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Helgeson, H. Thermodynamic relations. Acta 32 , — Holister, L. Jurewicz, S. Kerrich, R. Knipe, R. Kwak, T. Lachenbruch, A. Matthews, A. Acta 44, — Misch, P. Meike, A. Nicolas, A. Wiley, New York. Obreimoff, J.

Metamorphic processes: Reactions and microstructure development

London A , Orowan, E. Parks, G. Passchier, C. Petrovich, R. Deformation and dissolution of quartz under laboratory conditions. Acta 45 , — Poirier, J. Earth Planet. Reitan, P. Lithos 10 , — Robin, P-Y. Acta 42 , — Rosenfeld, J. Paper , p. Rutter, E. London , — Sacerdoti, M. Scholz, C. Sibson, R. Sodre Borges, F. Stumm, W. Wiley Interscience, New York. Vernon, R. Halshed Press, New York.

Wallace, R. Walton, M. Watts, M. White, S. Wintsch, R. Union EOS 64 , Wyart, J. Yund, R. Minerals 7 , — Metamorphism occurs because rocks undergo changes in temperature and pressure and may be subjected to differential stress and hydrothermal fluids. Metamorphism occurs because some minerals are stable only under certain conditions of pressure and temperature.

When pressure and temperature change, chemical reactions occur to cause the minerals in the rock to change to an assemblage that is stable at the new pressure and temperature conditions. But, the process is complicated by such things as how the pressure is applied, the time over which the rock is subjected to the higher pressure and temperature, and whether or not there is a fluid phase present during metamorphism.

Temperature Temperature increases with depth in the Earth along the Geothermal Gradient. Thus higher temperature can occur by burial of rock. Temperature can also increase due to igneous intrusion. 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. There are two kinds of differential stress. Normal stress causes objects to be compressed in the direction of maximum principal stress and extended in the direction of minimal stress. If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock. Shear stress causes objects to be smeared out in the direction of applied stress.

Differential stress if acting on a rocks can have a profound affect on the appearance or texture of the rock. Grade of Metamorphism Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form. Retrograde Metamorphism.

As temperature and pressure fall due to erosion of overlying rock or due to tectonic uplift, one might expect metamorphism to a follow a reverse path and eventually return the rocks to their original unmetamorphosed state.

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Such a process is referred to as retrograde metamorphism. If retrograde metamorphism were common, we would not commonly see metamorphic rocks at the surface of the Earth. Since we do see metamorphic rocks exposed at the Earth's surface retrograde metamorphism does not appear to be common. The reasons for this include: chemical reactions take place more slowly as temperature is decreased during prograde metamorphism, fluids such as H 2 O and CO 2 are driven off, and these fluids are necessary to form the hydrous minerals that are stable at the Earth's surface. Metamorphic Rock Types.

There are two major subdivisions of metamorphic rocks. Foliated — These have a planar foliation caused by the preferred orientation alignment of minerals and formed under differential stress. They have a significant amount of sheet silicate platy minerals and are classified by composition, grain size, and foliation type.

These are classified mainly by the minerals present or the chemical composition of the protolith. Foliated Metamorphic Rocks. Example - metamorphism of a shale, made up initially of clay minerals and quartz all of clay or silt size. The preferred orientation of these sheet silicates causes the rock to easily break along the planes parallel to the sheet silicates, causing a slatey cleavage. Note that in the case shown here, the maximum stress is applied at an angle to the original bedding planes, so that the slatey cleavage has developed at an angle to the original bedding.

Because of the nearly perfect breakage along planes, slates are useful for blackboards and shingles. Phyllite - Fine mica-rich rock, formed by low — medium grade metamorphism.

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In a phyllite, the clay minerals have recrystallized into tiny micas biotite and muscovite which reflect a satiny luster. Phyllite is between slate and schist. Schist - The size of the mineral grains tends to enlarge with increasing grade of metamorphism. Eventually the rock develops a near planar foliation caused by the preferred orientation of sheet silicates mainly biotite and muscovite.

Quartz and Feldspar grains, however show no preferred orientation.

Metamorphic Rocks

The irregular planar foliation at this stage is called schistosity. Schist often has other minerals besides micas. When these non-mica minerals occur with a grain size greater than the rest of the rock, they are called pophyroblasts. These dark colored minerals tend to become segregated in distinct bands through the rock, giving the rock a gneissic banding. Granulite - At the highest grades of metamorphism all of the hydrous minerals and sheet silicates become unstable and thus there are few minerals present that would show a preferred orientation.

