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Unformatted text preview: PART II: METAMORPHIC PETROLOGY INTRODUCTION What is metamorphism? "Metamorphism is the mineralogical and/or textural adjustments that take place in a rock in response to a new set of physicochemical conditions different from the ones under which the rock originally formed" Requirements : These changes 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 ); (iii) take place at conditions above those of diagenesis but below those of melting 1 . Range or realm of Metamorphism: The scope and range of metamorphism is not easy to define. By definition, metamorphism should exclude all sedimentary and igneous processes. Yet the boundary between diagenesis and metamorphism is poorly defined, and is definitely a function of the composition of the rock undergoing such changes. This means that a metamorphic texture or mineral may form for the first time in a basaltic rock at a temperature of for example 180°C, but a shale subjected to the same temperature may not show any textural or mineralogical changes that can be clearly identified as metamorphic . Similarly, the boundary between metamorphism and igneous activity is also unclear. Most mantle rocks could be considered metamorphic, but at the same time, partial melting within the mantle results in many igneous textures and minerals. Although a basaltic rock will not melt at temperatures of 700°C, a shale saturated with H 2 O will undergo "partial melting" at these temperatures to produce a granitic liquid that moves like a magma, leaving behind mafic minerals which can undergo further changes in the solid state. Nevertheless, the temperature "boundaries" between metamorphism and diagenesis on one hand and metamorphism and igneous activity on the other can be set...
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This note was uploaded on 02/28/2012 for the course GLY 421 taught by Professor Staff during the Fall '11 term at Marshall.
- Fall '11