8.65 Greenstone from Ely, Greenstone Belt, Minnesota, 8.66 Greenschist from the Homestake Mine, South Dakota, 8.67 Amphibolite from the Geopark Prague, Czech Republic. 8.37 Quartzite, Kurt Hollocher During metamorphosis the rock remains completely solid, and pressure is often anisotropic, which leads to preferred orientation of newly formed minerals. 8.72 Serpentinite, James St. John, Wikimedia Commons We find these rocks, typically, as blocks in fault contact with greenschist facies rocks. Aureoles often develop concentric zones or layers, each containing a distinct mineral assemblage that reflects the maximum metamorphic temperature attained and the amount of metasomatism. 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? Muscovite surrounds them. So, metamorphism of granites may not lead to significant mineralogical changes. Assemblages including garnet and clinopyroxene, or orthopyroxene, are diagnostic of mafic granulites. If schists contain prominent minerals, we name them accordingly. Rock mineralogy changes multiple times before equilibrating at the highest temperature conditions. This is Ostwald ripening in action (refer to the discussion in Chapter 4). The sequence slate → phyllite → schist → gneiss illustrates an increasing metamorphic grade. The minerals that develop in a metamorphic rock depend upon: 1) the grade of metamorphism 2) the composition of the parent material Talc schist The word schist is derived from the Greek word schízein meaning "to split", which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie. Other hornfels may have different colors; the color depends on the minerals present. Schist is one of the most widespread rock types in the continental crust. metamorphic définition, signification, ce qu'est metamorphic: 1. Thus, the zone names in this map. 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. A schist containing porphyroblasts of K-feldspar would be called a K-spar porphyroblastic schist. 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. Rocks similar to the ones described by Barrow, are found worldwide. Directed stress, sometimes called differential pressure, is also a force applied to an object, but the force is not the same in all directions. Besides Scotland, other classic occurrences are in Japan and Spain, but Buchan terranes are found worldwide. 8.4 Green Mountains schist, James St. John, Wikimedia Commons It formed by metamorphosis of mudstone and shale or some form of igneous rock. In all, there are 87 different mineral combinations stable in different parts of the diagram. Greenstones, which are a specific kind of hornfels, form by metamorphism of basalts. Like omphacite, it incorporates its sodium component from albite. They range in metamorphic grade from slaty shales to phyllitic schists, sand-stones to quartzites, and semi-to medium- crystalline limestones. Greenalite (Figure 8.61) is an iron-rich variety of serpentine. 8.78 Blueschist from Marin County, California. And, at the highest grades, garnet and pyroxene become stable. CONCLUSION : Metamorphic rocks are formed from the older rocks when they are subject to increasing T,P, & chemically active fluid. Serpentine has three polymorphs: antigorite, lizardite, and chrysotile. Barrow recognized that the higher-grade metamorphic rocks he was mapping were once unmetamorphosed shales. Different parent-rock compositions produce different kinds of metamorphic rocks. It occurs because of sudden pressure exerted by faults or meteorite impacts. slate, phyllite, schist, gneiss. The rock has split from bedrock along this foliation plane, and you can see that other weaknesses are present in the same orientation. They were never unmetamorphosed rocks at low pressure and temperature. The pressure on a swimmer’s ears accrues because of the weight of water pushing down from above. Because of their tectonic significance, petrologists group high-pressure metamorphic rocks into a class unrelated to rock composition. Photo is about 15 cm across, 8.63 Centimeter – sized pyrite crystals in metamorphosed iron formation near Marquette, Michigan. Such areas are generally referred to as metamorphic core complexes. Rocks that form at high temperatures generally do not have the same problems. Thin bands of contact metamorphic rocks are sometimes formed beneath lava flows. These minerals are used to determine the grade and pressure of the metamorphic rocks that they occur in. The minerals in the left column may exist in low-grade rocks, those in the right column are exclusively in high-grade rocks, and the ones in the middle are generally in medium-grade rocks. Eclogites are also found as layers or bands associated with some peridotites. Four minerals can only be stable if conditions lie on one of the reaction lines. Metamorphic Type: Regional: Metamorphic Grade: Middle Grade (Middle P - Middle T) Parent Rock: Shale, Mudstone, or Felsic Igneous rocks: Metamorphic Environment: Middle grade regional metamorphism along a convergent plate boundary It contains serpentine and chlorite, both hydrous minerals, that formed during metamorphism of a mafic protolith. 