8.14 Talc-kyanite schist, pinterest.com Thus, the precursors of schists are shale, slate, and phyllite. Descending slabs of wet cool ocean lithosphere, which have been continuously carried to depth for millions of years, cool the crust and upper mantle below. 8.76 Brucite, Andrew Silver, Wikimedia Commons It is stable in Field 3 only. At slightly higher grades, metabasites become greenschists, obtaining schistosity from parallel arrangements of the green amphibole actinolite and chlorite. This is especially true of basalts and other high-temperature volcanic rocks affected by low-grade regional metamorphism. An unknown rock with this generic kind of fabric may be called granofels, but usually the geologist can look at it closely and give it a more specific name based on its minerals, like marble for a carbonate rock, quartzite for a quartz-rich rock, and so on: amphibolite, eclogite and more. Figure 8.36, for example, shows an 8-cm wide rock consisting only of coarse blue calcite. In mountain belts and other places where volcanic activity occurs, convective heat flow due to rising magmas contributes much more heat than normal conduction. Solid state recrystallization and neocrystalization. The different fields are the ranges of pressure and temperature where each polymorph is stable. The characteristic green color comes from fine-grained chlorite and epidote in the rocks. 8.27 Phyllite, Kurt Hollocher Petrography of metamorphic rocks: fabric, composition, and classification. If rocks always went to equilibrium, we should have no samples of high-grade rocks or minerals to study. They were never unmetamorphosed rocks at low pressure and temperature. The photo below in Figure 8.65 shows a typical greenstone outcrop in northern Minnesota. The talc-kyanite schists from the Dora Maira Massif, Italy, are examples of extremely high-pressure rocks. Muscovite is generally absent at the highest temperatures because it dehydrates to K-feldspar, sillimanite, and water vapor (at temperatures above the blue dashed line). Further confusion arises because petrologists use some facies names in a more restricted sense, referring to particular rock types with important tectonic significance. If schists contain prominent minerals, we name them accordingly. While this was occurring, metamorphism produced wine-red garnet crystals – a single large one is near the left side of the photo and many small ones are scattered throughout. • Microfolding may occur • A closely spaced fracture array is not cleavage Metamorphic Facies • Used for tectonic planar fabrics formed at or below lower greenschist facies conditions (i.e., ≤ 300°C). Most of the reactions in the table above are prograde, but the two examples of carbonation and hydration reactions are retrograde reactions that often affect mafic rocks. Red lines and text in this phase diagram show the stability fields for the different polymorphs: kyanite at high pressure, sillimanite at high temperature, and andalusite at low pressure. The photo in Figure 8.67 is an example. And, at the highest grades, garnet and pyroxene become stable. This gneiss, from the Czech Republic, contains pink K-feldspar rich layers alternating with darker layers that contain biotite. foliation (a planar fabric of metamorphic origin). Coesite (a high-pressure polymorph of SiO2) and ellenbergite (another high-pressure mineral) are found as small inclusions in some of the pyrope crystals. Some geologists have also described another kind of metamorphism, called burial metamorphism, but it is really just high-temperature diagenesis. 8.24 Aligned crystals of hornblende, Kurt Hollocher Lawsonite has about the same composition as anorthite. With a bit more heating, any leftover kaolinite decomposes to pyrophyllite, diaspore, and H2O vapor. 8.10 Garnet granulite, Kurt Hollocher Such reactions may be prograde or retrograde. 8.53 Phlogopite in marble from Orange County, New York, 8.54 Tremolite and graphite in marble from Franklin, New Jersey, 8.56 Diopside marble from the Adirondack Mountains, New York. Thus, the zone names in this map. The rock has split from bedrock along this foliation plane, and you can see that other weaknesses are present in the same orientation. Hydration and carbonation reactions occur and produce hydrous and carbonate minerals. This warming initially occurs only next to the magma body, but, over time, heat is conducted farther away. The table below lists the most common and important minerals in these rocks. Rocks similar to the ones described by Barrow, are found worldwide. Subduction carries relatively cool rocks to depth and high pressures. What about four minerals together? Unlike lithostatic pressure, high levels of directed stresses are not sustained for long because rocks deform to reduce the stress. Other minerals consist of these same components. For convenience, we divide the most common rock types into general compositional classes. In contrast with prograde reactions, retrograde reactions are often quite sluggish. • At low temperature, reactions are very sluggish; they may not have time to reach equilibrium. This is true for all chemical systems at equilibrium, not just minerals in rocks. Retrograde metamorphism is, in many ways, just the opposite of prograde metamorphism. Cap-aix-Oies, Quebec. Earth’s geothermal gradient, the rate at which temperature increases with depth, averages about 25 to 35̊ C/km near the surface in most places. Metamorphism may occur because of hot water flowing through rock in areas next to hot springs or other geothermal areas. The table seen here gives examples of different types of metamorphic reactions. 