EART10 Geologic Principles
Lecture #5
Metamorphic Rocks

Geologists recognize three primary rock types: igneous, sedimentary and metamorphic rocks. Igneous rocks form from the crystallization of magma or lava. Sedimentary rocks form by the lithification of mineral and rock pieces, through deposition from a fluid, or through biochemical production. Metamorphic rocks form from the alteration of existing rocks of any type, usually at high pressure and temperature, sometimes in the presence of large volumes of water. Metamorphic rocks serve as "geothermometers" and "geobarometers", recording the history of burial, heating, uplift and cooling of the crust. Metamorphosed limestone yields marble, the rock of choice for sculpting. The characteristic blue and green colors of Franciscan rocks, common along most of the California coast, are the result of metamorphic change of sedimentary and basaltic rocks. In addition, many important ores are produced by metamorphic reactions with water.

This lecture is about metamorphism as a process and metamorphic rocks as a product.

Metamorphism

Metamorphism is defined as the process of changing the characteristics of a rock by changes in pressure, temperature or, sometimes, volatile content. Most changes do not affect rock composition. Metamorphism changes the mineral assemblage and texture of rock. It happens in solid state: there is no melting, although fluids (mostly water) can be important. It is differentiated from diagenesis by minimum temperatures and pressures. The boundary is vague, but 200 degrees C is sufficiently hot to reach metamorphism and 5 to 10 km depth sufficiently deep.

Given this definition, it is logical to ask if all rocks at mid-crustal depths undergoing metamorphosis? The answer is no. Prompting the question of what causes metamorphism?

Ultimately, heat and gravity. These are manifest as the increase in temperature with depth (or by proximity to a pluton); the increase in pressure with depth; and through the horizontal stresses applied by plate movements. To calibrate you, recall that the geotherm (variation of temperature with depth) is about 30¡/km at shallow depths. Pressure increases at a rate of roughly 30 MPa/km, or roughly 300 atmospheres/km. This implies that typical conditions at 10 km are 300 degrees C and 3000 atmospheres (3000 times surface pressure). Rocks formed at the surface at not happy at these conditions and will metamorphose, so the answer to the question, "What causes metamorphism?" is a change in the pressure and temperature condition of a rock. This can occur through burial or through one of several other mechanisms.

Physical and Chemical Factors

What do temperature and pressure do and what other controls are there on metamorphism? Increasing temperature causes minerals to change. Some will dissolve into fluids. Others will internally reorganize their crystalline structure (change from one polymorph to another). Still others will grow large by cannibalizing other crystals and fluid content. The mineral assemblage that results, one in equilibrium with the prevailing temperature, is a "geothermometer" that records temperature.

Changes in pressure change both a rock's mineralogy and its texture. Pressure comes in two varieties: confining pressure and differential pressure (or stress). Confining pressure is uniform, pushing equally hard in all directions. It is a result of the weight of overlying rocks, increasing at about 30 MPa/km (or 300 atmospheres/km. Stresses vary with direction and require a solid to act upon (fluids cannot maintain differential pressures). They are produced by plate motions which result in external forces applied to the rocks. Pressure of both varieties causes mineral assemblages to adjust. The equilibrium assemblage that results is a "geobarometer" that records pressure. As minerals recrystallize, they develop textures in response to stress, growing in preferred directions with respect to the applied forces.

The combination of pressure and temperature during metamorphism determines metamorphic grade. Low grade metamorphism extends to about 10 km depth along a normal geotherm. High grade metamorphism picks up at about 20 km. Conditions of metamorphism need not follow the geotherm (a particular curve relating pressure and temperature), instead they may occupy a broad range of pressure and temperature combinations. At the high end, pressure and temperature at limited by the wet partial melting point of rock. Remember, metamorphism is solid-state: no melting occurs. Melting of rock results in igneous rock, not metamorphic rock.

