6 Metamorphic Rocks
Contributing Author: Dr. Peter Davis, Pacific Lutheran University
- Describe the environment of temperature and pressure in which metamorphism takes place.
- Identify and describe the three principal agents of metamorphism.
- Describe what recrystallization is and how it affects mineral crystals.
- Explain what foliation is and how it results from directed pressure and recrystallization.
- Explain the relationship among slate, phyllite, schist and gneiss in terms of metamorphic grade.
- Define a metamorphic index mineral.
- Explain how metamorphic facies relate to plate tectonic processes.
- Describe what a contact aureole is and how contact metamorphism affects surrounding rock
- Describe the role of hydrothermal metamorphism in forming mineral deposits and ore bodies
Metamorphic rocks (meta=change and morphos=form) are one of the three rocks in the rock cycle and represent material that has been changed due to heat, pressure, and/or fluids. The rock cycle shows that both igneous and sedimentary rocks can become metamorphic rocks, metamorphic rocks can themselves be re-metamorphosed. This is due to the fact that metamorphism is ultimately caused by plate tectonic motion, so if a sedimentary rock is buried underground to great depths by slow plate motions, the rock could be constantly metamorphosed, in a conceptual way tracking the plate tectonic process that is causing the metamorphism. This is why metamorphic rocks are important. They can record how long-term, deep-seated tectonic processes that shape our planet work from a view inside the Earth a very long time ago .
6.1 Process of Metamorphism
Metamorphism occurs when a previously existing rock changes in composition and texture due to heat, pressure, and/or hot circulating fluids. The changes in composition and texture occur without melting the rock. This previously existing rock is called the parent rock, or protolith from proto=first and lithos=rock. Most metamorphic processes take place underground inside the earth’s crust. In general, the chemistry of the protolith can be changed by heat, a type of pressure called confining pressure, and fluids. While the texture is changed by a type of pressure called directed stress. The following sections will discuss each metamorphic process in more detail.
6.1.1 Heat (Temperature)
Heat is the amount of thermal energy of a substance. This may be the heat of a body of igneous rock trapped within a volcano, or the heat of ocean water. On the other hand, temperature is the measure of the vibrational (kinetic) energy of a substance. Therefore, as the temperature of a body increases, the vibrational energy rises . At the atomic scale, high temperatures cause atoms in the crystal structure to vibrate so vigorously that the atoms can jump from one position to another in the crystal. So, temperature can affect the chemical makeup of minerals in a rock by affecting the chemical equilibrium, or balance of cations in minerals.
Since temperature increases with increasing depth with the Earth (geothermal gradient), metamorphic rocks are affected by depth and these rocks can record these temperature changes within their minerals. Temperature increases from 15˚C (50˚F) at the surface to about 650-700˚C at the base of the continental crust. Metamorphic rocks lie in between sedimentary rocks and magma in the rock cycle diagram above. In terms of temperature, heat-driven metamorphism can be as cold as 200˚C, and as high as the temperature needed to melt a rock, generally between 700-1100˚C . However, the temperature at which a mineral melts is dependent on the pressure.
Pressure is the force exerted over a unit area on a material. Like heat, pressure can affect the chemical equilibrium. The pressure that affects metamorphic rocks can be grouped into confining pressure and directed stress. Stress is a scientific term indicating a force. Strain is the result of this stress, including metamorphic changes within minerals.
Pressure exerted on rocks under the surface is due to the simple fact that rocks lie on top of one another. When pressure is exerted from rocks above, it is balanced from below and sides, and is called confining or lithostatic pressure. Confining pressure has equal pressure on all sides (see figure) and is responsible for causing chemical reactions to occur just like heat, or temperature. These chemical reactions will cause new minerals to form.
Confining pressure is measured in bars and ranges from 1 bar at sea level to around 10,000 bars at the base of the crust . For metamorphic rocks, pressures range from a relatively low-pressure of 3,000 bars around 5-6 kilometers below the Earth’s surface to a relatively high pressure of 50,000 bars around 15-35 kilometers.