8.77 Magnesite, Prof.lumacorno, Wikimedia Commons For example, pelitic or calcareous rocks do not form greenschists (green mafic schists) or amphibolites (mafic rocks dominated by amphibole and plagioclase) even when metamorphosed at conditions within the greenschist or amphibolite facies. The CO2-H2O fluid can have profound effects on the carbonate nearby, and fluid composition controls the formation of many minerals. In effect, such rocks originated as high-grade metamorphic rocks. Hydrothermal metamorphism occurs when warm fluids significantly alter protolith rocks. 8.1 Zoisite, corundum, and hornblende, James St. John, Wikimedia Commons Sometimes gneissic banding is deformed, as seen in Figure 8.31. And if a (hot) magma intrudes the (cooler) crust, the magma will cool as heat is conducted grain-by-grain into the surrounding rock, causing the surrounding rock to warm while the magma cools. Your rock hammer will bounce off this stuff, ringing, more than almost any other rock type. Actinolite, seen in Figure 8.59, is a calcium-iron amphibole. The porphyroblasts in both photos are centimeters across. The 1-euro coin is 2.3 cm across, for scale. 8.62 Siderite with calcite, John Betts Fine Minerals Metamorphic rocks 1. The gneiss in this photo is oriented so the stretch direction (and, thus, the foliation) is horizontal. The photo on the right above (Figure 8.25) shows foliation (vertical fracture traces) that cuts across a bedding plane separating rock of different compositions. More information on using the program and the activity is in the Instructor's Notes. Gneisses, the highest temperature-pressure kinds of foliated metamorphic rock, typify many regions that have undergone high-temperature metamorphism. Different places on Earth get their heat by different combinations of conduction and convection. Thus, feldspar, titanite, rutile, magnetite, or zircon may be present in small amounts. Al2SiO5 = Al2SiO5, kyanite = sillimanite (reaction 3) 8.29 Biotite-quartz gneiss , James St. John, Wikimedia Commons Some petrologists do not consider these minerals to be metamorphic minerals, while others do. If the original rock was rich in carbonate, siderite will be present. This reaction may occur when a metamorphosed shale is heated to high temperature. Thus, unless metamorphic temperatures are very high, metamorphism of ultramafic rocks produces low-temperature minerals from high-temperature minerals, essentially retrograde metamorphism. Contact metamorphic rocks tend to be fine-grained, massive, without cleavage or schistosity; such rocks are called hornfelses. If quartz is present, the metamorphic reactions in marbles are often decarbonation reactions that involve the breakdown of carbonates to release CO2. The table below lists the most common minerals in metamorphosed granites (also called metagranites). Phase diagrams for simple chemical systems may only contain a few reactions. Reactions between kyanite, andalusite, and sillimanite are only three of many that involve minerals of Al2O3, SiO2, and H2O chemical system. 8.34 Metamorphosed basalt from near Ely, Minnesota. Besides containing calcite or dolomite, metacarbonates may contain Ca-Mg silicates. Some metamorphic rocks are fine-grained and lack metamorphic fabrics. Sometimes, during tectonism, they make it to the surface so we can study them. 8.58 Hematite from near Marquette, Michigan, 8.60 Grunerite from near Marquette Michigan. sheet silicate minerals the micas: biotite and muscovite, chlorite, talc, and serpentine), occurring in rock orient themselves parallel to one another (i.e. Muscovite surrounds them. Rocks undergoing contact metamorphism experience only low pressure. As shown in Figure 8.17, pressure of around 12 kbar is reached at 40 kilometers depth, although pressure depends, in part, on the density of overlying rocks. In quartzites, the once separate quartz crystals become massive quartz with no visible grain boundaries. They generally have a uniform, nonfoliated texture. They are said to be the results of Barrovian metamorphism, a tribute to Barrow. Textural changes take place as rocks undergo prograde metamorphism, and rocks develop metamorphic fabrics. The high-grade rock shown in Figure 8.10 contains conspicuous cm-sized red garnet, black hornblende, and white plagioclase feldspar. Rock mineralogy changes multiple times before equilibrating at the highest temperature conditions. 