Chapter 8: Metamorphism and Metamorphic Rocks: New Rocks from Old

What is Metamorphism? (1) Metamorphism is the change in form that happens in Earth’s crustal rocks in response to changes in temperature and pressure.

What is Metamorphism? (2) There are six major factors in metamorphism: Chemical composition. The change in temperature. The change in pressure. The presence or absence of fluids. How long a rock is subjected to high pressure or high temperature. Whether the rock is simply compressed or is twisted and broken during metamorphism.

Chemical Composition of Original Rock The greatest factor in determining the mineral assemblage of a metamorphic rock. The chemical composition of the original rock controls the mineralogy of the metamorphosed rock.

Temperature And Pressure (1) The heat source is Earth’s internal heat. Rock can be heated by burial or by nearby igneous intrusion. Burial is inevitably accompanied by an increase in pressure due to the weight of the overlying rocks. An intrusion may be shallow, resulting in low pressure, or deep, resulting in high pressure.

Fig 8.1

Figure B8.2

Temperature And Pressure (2) Low-grade metamorphism is the result of metamorphic processes that occur at temperatures from about 100oC to 500oC, and at relatively low pressures. High-grade metamorphism is the result of metamorphic processes at high temperatures (above 500oC), and at high pressure.

Figure 8.1

Stress Stress is applied pressure that results in deformation in a solid, and the development of new textures. Uniform stress occurs if pressure is equal in all directions. Differential stress occurs if pressure is different in different directions. Texture is controlled by differential versus uniform stress.

Figure 8.2A

Figure 8.2

Figure 8.2B

Figure 8.3

Fluids and Metamorphism (1) Sedimentary rocks have open spaces between their grains filled by a watery intergranular fluid. This fluid: Is never pure water. Always contains small amounts of dissolved gases and salts. Contains traces of all the mineral constituents in the enclosing rocks.

Fluids and Metamorphism (2) Some of the fluid in sedimentary rock is retained surface water buried with the rocks. Some of the fluid is released when hydrous minerals (containing water in the formula) such as clays, micas, and amphiboles, decompose and lose water as the temperature increases on burial.

Fluids and Metamorphism (3) When the temperature and pressure change in a rock that is undergoing metamorphism, so does the composition of the intergranular fluid. The intergranular fluid is an important transporting medium.

Fluids and Metamorphism (4) When intergranular fluids are absent, metamorphic reactions are very slow. When pressure increases due to burial of a rock, and as metamorphism proceeds, the amount of pore space decreases and the intergranular fluid is slowly squeezed from the rock.

Fluids and Metamorphism (5) Any fluid that escapes during metamorphism will carry with it small amounts of dissolved mineral matter. Minerals precipitated in a facture are called a vein. Metamorphic changes that occur while temperatures and pressures are rising (and usually while abundant intergranular fluid is present) are termed prograde metamorphic effects.

Fluids and Metamorphism (6) Metamorphic changes that occur as temperature and pressure are declining (and usually after much of the intergranular fluid has been expelled) are called retrograde metamorphic effects.

Role of Time in Metamorphism Coarse-grained rocks are the products of long sustained metamorphic conditions (possibly over millions of years) at high temperatures and pressures. Fine-grained rocks are products of lower temperatures, lower pressures or, in some cases, short reaction times.

The Upper And Lower Limits Of Metamorphism At the lower end, metamorphism occurs in sedimentary and igneous rocks that are subjected to temperatures greater than about 100oC, usually under pressures of hundreds of atmospheres, caused by the weight of a few thousand meters of overlying rock. At the upper end, metamorphism ceases to occur at temperatures that melt rock.

Role of Water in Determining the Limits of Metamorphism The water present controls the temperature at which wet partial melting commences and the amount of magma that can form from a metamorphic rock. When a tiny amount of water is present, only a small amount of melting occurs. Migmatites are composite rocks that contain an igneous component formed by a small amount of melting plus a metamorphic portion.

How Rocks Respond To Temperature and Pressure Change In Metamorphism Lower-grade Metamorphism: Slaty Cleavage. the newly forming sheet-structure minerals create foliation that tends to be parallel to the bedding planes of the sedimentary rock being metamorphosed. Higher-grade Metamorphism: Schistosity. At intermediate and high grades of metamorphism, grain size increases. Foliation in coarse-grained metamorphic rocks is called schistosity (the parallel arrangement of coarse grains of the sheet-structure minerals).

Figure 8.5

Figure 8.6

Mineral Assemblage Change As temperature and pressure rise, one mineral assemblage “morphs” into another. Each assemblage is characteristic of a given rock composition.