The role of fluids: Water is usually present in the pore space of rocks. In metamorphism, the water is usually not water that was included with the sediment during deposition, but rather water derived from the breakdown of clay minerals in the rock. The carbon dioxide content is usually obtained from the dissolution of limestones and dolostones. There are dissolved gases and salts also. In most environments, water is not abundant, with water to solid rock ratios of 1 to 10 or less. This water is intergranular (water between rock grains). Its primary action is to facilitate recrystallization by allowing ions to move about more freely. Metamorphism is solid-state. Diffusion of the atoms through solids is slow. Diffusion through a liquid is fast, so water facilitates the process. As to why you can have water involved in a solid-state process, remember that the minerals don't fully dissolve or melt: they simply transfer ions with the fluid.

Sometimes water is present in high abundance and moves freely. This can happen in joints and fractures where water concentrates, or in other environments where hot water percolates through the rock. In this case, rock composition can change as the water transports stuff in and out. This process is referred to as metasomatism.

Kinds of Metamorphism

Geologists in the field have identified different kinds of metamorphism. By this I mean, characteristic settings, scales and styles of metamorphism. Each is indicative of the processes that produced it and each is discussed briefly below.

  • Regional metamorphism: This is the most widespread type of metamorphism, usually affecting large volumes of rock. Grades range from intermediate to high. Regional metamorphism is associated with differential pressure (stress), producing distinct rock textures. Regional metamorphism occurs in the cores of mountain belts.

  • Contact metamorphism: This occurs in the high temperature, low pressure halo around igneous intrusions (plutons). In the field, it appears as a shell of metamorphic rock with metamorphic grade decreasing outward from the center. The shell is referred to as a contact aureole. Since water is commonly associated with magmatism, metasomatism is common.

  • Burial metamorphism: Diagenesis and lithification start when rocks reach several kilometers depth. Continued burial leads to low grade burial metamorphism. It is common for sedimentary structures in the unaltered rocks to remain in the metamorphosed rocks, indicating relatively little recrystallization. This style of metamorphism grades into regional metamorphism with increasing pressure and temperature. We find it in deep sedimentary basins.

  • Cataclastic metamorphism: When blocks of rock are faulted, the constituent mineral grains are crushed and sheared. Faulting requires high stresses, but not necessarily high pressures or high temperatures, so much cataclastic metamorphism is no more than the mechanical breaking of rock. This style of metamorphism is commonly associated with regional metamorphism since the stresses require to fold rocks and uplift mountains also produce faults.

  • Hydrothermal metamorphism: This style of metamorphism is distinguished by high fluid content and changes in rock composition. It occurs when hot water percolates (or convects) through rock. This happens around plutons and in association with underwater volcanism. Pressures are usually low and temperatures moderate. By dissolving components that are least compatible within the rocks, hydrothermal metamorphism can produce very exotic deposits. Sulfides and massive ore bodies are associated with it.

    • Kinds of Metamorphic Rock

    Having detailed the varieties of metamorphism observed in the field, we now turn to the variety of metamorphic rocks found there. As with igneous and sedimentary rocks, we classify metamorphic rocks one the combined basis of composition and texture. Composition, however, appears in two guises: as the assemblage of minerals found in the metamorphosed rock and as the minerals originally present within the source rock. The latter dependence (on source rock) is most conveniently handled by breaking the classification scheme up into categories based on the most common source rocks: shales and mudstones; basalts; and quartz sandstones and limestones. What we use to classify rocks within each category will vary because the rocks that result vary.

    Metamorphism of shales and mudstones: Metamorphism of shales and mudstones produces rocks with foliation and cleavage. Foliation is derived from folium (for page) and refers to sets of flat or wavy parallel planes produced by rock deformation. They are the result of platy minerals (micas usually) which align perpendicular to the applied force (perpendicular to the direction of shortening). This occurs by recrystallization and by rotation of the crystals themselves. Slaty cleavage is a type of foliation, characterized by closely spaced, parallel planes along which the rock easily cleaves. This is differentiated from shales (which will break into tabular pieces) by the close spacing of the planes and the greater ease of cleaving. Slaty cleavage is exploited in chalkboards, pool table beds and roofing tile (photo to right; Copyright B. Whittles).

    Because foliation is pervasive in metamorphosed shales and mudstones, we use it to classify the rocks. Size of crystals, metamorphic grade and the degree to which minerals have segregated are also considered.

    Slate: Is the lowest grade metamorphic rock. It has slaty cleavage and aphanitic crystals.