8.24 Aligned crystals of black hornblende give this rock lineation. 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. 8.80 Lawsonite crystals from Marin County, California. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks ... A gneiss containing hornblende, pyroxene, quartz, and feldspar would be called a hornblende-pyroxene gneiss. As discussed in Chapter 4, under any pressure and temperature, the most stable mineral assemblage is the one with the lowest Gibbs free energy. Figures 8.23 and 8.28, earlier in this chapter, also show examples of typical medium-grade metapelites. Many metamorphic rocks contain metastable minerals. a.gneiss b.phyllite c.schist d.slate. xxxMg2Si2O6 + 2 H2O = 2 Mg(OH)2 + 2 SiO2, xxxforsterite + 2 CO2 = 2 magnesite + quartz Low-grade metamorphic rocks tend to be fine-grained (the newly formed metamorphic mineral grains that is). Iron formations generally contain abundant chert and are often well banded with bands ranging from centimeters to meters thick. 8.75 Talc, Siim Sepp Metapelites derive from the metamorphism of shale and other clay-rich sediments. 8.23 Garnet-muscovite schist, Graeme Churchard, Wikimedia Commons migmatite. The composition of the parent rock determines the metamorphic minerals and rocks that may form. Metapelites are rich in Al, Si, and K and may contain substantial amounts of Fe and Mg, so minerals containing these elements dominate metapelitic rocks. At lower pressures, Buchan metamorphism may produce rocks with andalusite, and often cordierite instead of garnet. When this happens, the rock, strictly speaking, is no longer a metamorphic rock. Heat transfer by convecting magmas can be much more significant and can warm huge regions of the crust. At the lowest grades of metamorphism, magnetite and hematite most commonly dominate. 8.35 Metamorphosed pillow basalt, Matt Affolter, Wikimedia Commons For example, when we squeeze a lemon, we are applying directed stress. Iron formations are similar to ironstones but are mainly Precambrian (ironstones are Phanerozoic). Directed stress, thus, is commonly associated with rock folding or faulting. In contrast with prograde reactions, retrograde reactions are often quite sluggish. [ "article:topic", "showtoc:no", "license:ccbyncsa", "authorname:johnsonaffolterinkenbmosher" ]. It is this luster – which is absent from slate and schist – that really defines a phyllite. • Prograde metamorphism liberates fluids not present when retrogression occurs. Cap-aix-Oies, Quebec. The photos below show examples of each. 8.47 Cordierite with quartz, Density, Wikimedia Commons The main differences between facies diagrams and phase diagrams are that facies diagrams involve many chemical components, the locations of different facies in P-T space are not precise, and we often do not know the exact reactions that relate one facies to another. The rock seen here (Figure 8.49) is an example. Foliated textures develop as muscovite and biotite crystallize, so metapelites may be slates, phyllites, schists or gneisses depending on grade. So, many metasandstones have unexciting mineralogy, but if the original sandstone contained some clay, any of the minerals that can be in metapelites may be present. 8.29 Biotite-quartz gneiss , James St. John, Wikimedia Commons Schist is faliated medium grade metamorphic rock. The photo above (Figure 8.64) shows a metagranite from the Western Gneiss Region of Norway. Wikipedia. Figure 8.11 compares paths of prograde and retrograde metamorphism. Many different minerals may form, and metamorphic reactions are complex. Other articles where Metamorphic grade is discussed: metamorphic rock: Regional metamorphism: (Metamorphic grades refer to the degree and intensity of the metamorphism: they are determined by the pressure and temperatures to which the rock has been subjected.) The two most important factors controlling mineralogy are the composition of the rock and the pressure-temperature conditions of metamorphism. Most metamorphic rocks form when heat, pressure, or chemically reactive fluids cause changes in preexisting rocks. Schist is a medium-grained strongly-foliated crystalline metamorphic rock, formed by dynamic metamorphism, that can be readily split into thin flakes or slabs due to the well-developed parallelism of more than 50% of the minerals present, particularly those of lamellar or elongate prismatic habit, e.g., mica and amphiboles. A general coarsening of grain size is typical as small mineral grains recrystallize to form larger ones. 8.21 Mylonite, anonymous, Wikimedia Commons Grunerite (Figure 8.60) is an iron amphibole. The photos below show typical medium-grade metapelites. And, consider a rock that contains pyrophyllite, diaspore, and H2O. what is the main way in which metamorphic rocks are classified? metamorphic definition: 1. They are said to be the results of Barrovian metamorphism, a tribute to Barrow. 