Directed stress, or differential or tectonic stress, is an unequal balance of forces on a rock in one or more directions (see figure above). In contrast to confining pressure that cause chemical reactions, the magnitudes of directed stress are much lower and do not cause chemical reactions to occur . As a metamorphic process, directed stress modifies the arrangement, size, and/or shape of mineral crystals in a parent rock. Directed stresses are ultimately caused by the physical motion of large-scale plates of lithosphere. Most importantly, while minerals are being mechanically manipulated, they are not melted, and do not need to change chemically or compositionally at all. Just the arrangement of crystals. In simplistic terms, when a rock begins to change shape, it is really the minerals within the rock that change shape and/or orientation. This mineral arrangement is called texture such as foliations and lineations discussed below.
Directed stresses can produce rock textures in many ways. Stress can cause minerals to rotate and change their orientation in space, reduce their grain-size by physically breaking them, can deform their shape, and can make minerals actually grow in size. These processes rely on the fact that minerals can change shape through recrystallization. Dissolution recrystallization occurs when a mineral responds to a directed stress by dissolving from the part of a mineral with the highest stress and precipitates (or regrows) on a surface with lower stress. Rocks can also recrystallize to make their grain-size larger, much like soap bubbles that increase in size by absorbing smaller adjacent bubbles. Rocks can also recrystallize by allowing the grain-size to reduce, but not allow the rock to break. In this case rocks deform much like silly putty, and are very important in understanding how rock faults work in the deep earth.
A third agent of metamorphism is chemically reactive fluids which can be expelled from magma as it crystallizes and nearby metamorphic reactions. These reactive fluids are made of mostly water (H2O) and carbon dioxide (CO2), and smaller amounts of elements such as potassium (K), sodium (Na), iron (Fe), magnesium (Mg), calcium (Ca), and aluminum (Al). These fluids react with minerals in the protolith causing compositional changes because the fluid is out of chemical equilibrium with the minerals. This is similar to equilibrium reactions driven by heat and pressure discussed above. This process uses elements from the existing chemistry of the protolith or new elements from the fluids to contribute to the growth of new minerals. One example of a compositional change is emplacement of economically important mineral deposits and ores in veins surrounding magma bodies. For example, the mining districts in the Cottonwood Canyons and Mineral Basin of northern Utah resulted from the Little Cottonwood Stock (granite) intruding limestone and dolostone country rock. The circulating fluids easily react (or dissolve) the limestone and precipitate minerals such as gold (native element), argentite (silver sulfide), galena (lead sulfide), and chalcopyrite (copper iron sulfide) . The Rio Tinto Kennecott Mine near Salt Lake City is mining ore that formed from hydrothermal alteration from an igneous intrusion called the Bingham Stock. General alteration of surrounding rocks by these hot fluids is called hydrothermal metamorphism and accompanies igneous activity wherever it occurs. This process is discussed in detail in Chapter 16.
An example of hydrothermal fluids that remove elements rather than deposit them are mid-ocean spreading centers where new crust is forming and interacting with seawater. As seawater circulates down through fractures in the fresh basalt, fractures act like a channel system that allows seawater to interact with the hot basalt and picks up ion in solution. The ion most commonly harvested in this manner are those in between crystals that don’t fit into the typical silicate mineral structures such as copper. When this hot chemically-laden water emerges at the sea floor, hydrothermal vents called black smokers are produced, so named because of the dark chemical precipitates produced as the hot fluid interacts with cold seawater. Deposits from black smokers include ores of important metals. Ancient black smokers are responsible for copper ores mined on Cyprus as early as 4000 BCE and later by the Romans , an important contribution to the development of human civilization. Hydrothermal metamorphism of basalt near mid-ocean ridges creates the metamorphic rock serpentinite , which includes minerals from the serpentine subgroup of minerals.
6.2 Metamorphic textures
Foliated rocks are defined by the parallel alignment of mineral grains that are mostly thin and planar such as micas or in some cases hornblende. Foliated rocks typically appear as if the minerals are stacked like pages of a book, thus the use of the term ‘folia’, like a leaf, for foliation. Some minerals are longer in one direction, linear like a pencil or a needle, rather than a planar-shaped book. These linear objects can also be aligned within a rock. This is referred to as a lineation. Linear objects, such as hornblende grains, tourmaline, or stretched quartz grains, can be arranged as a foliation, lineation, or foliation/lineation together. First they can lie on a plane, but with no common or preferred direction, this is foliation. Secondly, minerals can line up and point in a common direction, this is lineation. Finally, minerals can lie on a plane AND point in a common direction; this is both foliation and lineation.