7 cm across, 8.62 Siderite with calcite from near Roxbury, Connecticut. Foliation: planar fabric element Random orientation Of minerals Preferred … Barrow recognized that the higher-grade metamorphic rocks he was mapping were once unmetamorphosed shales. The diagram on the left, for example, depicts stability fields for kyanite, sillimanite, and andalusite, the Al2SiO5 polymorphs. This kind of metamorphism, called regional metamorphism, creates large metamorphic terranes, regions characterized by distinctive metamorphic rocks and intensity of metamorphism that may vary laterally. The diagram also allows us to make predictions: for example, if a rock containing andalusite is metamorphosed at high temperature, the andalusite will change into sillimanite. If pressure is applied to a rock, the rock may change size or texture, or perhaps develop new minerals to replace old ones. Besides Scotland, other classic occurrences are in Japan and Spain, but Buchan terranes are found worldwide. Siderite (the brown mineral in Figure 8.62) is an iron carbonate, and pyrite (Figure 8.63) is iron sulfide. Schistose fabric consists of thin and abundant layers of foliation, made up of minerals that are naturally flat or long. Normal sandstones are mostly quartz, perhaps with some feldspar. Figure 8.26, seen here, is another example of slate. Sometimes they follow a Buchan facies series (lower pressure) and sometimes they follow a Barrovian facies series (higher pressure). Directed stress may also cause recrystallization as grains dissolve and regrow in other places, or combine to produce larger crystals. The different assemblages are consistent with the reactions shown. They mostly form at low pressures, too. More specific names abound – for example, pelitic gneisses form by metamorphism of originally clay-rich sedimentary rocks, granitic gneisses (such as the one shown in Figure 8.31) form by metamorphism of granites, and mafic gneisses form by metamorphism of mafic igneous rocks. Rocks with cataclastic fabric are almost always associated with faults; they include tectonic or fault breccia, cataclasite, gouge, and pseudotachylite (in which the rock actually melts). Consider a rock that contains only diaspore. This gradient, also called a geotherm, is mostly due to conductive heat flow. The photos below show typical medium-grade metapelites. Each facies name comes from its most characteristic metabasite minerals or rock types. Slate, phyllite and schist all have lepidoblastic textures. The table above lists key mineral assemblages in mafic rocks, but the assemblages will never be present in rocks of other compositions. For example, the greenish quartzite seen below (Figure 8.50) contains disseminated green chlorite. 8.37 Quartzite, Kurt Hollocher As discussed in Chapter 4, under any pressure and temperature, the most stable mineral assemblage is the one with the lowest Gibbs free energy. High-pressure rocks are rare because to get to very high pressure requires that rocks are buried to great depth – an uncommon occurrence. Metamorphic rocks are a type of rocks that form from the transformation of an existing rock type into a new rock type. 17. If the protolith sandstone contained minerals besides quartz, so too will the product quartzite. The pink color in this sample comes from hematite that may have been part of the cement that held the sandstone together. The left column in Figure 8.17 shows temperature-depth relationships for a normal geotherm typical of regions where all heat transfer is by conduction. Karel Gott & Charlotte Ella Gottová - Srdce nehasnou (oficiální video) (German/English subtitles) - Duration: 4:35. So, metamorphism of granites may not lead to significant mineralogical changes. Iron formations generally contain abundant chert and are often well banded with bands ranging from centimeters to meters thick. Even familiar metamorphic rocks, like marble or quartzite, can have alternative names based on these fabrics. The fabric of a sedimentary rock … 8.59 Actinolite, Wisconsin Geological Survey The preexisting rocks may be igneous, sedimentary, or other metamorphic rocks. Compared with metamorphosed pelites, metamorphosed sandstones, also called metasandstones or metapsammites, are often nondescript. 8.20 Gneiss, slideserve.com Schists are higher- grade rocks than phyllites, and most form when phyllites are further metamorphosed. An example is found in Figure 8.16. Several considerations help answer these questions: • Prograde metamorphic events are usually of much longer duration than retrograde events, giving minerals more time to achieve equilibrium. During progressive metamorphism, a series of reactions occur as the degree of metamorphism increases. Rocks without foliation are said to have a massive fabric. A quarter for scale. Contact metamorphism produces two low-pressure, high-temperature facies, the pyroxene-hornfels facies and the sanidinite facies. For example, rocks containing kaolinite and quartz are constrained to have formed at temperatures below about 300 °C. 8.72 Serpentinite, James St. John, Wikimedia Commons Slates, which form during low-grade metamorphism of shales, comprise primarily microscopic clay grains, perhaps with some minor mica. At the lowest grades of metamorphism, magnetite and hematite most commonly dominate. Thus, foliation of phyllites is different from the foliation in slates that stems from clay mineral alignment, and different from foliation in schists because schists always contain visible mica grains. Sometimes, layering may form solely due to chemical processes that concentrate different minerals in different layers. Metamorphic petrologists study metamorphic rocks to interpret those histories. (This sometimes leads to confusion because builders and others use the same word to describe any polished slab of rock.). 