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metatexture - Metamorphic Textures EENS 2120 Prof Stephen A...

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This document last updated on 10-Mar-2011 EENS 2120 Petrology Prof. Stephen A. Nelson Tulane University Metamorphic Rock Textures Metamorphic rocks exhibit a variety of textures. These can range from textures similar to the original protolith at low grades of metamorphism, to textures that are purely produced during metamorphism and leave the rock with little resemblance to the original protolith. Textural features of metamorphic rocks have been discussed in the previous lecture. Here, we concentrate on the development of foliation, one of the most common purely metamorphic textures, and on the processes involved in forming compositional layering commonly observed in metamorphic rocks. Foliation Foliation is defined as a pervasive planar structure that results from the nearly parallel alignment of sheet silicate minerals and/or compositional and mineralogical layering in the rock. Most foliation is caused by the preferred orientation of phylosilicates, like clay minerals, micas, and chlorite. Preferred orientation develops as a result of non-hydrostatic or differential stress acting on the rock (also called deviatoric stress ). We here review the differences between hydrostatic and differential stress. Stress and Preferred Orientation Pressure increases with depth of burial, thus, both pressure and temperature will vary with depth in the Earth. Pressure is defined as a force acting equally from all directions. It is a type of stress , called hydrostatic stress or uniform stress. If the stress is not equal from all directions, then the stress is called a differential stress. Normally geologists talk about stress as compressional stress. Thus, if a differential stress is acting on the rock, the direction along which the maximum principal stress acts is called σ 1 , the minimum principal stress is called σ 3 , and the intermediate principal stress direction is called σ 2 . Note that extensional stress would act along the direction of minimum principal stress. Metamorphic Textures 3/10/2011 Page 1 of 6
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If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock. z Rounded grains can become flattened in the direction of maximum compressional stress. z Minerals that crystallize or grow in the differential stress field may develop a preferred orientation. Sheet silicates and minerals that have an elongated habit will grow with their sheets or direction of elongation orientated perpendicular to the direction of maximum stress. This is because growth of such minerals is easier along directions parallel to sheets, or along the direction of elongation and thus will grow along σ 3 or σ 2 , perpendicular to σ 1 .
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