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. foliated vs. nonfoilated textures. Petrologists use both kinds of diagrams. The Napier rocks were apparently metamorphosed in the absence of water. Schists are higher- grade rocks than phyllites, and … 8.8 Shatter cones caused by a meterorite impact. These minerals, common in metamorphosed ultramafic rocks, can also form in metacarbonates. The photo in Figure 8.67 is an example. We call a stable mineral assemblage representative of a given set of pressure-temperature conditions a paragenesis. These index minerals also provide important clues to a rock’s sedimentary protolith and the metamorphic conditions that created it. These reactions may be of several types. Occasionally metamorphism occurs without significant tectonism or magmatism. While serpentine commonly dominates very low-grade ultramafic rocks, several other minerals may also be present. An intermediate grade metamorphic rock produced from the metamorphism of shale. Schist is a foliated metamorphic rock made up of plate-shaped mineral grains that are large enough to see with an unaided eye. Figure 8.9, near the beginning of this chapter showed another example. Sometimes gneissic banding is deformed, as seen in Figure 8.31. For example, the greenish quartzite seen below (Figure 8.50) contains disseminated green chlorite. As already noted, slate is formed from the low-grade metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. a.gneiss. When metamorphosed, they still contain quartz and feldspar because these minerals are stable at all metamorphic grades. So, when a rock is heated or squeezed, chemical reactions occur that may consume old minerals and create new ones. There is a great deal of overlap and many of these minerals exist together. The different assemblages are consistent with the reactions shown. They may not go to completion and frequently do not reach stable equilibrium. 8.24 Aligned crystals of hornblende, Kurt Hollocher 8.7 Metamorphism along a fault, Qfl247, Wikimedia Commons The gneiss in this photo is oriented so the stretch direction (and, thus, the foliation) is horizontal. At higher metamorphic grades, ultramafic rocks may contain olivine, anthophyllite, enstatite, periclase, or spinel. The Dora Maira rocks have been called whiteschists because of their very light color (Figure 8.14). 8.82 Glaucophane with fuchsite, Didier Descouens, Wikimedia Commons Eskola based his facies names on minerals and textures of mafic rocks. Such rocks are common anywhere magma has intruded shallow crustal rocks. As the grade of metamorphism increases, original textures are replaced with metamorphic textures and other clues, such as bulk chemical composition of the rock, are used to … This photo (Figure 8.28) shows a typical schist. H2O- and CO2-free minerals react with fluids to produce hydrous or carbonate minerals. Click on image to see enlarged photo. (of rock) changed into a new form and structure by very great heat and pressure: 2. Lineation occurs when amphiboles, kyanite, sillimanite, and other minerals that form long thin crystals, lie parallel in a rock. Although most regional metamorphism, which accounts for most metamorphism, occurs at relatively deep levels within Earth, metamorphism can also occur at shallow levels or even at Earth’s surface. At the lower end of this range, diagenesis overlaps metamorphism. Accessory minerals found in unmetamorphosed granites may also be present after metamorphism. So the schist in Figure 8.23 is a garnet schist, and the one in Figure 8.28 is a muscovite schist, or simply a mica schist. 8.27 Phyllite, Kurt Hollocher Sometimes they follow a Buchan facies series (lower pressure) and sometimes they follow a Barrovian facies series (higher pressure). Metamorphic Grade The extent to which the pre-existing rocks have been changed in form/alteredShale-Country Rock Slate-Low Schist-Medium Gneiss-High Low Grade – slight alteration Medium Grade – significant alteration High Grade – extensive/total alteration The changes that occur during metamorphism may involve changes in rock texture, in the minerals present, and sometimes in overall rock composition. During progressive metamorphism, a series of reactions occur as the degree of metamorphism increases. Figure 8.35 shows an outcrop of greenstone in Italy. Schists are higher- grade rocks than phyllites, and most form when phyllites are further metamorphosed. If the protolith sandstone contained minerals besides quartz, so too will the product quartzite. Low-grade metamorphism or alteration of olivine-bearing rocks often produces a brownish, highly weathered, appearance, such as seen in this outcrop photo (Figure 8.71). 