Figure 8.8

Metamorphism of Shale and Mudstone Slate (low grade): The low grade metamorphic product of shale. Phyllite (intermediate grade): Pronounced foliation, larger mica grains. Schist and gneiss (high grade): Schist is a coarse-grained rock with pronounced schistosity. Gneiss is a high grade, coarse grained rock with layers of micaceous minerals segregated from layers of minerals such as quartz and feldspar.

Metamorphism of Basalt Greenschist has pronounced foliation like phyllite, but also a very distinctive green color because of its chlorite content. Amphibolite and granulite. When greenschist is subjected to intermediate-grade metamorphism, amphibole replaces the chlorite. Foliation is present in amphibolites, but is not pronounced because micas and chlorites are usually absent. At the highest grade of metamorphism, amphibole is replaced by pyroxene and an indistinctly foliated rock called a granulite develops.

Figure 8.9

Figure 8.19

Metamorphism of Limestone Marble is the metamorphic derivative of limestone. Coarsely crystalline. Pure marble is snow white. Pure grains of calcite. Many marbles contain impurities that result in various colors.

Metamorphism of Sandstone Quartzite is the metamorphic derivative of quartz. It is derived from quartz sandstone by filling of the spaces between the original grains with silica and by recrystallization of the entire mass.

Types of Metamorphism (1) There are four types of metamorphism: Cataclastic metamorphism Dominated by mechanical deformation. Contact metamorphism Dominated by recrystallization due to contact with magma.

Figure 8.13 A

Figure 8.13 B

Types of Metamorphism (2) Burial metamorphism Dominated by recrystallization aided by water. Regional metamorphism Both mechanical deformation and chemical recrystallization.

Cataclastic Metamorphism Mechanical deformation of a rock can occur with only minor chemical recrystallization. Usually localized and seen in igneous rocks when a coarse-grained granite undergoes intense differential stress. Grain and rock fragments become elongated and a foliation develops.

Contact Metamorphism (1) Occurs when bodies of hot magma intrude into cool rocks of the crust. Vapors given off by the intruding magma play a role. Mechanical deformation is minor or absent.

Figure 8.14

Contact Metamorphism (2) Rock adjacent to the intrusion becomes heated, developing a metamorphic aureole. Hornfels. Aureoles reach more than 100 m in thickness. Metamorphism that involves a lot of fluid and a large change in rock’s composition is called metasomatism.

Burial Metamorphism When buried deeply in a sedimentary basin, sediments may attain temperatures of a few hundred degrees Celsius, causing burial metamorphism. Zeolites are group of minerals with fully polymerized silicate structures containing the same chemical elements as feldspars, plus water. As temperatures and pressures increase, burial metamorphism grades into regional metamorphism.

Regional Metamorphism—A Consequence of Plate Tectonics Regional metamorphism results from tectonic forces that build mountains. It results from pronounced differential stresses and extensive mechanical deformation in addition to chemical recrystallization. Regional metamorphism produces greenschists and amphibolites.

Figure 8.15

Metamorphic Facies Mineral assemblages caused by specific sets of temperature/pressure conditions: Granulite facies, - hornfels facies Amphibolite facies, - zeolite facies Epidote-amphibolite facies, Greenschist facies, Blueschist facies, Eclogite facies

Figure 8.16

Metasomatism Metasomatism is the process in which rock compositions are distinctively altered through exchange with ions in solution. Metasomatic fluids may carry valuable metals and form mineral deposits.

Figure 8.17

Plate Tectonics And Metamorphism (1) There are five geologic settings where plate tectonics encourages metamorphism: Burial metamorphism. Subduction (blueschist and eclogite metamorphism). Regional metamorphism. Zone where wet fractional melting starts. Contact metamorphism.

Figure 8.18

Plate Tectonics And Metamorphism (2) Burial metamorphism occurs today in the sediment accumulated in ocean-floor trenches, such as those off the coasts of Peru and Chile. When oceanic crust with a covering of sedimentary rocks is dragged down by a rapidly subducting plate, pressure increases faster than temperature, subjecting the rock to high pressure but relatively low temperature. This is observed today along the subduction margin of the Pacific Plate where it plunges under the coast of Alaska and the Aleutian Islands.

Plate Tectonics And Metamorphism (3) Regional metamorphism: where continental crust is thickened by plate convergence and heated by rising magma, greenschist and amphibolite facies metamorphic condition occur. Examples include the Appalachians, Alps, Himalayas, and Andes. If the crust is sufficiently thick, when 10 percent or more of the crust has melted the magma so formed will rise forming stock or batholith. As the granitic magma formed by wet partial melting rises, it heats and metamorphoses the rocks with which it comes in contact.

Figure 8.19