N the Change of One Rock Type Into Another by Heat And/Or Pressure

Chapter 8 Metamorphism and Metamorphic Rocks

Metamorphism  The change of one rock type into another by heat and/or pressure  Metamorphic rocks are produced from igneous and sedimentary protoliths  Most common rock type in the continental crust, but often not seen at the surface  During metamorphism the rock must remain essentially solid. If it progresses to the point of being largely melted, it is considered igneous.  Metamorphic settings  Contact or thermal metamorphism – driven by a rise in temperature within the host rock  Hydrothermal metamorphism – chemical alterations from hot water  Regional metamorphism  Occurs during mountain building  Produces the huge volumes of metamorphic rock Agents of metamorphism  Heat • Most important agent • Recrystallization results in new, stable minerals • Two sources of heat • Contact metamorphism – heat from magma • An increase in temperature with depth due to the geothermal gradient  Pressure and differential stress • Increases with depth • Confining pressure applies forces equally in all directions • Rocks may also be subjected to differential stress which is unequal in different directions  Chemically active fluids • Mainly water with other volatile components • Enhances migration of ions • Aids in recrystallization of existing mineral • Sources of fluids • Hydrated minerals such as clays and micas • Fluids purged from crystallizing melts • Metasomatism • Dissolved ions migrate with fluids and react  The importance of parent rock • Most metamorphic rocks have the same overall chemical composition as the parent rock from which they formed Metamorphic textures  Foliation • Examples of foliation  Parallel alignment of flattened mineral grains and pebbles  Compositional banding  Slaty cleavage where rocks can be easily split into thin, tabular sheets  Foliated textures • slaty cleavage (slates): fissile, aphanitic • Schistosity (schists): fissile, micaceous or parallel-aligned elongated minerals, phaneritic • Gneissic (gneiss): not fissile, phaneritic  Other metamorphic textures • Those metamorphic rocks that lack foliation are referred to as nonfoliated  Quartzite  Marble • Porphyroblastic textures  Large grains, called porphyroblasts, surrounded by a fine-grained matrix of other minerals  Foliated rocks • Slate – dull, aphanitic, from shale or mudstone • Phyllite – shiney, aphanitic, from shale or mudstone • Schist – phaneritic, from shale or mudstone, and sometime other protoliths • Gneiss – phaneritic, generally high in grade, phaneritic, from many protoliths. Can be banded, but isn’t always.

 Common metamorphic rockNonfoliated rocks • Marble • Coarse, crystalline • Parent rock was limestone or dolostone • Composed mainly of calcite or dolomite crystals • Used as a decorative and monument stone • Exhibits a variety of colors  Nonfoliated rocks • Quartzite • Formed from a parent rock of quartz-rich sandstone • Quartz grains are fused together • Hornfels • Non-foliated rock from shale or mudstone that forms in contact aureoles Metamorphic environments  Contact or thermal metamorphism • Results from a rise in temperature when magma intrudes country rock • A zone of metamorphism called an aureole forms in the rock surrounding the magma • Most easily recognized when it occurs at the surface, or in a near-surface environment  Regional metamorphism • Produces the greatest quantity of metamorphic rock • Associated with mountain building Metamorphic zones  Index minerals and metamorphic grade • Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form • Migmatites • Highest grades of metamorphism that is transitional to igneous rocks • Contain light bands of igneous components along with areas of unmelted metamorphic rock Figure 8.25

Metamorphic facies  Based on metamorphism of basalts: greenschist, blueschist, amphibolite, granulite, eclogite Metamorphism and plate tectonics  Most metamorphism occurs along convergent plate boundaries • Faulting thickens the continental crust and deeply buries sediments • Formation of Earth’s major mountain belts including the Alps, Himalayas, and Appalachians  Other metamorphic environments • Burial metamorphism  Associated with very thick sedimentary strata • Deformation along shear zones  Occurs at depth and high temperatures  Pre-existing minerals deform by ductile flow  Other metamorphic environments • Impact metamorphism  Occurs when meteorites strike Earth’s surface  Products are called impactites