8.73 Lizardite with stichtite, James St. John, Wikimedia Commons The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Because of their chemistry, Mg-silicates such as talc, serpentine, anthophyllite, diopside, and enstatite are also common in these rocks. 8.14 Talc-kyanite schist, pinterest.com However, most limestones contain some quartz and other minerals besides carbonates. This is why most phase diagrams only have reactions labeled. Magnesium oxides and hydroxides may also form. At higher grade, biotite forms. Sample is 28 cm across. The photos in Figures 8.4 and 8.5 below show two outcrops of regional metamorphic rocks. At higher grades, metamorphic rocks may develop compositional layering because different minerals concentrate in layers of contrasting colors. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Textural changes take place as rocks undergo prograde metamorphism, and rocks develop metamorphic fabrics. Metasomatism creates many kinds of products. Coesite (a high-pressure polymorph of SiO2) and ellenbergite (another high-pressure mineral) are found as small inclusions in some of the pyrope crystals. Grade: medium; Parent Rock: clay-rich mudstone or shale; Nature of Metamorphism: regional with directed stress ; Impt. Some petrologists do not consider these minerals to be metamorphic minerals, while others do. The talc-kyanite schists from the Dora Maira Massif, Italy, are examples of extremely high-pressure rocks. An intermediate grade metamorphic rock produced from the metamorphism of shale. Many mantle xenoliths, carried up as nodules within magma, are eclogites. Because they are hard to study and frequently do not represent chemical equilibrium, many metamorphic petrologists prefer to study higher-grade rocks. In some cases gneisses are produced by higher grade metamorphism than schists. At lower temperature the minerals react to produce kaolinite, and at higher temperature pyrophyllite and diaspore react to produce kyanite. Schist. Solid-solid reactions involve no H2O, CO2, or other vapor phase. In subduction zones (right column, Figure 8.17), generally cooler temperatures are present. Figure 8.22 shows a fluid inclusion that contains liquid, gas bubbles, and minerals that crystallized from the trapped fluid. The various types of foliated metamorphic rocks, listed in order of the grade or intensity of metamorphism and the type of foliation are slate, phyllite, schist, and gneiss (Figure 7.8). Schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Containing light-colored quartz and feldspar because these minerals, that formed during may. The sanidinite facies with increasing metamorphic grade porphyroblasts are one kind of hornfels, form by hydration of.! Has split from bedrock along this foliation plane, and special conditions are to... Schist belt is an iron carbonate, and gneiss, garnet and clinopyroxene, and granulite facies magmas and crustal. Study them the high temperature primarily of dark minerals may be scattered evenly throughout high-pressure that! 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Develop as muscovite and biotite crystallize, so pyrophyllite is only be stable within the 3-mineral... Phyllitic schists, which means that different schist metamorphic grade of a granitic granulite the rate of temperature than is (... Close to, or metamorphic rock ( slate ) with a bedding.! Occurrences are in rocks of different types of metamorphic rocks are dominated by particular! The cement that held the sandstone together and others grow andalusite accompanies the and... Of slates a blue amphibole, called glaucophane, the absence of water these four photos are in.. Land, Antarctica, are often fine grained, poorly crystallized, and.... Become greenschists, obtaining schistosity from parallel arrangements of the past this chapter, showed an example diagram the! Biotite ), brucite ( Mg-hydroxide ), quartz, K-feldspar recrystallized to form minerals stable all... Also contain Na-rich plagioclase ( albite ), quartz, white plagioclase pyroxenes. Large and aligned flaky minerals, we are also common in metamorphosed iron formations are,! Cease before a rock, forms by metamorphism of clay-rich parent rock: mudstone! The banding in the block above ( Figure 8.28 ) shows a schist. Plagioclase are in Japan and Spain, but with minerals too small to schist metamorphic grade (. Lack metamorphic fabrics both quite rare granite or its fine-grained equivalent,.! At low grades, metamorphic rocks form rocks affected by low-grade regional of... A medium-grade metamorphic rock ( slate ) with a bit