metam - 1 Metamorphism and Metamorphic Rocks A-...

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1 Metamorphism and Metamorphic Rocks A- Metamorphism What is metamorphism? "Metamorphism is the mineralogical and/or textural changes that take place in a rock in response to changes in its physicochemical environment." As such, this definition is incomplete . Changes that lead to metamorphism take place predominantly in the solid state , and outside the ranges of diagenesis and melting . Moreover, metamorphism results in very little if any changes in the chemical composition of the affected rock. Accordingly, for a mineralogical or textural change in a rock to be described as "metamorphic" it should: (i) occur in the solid state (ii) be isochemical, i.e. involve no chemical changes in the bulk rock composition (with the exception of loss or gain of volatiles such as H 2 O and CO 2 ). If this condition is not fulfilled, and metamorphism is accompanied by a change in the chemical composition of the rock, the process is called " metasomatism ". (iii) take place at conditions above those of diagenesis but below those of melting. Protoliths : The protolith of a metamorphic rock is its precursor, or original rock before metamorphism. Because metamorphism is predominantly isochemical, chemical analysis of a metamorphic rock yields clues as to its protolith. A good metamorphic petrologist can therefore guess the protolith of the metamorphic rock from its mineralogy . Texture may also be useful in some cases, as some metamorphic rocks preserve textures inherited from their protoliths. Why do we study metamorphic rocks? II- Range of Metamorphism: The temperature "boundaries" between metamorphism and diagenesis on one hand and metamorphism and igneous activity on the other are not clearcut, or well defined, but will depend on many factors. Nevertheless, the range of metamorphism can be set at >150- 200°C and <700-800°C. III- Factors controlling metamorphism: 1- Temperature : Importance : metamorphic reactions stability of different minerals and mineral assemblages .
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2 Temperature and depth of burial : types of geotherms (Fig. 1) Sources of heat: (i) the mantle, (ii) radioactive decay in the crust (iii) igneous intrusion. 2- Pressure: Types: Lithostatic Directed Importance: 3- Time: 4- Original chemical composition of the rock: Types of protoliths "Best" compositions or rock types for studying metamorphism 5- Composition of the fluid attending metamorphism : IV- Types of Metamorphism 1- Contact (thermal) metamorphism: 2- Cataclastic (dynamic): 3- Regional (dynamothermal) metamorphism: 4- Burial metamorphism: 5- Ocean floor metamorphism: 6- Impact metamorphism: Related to meteoritic impacts. Very high P and T, lasting for a
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metam - 1 Metamorphism and Metamorphic Rocks A-...

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