Slate is a fine-grained metamorphic rock that exhibits a foliation called slaty cleavage that is the flat orientation of the small platy crystals of mica and chlorite forming perpendicular to the direction of stress. The minerals in slate are two small to see with the unaided eye. The thin layers in slate may resemble sedimentary bedding, but they are a result of directed stress and may lie at angles to the original strata. In fact, original sedimentary layering may be partially or completely obscured by the foliation. Thin slabs of slate are often used as a building material for roofs and tiles.
Phyllite is a foliated metamorphic rock in which platy minerals have grown larger and the surface of the foliation shows a sheen from light reflecting from the grains, perhaps even a wavy appearance, called crenulations. Similar to phyllite but with even larger grains is the foliated metamorphic rock schist, which has large platy grains visible as individual crystals. Common minerals are muscovite, biotite, and porphyroblasts of garnets. A porphyroblast is a large crystal of a particular mineral surrounded by small grains. Schistosity is a textural description of foliation created by the parallel alignment of platy visible grains. Some schists are named for their minerals such as mica schist (mostly micas), garnet schist (mica schist with garnets), and staurolite schist (mica schists with staurolite).
Gneissic banding is a metamorphic foliation in which visible silicate minerals separate into dark and light bands or lineations. These grains tend to be coarse and often folded. A rock with this texture is called gneiss. Since gneisses form at the highest temperatures and pressures, some partial melting may occur. This partially melted rock is a transition between metamorphic and igneous rocks called a migmatite .
Migmatites appear as dark and light banded gneiss that may be swirled or twisted some since some minerals started to melt. Thin accumulations of light colored rock layers can occur in a darker rock that are parallel to each other, or even cut across the gneissic foliation. The lighter colored layers are interpreted to be the result of the separation of a felsic igneous melt from the adjacent highly metamorphosed darker layers, or injection of a felsic melt from some distance away.
Non-foliated textures do not have lineations or alignment of mineral grains. Non-foliated metamorphic rocks are typically composed of just one mineral, and therefore, usually show the effects of metamorphism with recrystallization in which crystals grow together but with no preferred direction. The two most common examples of non-foliated rocks are quartzite and marble. Quartzite is the metamorphic equivalent of the protolith quartz sandstone in which the quartz grains are enlarged and interlocked by recrystallization. A defining characteristic for distinguishing quartzite from sandstone is that when broken with a rock hammer, the quartzite crystals break across the grains rather than around the grains as in sandstone. Compared with marble, quartzite is very hard and resistant to weathering. Marble is metamorphosed limestone or dolostone composed of calcite or dolomite. Recrystallization has simply generated larger interlocking crystals of calcite or dolomite. These minerals are considerably softer than quartz. Another way to distinguish marble from quartzite is with a drop of dilute hydrochloric acid. Marble will effervesce (fizz) if it is made of calcite. A third non-foliated rock is hornfels identified by its dense, fine grained, hard, blocky or splintery texture composed of several silicate minerals. Crystals in hornfels are so small that specialized study is required to identify them. Most importantly this growth or reduction in size takes place without melting the rock.
6.3 Metamorphic Grade
The range of metamorphic change that a rock may undergo is typically referred to as a metamorphic grade. Rocks start as a protolith and progress up from low grade to high grade. Low-grade metamorphism begins at temperatures and pressures just above sedimentary rock conditions. Among foliated metamorphic rocks, increasing metamorphic grade is seen in the sequence: slate→phyllite→schist→gneiss. These rock textures are made because new minerals are formed that are stable at different temperatures and pressures within the earth.