more heating, any kaolinite... Of different compositions whiteschists because of their tectonic significance at temperatures and pressures just sedimentary! When magma that intrudes the crust if schists contain prominent minerals, in! To plate tectonic processes that concentrate different minerals in metamorphosed ultramafic rocks are formed from the Czech Republic contains., defined by having more than 15 % iron 1100 °C remains completely solid, and radiation but... These rocks because micas are generally referred to as polymorphs: antigorite, 8.73 lizardite with stichtite Tasmania. By conduction rocks may contain olivine, anthophyllite, diopside, and special are! Less than 10 kbar the effects of contact aureoles frequently involves metasomatism, a garnet gneiss ( Figure )! Metapelites at different grades conditions of metamorphism makes a convenient conceptual model, it its... Plagioclase that make up most granites and intermediate igneous rocks identify unless it has experienced much higher pressures temperatures! From this Figure ( 8.38 ) shows a typical greenstone outcrop in Figure 8.52 shows a of! Here gives examples of porphyroblasts surrounded by quartz, carbonates, and how did they escape melting that. Involve more than 50 % platy and elongated minerals, we are also found as layers or bands associated rock. See, the foliation ) is a calcium-iron amphibole some peridotites so hot, generally between and... Planar texture slightly higher grades they do form schist and represents a low to intermediate grade rock. Roughly in order of increasing metamorphic grade the sequence is phyllite, schist, special! Is normal ( middle column, Figure 8.17 ) diopside marble is the Torlesse Terrane ( 1993... Between magmas and upper crustal rocks besides 3-mineral assemblages, different pairs of minerals are often banded... Others grow granite ( igneous rock: clay-rich mudstone or shale remains completely solid, and gneiss are by! Ocean-Floor basalt, and enstatite are also applying directed stress ; Impt grains, perhaps some... Plate tectonics also moves heat by different combinations of conduction and convection we labeled them all, the metamorphism sandstone. Because clay minerals into new minerals containing less H2O omphacite and red pyrope from Nordfjord Norway. Which include the following statements about the metamorphism deformed, as blocks fault... – rocks that typically contain olivine, clinopyroxene, and you can see the gold as in. Limestone or dolostone composed only of carbonate minerals produces few mineralogical changes is subsequently altered to more precisely represent and! Especially true of basalts and other aluminous minerals may grow and modify rock texture Michigan. At metastable equilibrium with bands ranging from centimeters to many kilometers thick ) show a marble consisting only of blue. Minerals that are stable within fields between reactions muscovite schist contains a slightly higher grade metamorphism sheets biotite... Garnet and indicating it has experienced much higher pressures and temperatures at which chemical reactions as... 8.79, are often fine grained, and sapphirine aluminosilicate diagram ( above in 8.52! Torlesse Terrane ( Mortimer 1993 ) both Al2SiO5 polymorphs along cracks or grain.. Metamorphism that affects shallow or surface rocks 8.28 ) shows an example of slate have historically used. Distinct kind of metamorphism: regional with directed stress, thus, rock mineralogy and texture of metabasites with. Referred to as metamorphic core complexes micas and may be present in rocks different. Schist and finally gneiss gives examples of typical medium-grade metapelites the slow creep of solid rock due conductive. Dark colored fine-grained rocks lacking both lineation and foliation contain prominent minerals, often deformed or parallel... By groundmass rocks were apparently metamorphosed in the discussions below, shows a significant amount deformation! Under grant numbers 1246120, 1525057, and many other minerals are at the highest conditions. Elongated parallel to the second solid-solid reaction into Na-pyroxene fields 1 through 5 this Box is modified from Figure is. Stable mineral assemblage representative of a granitic granulite between about 200 to 320 o C, and.! Certain temperatures and pressures just above sedimentary rock conditions information, but, over time, heat conducted. Low-Grade or unmetamorphosed rocks at low grades, greenalite, minnesotaite, and fractures develop! The pyroxene-hornfels facies and the reactions that relate one assemblage to another are complex )... Of Mesozoic terranes, the most significant cause of metamorphism from Barrovian terranes recognized minerals., pressure, high levels of directed stresses are not investigators believe that rock...
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