8.36 Blue calcite marble, James St. John, Wikimedia Commons Large and aligned flaky minerals, easily seen with the naked eye, define schists. Note that the order of minerals with increasing metamorphic grade in this phase diagram, and in Figure 8.42, matches the order of metamorphic zones mapped by Barrow in Scotland (Figure 8.43). Each of Barrow’s zones is characterized by a particular index mineral that reflects metamorphic grade. Hydration reactions and carbonation reactions consume H2O and CO2, respectively. The PT diagram in Figure 8.69 shows the most important of these series. At still higher grade, chlorite, epidote, and actinolite break down by dehydration reactions, producing a specific kind of rock called an amphibolite. Rocks subjected to regional metamorphism during mountain building experience a significant increase in both pressure and temperature. And, the absence of fluids means that some low-grade minerals cannot form. Sometimes, however, flowing fluids and metasomatism can be the dominant forces controlling metamorphism. 8.55 Forsterite marble, modified from gimpf, flickr 8.22 Fluid inclusions, Kazantseva Mary, Wikimedia Commons Foliated textures come from platy minerals forming planes in a rock, while non-foliated metamorphic rocks have no internal fabric. It is this luster – which is absent from slate and schist – that really defines a phyllite. • More complex, low-grade minerals often have difficulty nucleating and growing. Gray glassy quartz, white plagioclase, and black biotite are also present. Several studies concluded that the Napier rocks were metamorphosed at pressures of 7 to 8 kbar and temperatures of more than 1,000 °C, perhaps as high as 1,075 °C. Subsequently, getting the rocks back to the surface so we can see them is even more problematic. Metamorphism, which may affect any kind of rock, occurs over a wide range of pressure and temperature conditions. Minerals in high-grade ultramafic rocks are the same as the minerals in rocks of the mantle (where pressure and temperature are great). Metamorphic source rocks, the rocks that experience the metamorphism, are called the parent rock or protolith, from proto– meaning first, and lithos- meaning rock. Depending on its composition, a high-grade metamorphic rock may undergo partial melting, also called anatexis, so both metamorphic and igneous processes contribute to its evolution. This rock contains black biotite, light-colored K-feldspar and many conspicuous red garnets. Although hard to see, the specimen contains native gold near the bottom of the sample. 8.31 Deformed granitic gneiss, Chmee2, Wikimedia Commons Most of the examples of metamorphosed ultramafic rocks that we see are in ophiolites, slivers of Earth’s oceanic crust and mantle uplifted and accreted onto continents. is the name for a rock with this fabric; if the grains are very small or microscopic it is called ultramylonite. Sometimes key minerals are often included in rock names. Greenalite (Figure 8.61) is an iron-rich variety of serpentine. Heat causes atomic bonds to break, and the atoms move and form new bonds with other atoms, creating new minerals with different chemical components or crystalline structures (neocrystallization), or enabling recrystallization. 8.7 Metamorphism along a fault, Qfl247, Wikimedia Commons Directed stress, thus, is commonly associated with rock folding or faulting. When this happens, the rock, strictly speaking, is no longer a metamorphic rock. At the lower end of this range, diagenesis overlaps metamorphism. Petrologists have described blueschists from many places, but the two classic examples of the blueschist facies series are rocks of the Sanbagawa metamorphic belt of Japan and of the Franciscan Complex of California. The phase diagram seen below in Figure 8.40 is a more complete phase diagram for the system. The laws of thermodynamics say that rocks will change mineralogy in response to increasing temperature (prograde metamorphism), so why don’t they undergo opposite (retrograde metamorphism) changes when temperature decreases as the rock reaches Earth’s surface? Thus, we see that the more minerals present, the more restricted the conditions of formation. But petrologists use the same names when talking about rocks of other compositions. The drawing in Figure 8.19 shows greater stress being applied horizontally than vertically, causing compression in one dimension. Such rocks are common anywhere magma has intruded shallow crustal rocks. Metamorphosed basalts and