Index minerals are special minerals that only form at certain temperatures and pressures and therefore can be used to identify the degree of metamorphism to which the rocks have been exposed. The particular index mineral that forms depends on the original chemistry of the parent rock. Geologists finding these minerals in metamorphic rocks have an important clue to the protoliths and the metamorphic conditions they were exposed.In some cases, from low-grade to high-grade common index minerals are chlorite, muscovite, biotite, garnet, and staurolite. In other cases, the index minerals may include sillimanite, kyanite, and andalusite which exhibit a property called polymorphism, that is they all have the same chemical formula (Al2SiO5) but under different conditions of temperature and pressure, form different crystal structures with different properties. The figure below is a phase diagram of these three minerals showing the realms of temperature and pressure in which each is stable. Phase diagram of sillimanite, kyanite, and staurolite.
Some metamorphic rocks are named based on the highest grade index mineral present. A chlorite schist is a schist with the chlorite index mineral that indicates a low-grade of metamorphism. At a little higher grade, a muscovite schist has the index mineral muscovite. At an even higher grade garnet forms as the index mineral making garnet schist.
6.4 Metamorphic Tectonic Environments
As with igneous processes in the Earth’s upper mantle, metamorphic rocks form at different zones of pressure (depth) and temperature as shown on the pressure-temperature (P-T) diagram. The term facies is an objective description of a rock. In metamorphic rocks facies are groups of minerals called mineral assemblages. The names of metamorphic facies on the pressure-temperature diagram reflect minerals and mineral assemblages that are stable at these pressures and temperatures and provide information about the metamorphic processes that have affected the rocks. This is useful when interpreting the history of a metamorphic rock.
In the late 1800s, British geologist George Barrow mapped zones of index minerals in different metamorphic zones of an area that underwent regional metamorphism. Barrow outlined a progression of index minerals, named the Barrovian Sequence, that represents increasing metamorphic grade: chlorite (slates and phyllites) -> biotite (phyllites and schists) -> garnet (schists) -> staurolite (schists) -> kyanite (schists) -> sillimanite (schists and gneisses).
The first of the Barrovian sequence has a mineral group that is commonly found in the metamorphic greenschist facies. Greenschist rocks form under relatively low pressure and temperatures and represent the fringes of regional metamorphism. The “green” part of the name is derived from green minerals like chlorite, serpentine, and epidote, and the “schist” part is applied due to the presence of platy minerals such as muscovite. Depending on the original rock chemistry, regional metamorphism will progress from the zeolite facies through greenschist and amphibolite facies, and finally granulite and eclogite facies.
6.4.1 Burial Metamorphism
Burial metamorphism occurs when rocks are deeply buried (> 2000 meters) . Burial metamorphism commonly occurs in sedimentary basins, where rocks are buried deeply by overlying sediments. As an extension of diagenesis, a process that occurs during lithification (Chapter 5), burial metamorphism can cause clay minerals, such as smectite, in shales to change to another clay mineral illite. Or it can cause quartz sandstone to metamorphose into the quartzite such the Big Cottonwood Formation in the Wasatch Range of Utah. This formation was deposited as ancient near-shore sands in the late Proterozoic, deeply buried and metamorphosed to quartzite, folded, and later exposed at the surface in the Wasatch Range today. Increase of temperature with depth in combination with an increase of confining pressure produces low-grade metamorphic rocks with a mineral assemblages indicative of a zeolite facies .
6.4.2 Contact Metamorphism
Contact metamorphism occurs when rocks are exposed to high temperatures and low pressures. For example when a magma intrudes into pre-existing rocks, or a lava flow spreads over a surface. Like all metamorphic rocks, the chemistry of the parent rock is a major factor in determining the final metamorphic rock and index minerals. If the parent rock is fine-grained like shale or basalt, the high temperatures can recrystallize the minerals characteristically producing hornfels. Surrounding igneous intrusions, contact metamorphism can create a quartzite out of a sandstone and marble out of a limestone. Deep intrusions can create a chilled zone or metamorphic aureole of altered rock in which the degree of metamorphism diminishes with distance away from the igneous contact. This results in rings of various index minerals surrounding the intrusion such as with the Alta Stock in Little Cottonwood Canyon, Utah. The Alta Stock intruded the surrounding siliceous dolostones with index minerals with talc on the outside, tremolite (an amphibole), forsterite (an olivine), and others located closer to the granite .
The low pressures and high temperatures associated with contact metamorphism can produce numerous facies. The hornfels facies involves the lowest pressures. At slightly higher pressures greenschist, amphibolite, or granulite facies can be produced, depending on the temperature and protolith mineralogy.
