,Na+,Si+4 and O-2. In this way, large crystal of Feldspar may grow in SCHIST due to addition of potassium or sodium ions.

  15. 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.

  16. Convective circulation of hydrothermal fluids in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. The fluids eventually escape through vents in the ocean floor known as black smokers. The patterns of this hydrothermal alteration is used as a guide in the search for deposits of valuable metal ores.

  17. Hydrothermal metamorphism

  18. Dynamic Metamorphism Dynamic metamorphism is associated with zones of high to moderate strain such as fault zones. Cataclasis, crushing and grinding of rocks into angular fragments, occurs in dynamic metamorphic zones, giving cataclastic texture. The textures of dynamic metamorphic zones are dependent on the depth at which they were formed, as the temperature and confining pressure determine the deformation mechanisms which predominate.

  19. Metamorphism along a fault zone

  20. Within depths less than 5 km, 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. Within the depth range of 5–10 km pseudotachylite is formed, as the confining pressure is enough to prevent brecciation and milling and thus energy is focused into discrete fault planes. Frictional heating in this case may melt the rock to form pseudotachylite glass.

  21. Within the depth range of 10–20 km, 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 porphyroblasts in mylonite zones.

  22. Overthrusting 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.

  23. IMPACT METAMORPHISM

  24. Falling Meteorite, if they are of larger size,can cause impact metamorphism. • It creates locally metamorphic zone at the site of strike. • It also creates progressive metamorphism. • Products are called impactites.

  25. Metamorphic Facies 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 : Prehnite-Pumpellyite Low T - High P : Blueschist Mod to High T - Mod P : Greenschist - Amphibolite - Granulite Mod - High T - High P : Eclogite

  26. Metamorphic Grades In the Barrovian sequence (described by George Barrow in zones of progressive metamorphism in Scotland), metamorphic grades are also classified by mineral assemblage based on the appearance of key/index minerals in rocks of pelitic (shaly, aluminous) origin.

  27. Low grade ------ Intermediate ------ High grade Greenschist -------- Amphibolite -------------- Granulite Slate --- Phyllite ---- Schist ---- Gneiss ---Migmatite(partial melting) >>>melt Chlorite zone Garnet zone Staurolite zone Kyanite zone Biotite zoneSillimanite zone

  28. Identification of Metamorphic/Metamorphic Structures Foliation/Foliated Rocks Under effect of metamorphism, minerals in rocks are arranged in line/direction. If rock splits easily along nearly flat and parallel planes, indicating that platy mineral were pushed into alignment, we say rock is SLATY or it has SLATY CLEAVAGE. If visible platy minerals or needle like mineral have grown parallel to a plane due to differential stress, the rock is SCHISTOSE.

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