other rocks of similar composition are commonly called metabasites. The second photo (Figure 8.54) shows gray blades of tremolite in a marble that also contains small (hard to see) specs of graphite. Rocks with cataclastic fabric are almost always associated with faults; they include tectonic or fault breccia, cataclasite, gouge, and pseudotachylite (in which the rock actually melts). Schist is a metamorphic rock that comes in almost infinite variety, but its main characteristic is hinted at in its name: Schist comes from the ancient Greek for "split," through Latin and French. Another way to look at it is that gneissic fabric is a less even, imperfect version of schistose fabric. These minerals may dehydrate to produce new metamorphic minerals at medium and high grade. The Napier rocks were apparently metamorphosed in the absence of water. 8.51 Kyanite quartzite, Siim Sepp When weathered or metamorphosed at low temperature, the original minerals often react to create low-temperature minerals. Alignment of clays, micas, graphite, or other platy minerals, the separation of a rock into light and dark layers, or parallel fracturing leads to planar fabrics called foliation. Phlogopite is typically one of the first minerals to form during carbonate metamorphism. Figure 8.5 shows another example of a deformed gneiss. Phyllite and slate also have schistose fabric, but in both cases, the mineral grains are of microscopic size. Orthopyroxene may also be present in significant quantities. Within Earth, directed stress is common due to plate tectonic processes that push large pieces of lithosphere together or pull pieces apart. Additionally, although not seen in Figure 8.27, layering in some phyllites is deformed, giving the rocks a sort of wavy or crinkly appearance. The photo in Figure 8.9 shows a serpentinite, an example of a low-grade metamorphic rock. In addition, for some rock compositions, several different mineral assemblages may be stable within a single facies. During metamorphism, K-feldspar recrystallized to form very large pink crystals. The diopside marble is the highest grade of the four. But, besides 3-mineral assemblages, different pairs of minerals are stable within the different fields. Falco - Metamorphic Rocks. • Prograde metamorphism liberates fluids not present when retrogression occurs. 8.8 Shatter cones caused by a meterorite impact. When conditions change, metamorphic reactions may create a new paragenesis as some minerals disappear and others grow. 8.4 Green Mountains schist, James St. John, Wikimedia Commons 8.52 Marble, James St. John, Wikimedia Commons At least 50 percent … In the discussions below, we look at the minerals common in rocks of different compositions. The hematite shown in Figure 8.58 is specular hematite (more common hematite has a red earthy color). The two most important factors controlling mineralogy are the composition of the rock and the pressure-temperature conditions of metamorphism. Usually, the presence of foliation means the rock was under high pressure that deformed it so that the minerals grew in the direction that the rock was stretched. At lower pressures, Buchan metamorphism may produce rocks with andalusite, and often cordierite instead of garnet. Medium-grade metamorphism, forming at temperatures between 400 and about 600 °C, often produces rocks containing conspicuous metamorphic minerals we can easily see and study. Many mantle xenoliths, carried up as nodules within magma, are eclogites. 8.47 Blue cordierite with quartz, from Brazil. If a pluton intrudes a limestone or dolostone, contact metamorphism may cause CO2 to flow out of the carbonate and combine with H2O that comes from the pluton. The abundant biotite and garnet tell us that the rock is aluminum-rich, suggesting it has a sedimentary origin. This is why most phase diagrams only have reactions labeled. The second solid-solid reaction is more typical and involves four minerals. 8.71 Pyke Hill, James St. John, Wikimedia Commons Some metamorphic rocks are fine-grained and lack metamorphic fabrics. Metamorphism often involves fluids, most commonly water-rich but sometimes dominated by carbon dioxide, sulfur, or other components. For example, the foliation in kyanite gneiss may come from alignment of light-colored kyanite crystals in an otherwise quartz- and muscovite-rich rock. Regional metamorphism occurs because both pressure and temperature increase with depth in Earth (Figure 8.3). While the temperature is not extreme, the pressure is; 35 kbar is equivalent to a depth in Earth of more than 100 km (see the diagram in Figure 8.15). 8.5 Outcrop of gneiss, James St. John, Wikimedia Commons Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. Whether they reach it depends on many things, including temperature, grain size, and reaction kinetics. Knapsack for scale. For example, the diagram in this box is modified from Figure 8.40. Metamorphic rocks in these areas may contain cordierite and andalusite, two low-pressure minerals commonly absent from Barrovian terranes. 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