Chapter 7 - Metamorphic rocks

Abstract

Metamorphic rocks are the product of transformation or solid-state recrystallization of existing (protolith) igneous, sedimentary, and metamorphic rocks, by the change in physical and chemical conditions, principally temperature, pressure, and introduction of chemically active fluids. Metamorphism alters the mineral composition including the formation of new minerals (garnet, zoisite, kyanite, chlorite, biotite, sericite, staurolite, sillimanite, talc, and andalusite). The sources of temperature are geothermic gradient, effect of magmatic body, and friction in rock masses of tectonic movements following prograde or retrograde mechanism. The pressure is caused by the weight of sediments or crust. Common textures are crystalline, granular, xenoblasts, idioblasts, granoblastic, and porphyrpblastic. Structures include gneissic, schistose, and slaty. Types of metamorphism include dynamic/kinetic, contact, regional, and plutonic. Dynamic metamorphism is due to mechanical deformations during tectonic movements forming mylonites, flazer, and augen gneisses. Contact metamorphism by the thermal effect of magma/lava generates skarns. Regional metamorphism, under extreme high temperature and uniform pressure over large areas of continental crust forms high-grade metamorphic rocks like slate, phyllite, amphibolites, varieties of schists, para- and ortho-gneisses, quartzite, and marble. Plutonic metamorphism occurs at high temperatures and strong pressure in deeper parts of the lithosphere producing granulites, eclogites, and migmatites. Metamorphic rocks are exceptionally appreciated as decorative and building stone due to crystalline texture, layering, brilliant colors, and excellent polishing capabilities.