6.4.3 Regional Metamorphism
Regional metamorphism occurs when temperatures and pressures are exerted on a rock over a large geographic area. This is often associated with mountain belts from converging continental tectonic plates. Increasing metamorphic grade can be observed as one travels from the edge of a mountain belt into its core. This setting for metamorphism can produce the Barrovian sequence mentioned above, with the lowest-grade metamorphism on the flanks of the mountains and the highest grade near the core of the mountains.
An example is a drive traveling eastward from New York state through Vermont into New Hampshire, in the northern Appalachian Mountains. Along this route, you can see the gradual increase of metamorphism from sedimentary rocks to low-grade metamorphic rocks, then higher-grade metamorphic rocks, and eventually the granitic core. The rock sequence is sedimentary rocks then slates, phyllites, schists, gneisses, and migmatites and granites of New Hampshire, “the Granite State.” The reverse sequence can be seen heading east through eastern New Hampshire to the coast .
6.4.4 Subduction Zone Metamorphism
Another way that regional metamorphism can occur is subduction zone metamorphism. Subduction zone metamorphism takes place when a slab of oceanic crust is subducted into a realm of higher pressure and low temperature. Rock is a pretty good insulator so temperature within the subducting slab increases fairly slowly relative to pressure which increases instantaneously with burial. Thus the metamorphic realm of the subduction zone is high pressure/low temperature. One index mineral that forms at high pressure but low temperatures is glaucophane, which has a distinctive blue color. Thus, this zone is called the blueschist facies on the P-T diagram above. The California Coast Range near San Francisco has blueschist facies rocks associated with past subduction such as actual blueschists, greenstones (chlorite minerals in metamorphosed basalt), and folded red chert beds .
6.4.5 Fault Metamorphism
There are a range of metamorphic rocks made along faults that range from near the surface to the bottom of the crust, and into the mantle. Near the surface, rocks involved in repeated or extending faulting produce a material called rock flour, which is rock ground up to the particle size of flour used for food. At lower depths, faulting create cataclasites . At depths below cataclasites, mylonites are formed. Mylonites are metamorphic rocks created by dynamic recrystallization through directed shear forces, generally resulting in a reduction of grain size . When larger, stronger crystals (like feldspar, quartz, garnet) embedded in a metamorphic matrix are sheared into an asymmetrical eye-shaped crystal, an augen is formed .
6.4.6 Shock Metamorphism
Shock (also known as impact) metamorphism is metamorphism resulting from meteor or other bolide impacts, or from a similar high-pressure shock event. Shock metamorphism is the result of very high pressures delivered relatively rapidly. Shock metamorphism produces planar deformation features, tektites, shatter cones, and quartz polymorphs. Shock metamorphism produces planar deformation features (shock laminae), which are narrow planes of glassy material with distinct orientations found in silicate mineral grains. Shocked quartz has planar deformation features .
Shatter cones are cone-shaped pieces of rock created by dynamic branching fractures caused by high impact . Their diameter can range from microscopic to several meters. Fine-grained rocks with shatter cones show a distinctive horsetail pattern. The quartz polymorphs coesite and stishovite are indicative of impact metamorphism . As discussed in the Minerals Chapter, polymorphs are minerals with the same composition but different crystal structures. Intense pressure (> 10 GPa) and moderate to high temperatures (700-1200 °C) are required to form these minerals.
Impacts and other sudden high energy events can produce glass. Tektites are gravel-size glass grains ejected during an impact event. They resemble volcanic glass but, unlike volcanic glass, tektites contain no water or phenocrysts, and have a different bulk and isotopic chemistry. Tektites contain partially melted inclusions of shocked mineral grains . Although all are melt glasses, tektites are also chemically distinct from trinitite, which is produced from thermonuclear detonations , and fulgurites, which are produced by lightning strikes . All geologic glasses not derived from volcanoes can be called with the general term pseudotachylytes , a name which can also be applied to glasses created by faulting. The term pseudo in this context means ‘false’ or ‘in the appearance of’, a volcanic rock called tachylite because the material observed looks like a volcanic rock, but is produced by significant shear heating.