The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks. Migmatites — If the temperature reaches the solidus temperature first melting temperature , the rock may begin to melt and start to co-mingle with the solids. Usually these melts are felsic with the mafic material remaining metamorphic. Non-foliated Metamorphic Rocks. Non-foliated rocks lack a planar fabric.

Absence of foliation possible for several reasons:. Non-foliated rocks are given specific names based on their mineralogy and composition: Amphibolite - These rocks are dark colored rocks with amphibole usually hornblende as their major mineral. They are usually poorly foliated and form at intermediate to high grades of metamorphism of basaltic or gabbroic protoliths. Hornfels - These are very fine grained rocks that usually form as a result of magma intruding into fined grained igneous rocks or shales. The magma causes a type of metamorphism called contact metamorphism to be discussed later.

Quartzite - A rock made up almost entirely of quartz. They are formed by metamorphism of quartz arenites sandstones. Since quartz is stable over a large range of temperatures and pressures, no new minerals are formed during metamorphism, and the only metamorphic effect that occurs is recrystallization of the quartz resulting in interlocking crystals that make up a very hard rock.

Marble - A limestone or dolostone made up only of calcite or dolomite will metamorphose to a marble which is made mostly recrystallized calcite or dolomite. The Recrystallization usually obliterates all fossils. Marbles have a variety of colors and are often complexly banded. They are commonly used as a decorative stone. Although textures and structures of the protolith are usually destroyed by metamorphism, we can still get an idea about the original rock from the minerals present in the metamorphic rock.

Minerals that form, do so because the chemical elements necessary to form them are present in the protolith. General terms used to describe the chemical composition of both the protolith and the resulting metamorphic rock are:. Pelitic Alumina rich rocks, usually shales or mudstones. Because of the abundance of sheet silicates, pelitic rocks commonly form slates, phyllites, schists, and gneisses during metamorphism.

Mafic - These are Mg and Fe rich rocks with low amounts of Si. Minerals like biotite, hornblende and plagioclase form during metamorphism and commonly produce amphibolites. Calcareous - These are calcium-rich rocks usually derived from limestones or dolostones, and thus contain an abundance of Calcite.

Marbles are the type of metamorphic rock that results. Quartzo-Feldspathic - Rocks that contain an abundance of quartz and feldspar fall into this category. Protoliths are usually granites, rhyolites, or arkose sandstones and metamorphism results in gneisses containing an abundance of quartz, feldspar, and biotite. Metamorphism can take place in several different environments where special conditions exist in terms of pressure, temperature, stress, conditions, or chemical environments. We here describe several diff rent types of metamorphism that are recognized.

A map of a hypothetical regionally metamorphosed area is shown in the figure below.


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These lines are called isograds meaning equal grade and represent lines really surfaces where the grade of metamorphism is equal. A map of a regionally metamorphosed areas are can be seen in figure 8. Since oceanic ridges are areas where new oceanic crust is created by intrusion and eruption of basaltic magmas, these water-rich fluids are heated by the hot crust or magma and become hydrothermal fluids. The hydrothermal fluids alter the basaltic oceanic crust by producing hydrous minerals like chlorite and talc. Because chlorite is a green colored mineral the rocks hydrothermal metamorphic rocks are also green and often called greenstones.

But, because the oceanic crust by the time it subducts is relatively cool, the temperatures in the crust are relatively low. Under the conditions of low temperature and high pressure, metamorphism produces an unusual blue mineral, glaucophane. Compressional stresses acting in the subduction zone create the differential stress necessary to form schists and thus the resulting metamorphic rocks are called blueschist.

Shock Metamorphism - When a large meteorite collides with the Earth, the kinetic energy is converted to heat and a high pressure shock wave that propagates into the rock at the impact site. The heat may be enough to raise the temperature to the melting temperature of the earth rock.

The shock wave produces high enough pressure to cause quartz to change its crystal structure to more a dense polymorph like coesite or stishovite. Ancient meteorite impact sites have been discovered on the basis of finding this evidence of shock metamorphism.

In general, metamorphic rocks do not undergo significant changes in chemical composition during metamorphism. The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism.


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Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to.