    Phyllite: Low grade rock (higher than slate), with slaty cleavage, and just visible minerals that are often shiny, reflecting the larger crystals of mica and chlorite (also a mica, but present only in metamorphic rocks).

    Schist: Moderate to high grade rock with often wavy foliation and large crystals. This is the most abundant metamorphic rock type. Cleavage is not as strong as phyllite.

    Gneiss: Also moderate to high grade rock, like schists, but with distinct mineral segregation forming bands of light and dark minerals. Gneiss is not strongly foliated and crystals within it are large. Quartz and feldspar have replaced mica and chlorite at this grade (see photo to right; Copyright M.L. Bevier).



    Metamorphism of basalt: The classification of metamorphosed basalt is based on mineral assemblage more than texture though texture varies throughout. We recognize three categories:

    Greenschist: Equivalent in grade to slate, greenschist is low grade metamorphic basalt. Pyroxene and olivine in the source rock have reacted with water to produce chlorite (which is bright green, hence the name), plagioclase, epidote and calcite.

    Amphibolite: Amphibolites are medium to high grade rocks composed primarily of amphibole and plagioclase. They are not strongly foliated.

    Granulite: Granulites are very high grade, granular rocks containing little water (in water bearing minerals). Plagioclase, pyroxene, garnets and sillimanite (a alumino-silicate found only in metamorphic rocks) are common.


    Metamorphosed Basalts

    Greenschist

    Amphibolite

    Granulite

    images Copyright EOS U. British Columbia

    Metamorphism of Limestones and Quartz Sandstones: Classification of these rocks is based on the extent of recrystallization. Both are nearly monomineralic (limestone is mostly calcite; quartz sandstone is mostly quartz). The rocks which result are nonfoliated.

    Marble: Completely recrystallized limestone. Marble is composed of large, interlocking crystals of calcite. Grade varies, but the presence of remnant structures (grains, shell forms reefs structures) implies low grade. Marble is the choice rock of sculptors for its uniform texture, softness and translucency (see figure to right, Copyright EOS U. British Columbia).

    Quartzite: Recrystallized sandstone (quartz arenite source), with little or no remaining porosity. Grade varies, but as with marble, the presence of remnant structures implies low grade. Quartzite is used extensively as a railroad grade (bed rock).


    Metamorphic Facies

    For a sedimentary rock, facies is the sum of all observable characteristics of the sedimentary strata. It reflects sediment input, the amount and type of water available, the degree of biologic input, etc.-all the factors that control deposition. The concept of a metamorphic facies is similar, but not identical. We define a metamorphic facies as the assemblages of minerals that equilibrate under similar metamorphic conditions. To understand this, imagine a metamorphosed shale and a metamorphosed basalt side by side in a metamorphic terrain. Juxtaposition of the rocks implies similar metamorphic conditions. However, the minerals we would find in each would differ because of the different starting compositions. The different mineral assemblages in the two rocks comprise a metamorphic facies and are indicative of the pressure and temperature conditions that the rocks experienced. (Blueschist; Copyright EOS, U. British Columbia)

    There are seven commonly recognized metamorphic facies. The minerals that define them for different source rocks are tabulated below.

    Metamorphic Facies

    Facies

    Mineral Assemblage
    Basalt Source Rock

    Mineral Assemblage
    Shale Source Rock

    Granulite

    Pyroxene, plagioclase, garnet

    Biotite, K-feldspar, quartz, sillimanite

    Amphibolite

    Amphibole, plagioclase, garnet, quartz

    Garnet, biotite, muscovite,
    kyanite or sillimanite, quartz

    Greenschist

    Chlorite, amphibole, plagioclase, epidote

    Chlorite, muscovite, plagioclase, quartz

    Blueschist

    Blue-amphibole, chlorite, Ca-rich silicates

    Blue-amphibole, chlorite, quartz, muscovite

    Eclogite

    Pyroxene, garnet, kyanite

    Who knows?

    Hornfels

    Pyroxene, plagioclase

    Andalusite, biotite, K-feldspar, quartz

    Zeolite

    Calcite, chlorite, zeolite

    Zeolites, pyrophyllite (a clay), Na-rich mica


    The pressures and temperatures that exists within each facies are shown in Figure 5.16 of the book and below.

    Metamorphic minerals: Many of the minerals found in the different metamorphic facies are unique to metamorphic rocks, meaning that they don't occur or are very rarely found in primary igneous or sedimentary rocks. These include garnet, a dark, very hard silicate mineral (isolated tetrahedra); epidote, a green, waxy feeling silicate mineral (paired tetrahedra); and the zeolite minerals (hydrous silicates with Na, Ca and Al cations). Glaucophane, a rich blue colored amphibole, is common in very low temperature, high pressure metamorphism. Its color provides the blueschist facies with its name. Chlorite is a bright green colored mica. Despite the name, it has no chlorine (Cl) in it. Its color provides the greenschists facies with its name.

    Lastly, we have three alumino-silicate polymorphs: andalusite, kyanite and sillimanite. The chemical composition of the three is: Al2SiO5. Each occupies a unique pressure and temperature stability region (see figure). Andalusite is the polymorph in equilibrium with surface conditions. Sillimanite is stable at high temperatures and high pressures, whereas kyanite (image to the left) is stable at high pressures and low temperatures. The presence of these minerals in a rock is conclusive evidence of metamorphism. Which of the three polymorphs is found determines the pressure and temperature conditions prevailing during metamorphism.

    As we have seen, the different metamorphic facies occupy a wide range of pressure and temperature conditions. This leaves us to wonder how these different conditions are produced. Perhaps surprisingly, they are all produced at convergent plate margins (boundaries).

    Plate Tectonic Settings of Metamorphism

    At a convergent plate margin, two plates come together. This can be a collision of oceanic plates, an ocean and a continent, or two continents. Because plates cannot stack on the surface, continued convergence requires one plate to subduct into the mantle beneath the other. The one that does is always the densest. Oceans are denser than continental plates, so they subduct when the two converge. When oceanic plates converge, the older, colder, denser plate subducts. Continents are too buoyant to subduct. To accommodate convergence, the crust and lithosphere thicken (conserving volume but reducing surface area). At all three types of convergent boundary (ocean-ocean, ocean-continent, continent- continent), high stresses, high deposition rates and volcanism can be found.

    Convergent boundaries produce a variety of metamorphic facies and styles of metamorphism. The uplift of mountains results in regional metamorphism. Baking of "country" rock by igneous intrusions produces contact metamorphism. Faulting of highly stressed crustal rocks results in cataclastic metamorphism. Rapid sedimentation and subsidence offshore produces burial metamorphism. Attendant with these styles of metamorphism are all the seven metamorphic facies. Amphibolite to granulite facies are found within the cores of mountain belts. Greenschists occur at shallower depths within the belts. Blueschists are produced by the rapid subduction of sediments and oceanic crust. Here high pressures can be reached before temperatures within the subducted crust have had time to rise. Eclogite facies are reached within the subducting crust when it reaches depths of 20 to 25 km. Hornfels are found in contact aureoles around shallow intrusions where hot magma heats the surrounding rocks. Lastly, zeolite facies metamorphism occurs within the accretionary prism located arc ward of the trench. The prism consists of a wedge-shape pile of sediments scraped off the down going plate. Water is abundant; temperatures and pressures are low-just sufficient to reach metamorphism (rather than diagenesis).

    A unique form of metamorphism occurs at divergent plate boundaries. Here, new plate is created by the upwelling of hot mantle. Partial melting produces new oceanic crust through which water percolates, or convects, and is heated. Where it exits the rock, water temperatures can be as high 450¡C, and are commonly as high as 350¡C (high water pressure at the sea floor prevents boiling). As the heated water passes through the fresh basalt, it leaches out silica, iron, sulfur, manganese, copper and zinc. The basalt incorporates magnesium and sodium from the water, altering its composition and mineralogy. As the hot water ascends, sulfides (such as pyrite, FeS), sulfates (such as anhydrite, CaSO4) and ores are deposited, which someday may be mined. Sulfur in the water feeds chemo-synthetic plant and animal communities that are completely independent of photosynthesis. They survive in sunless conditions, in hot water at high pressures around the black smokers that supply them with energy to live. (Image Copyright J.R. Delaney, U. of Washington).


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    Document Last Modified Monday, November 17 1997 09:01