Image A Image B . Thin bands of contact metamorphic rocks are sometimes formed beneath lava flows. 8.57 Banded iron formation, Graeme Churchard, Wikimedia Commons Schist is a foliated metamorphic rock made up of plate-shaped mineral grains that are large enough to see with an unaided eye. 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. Schist is faliated medium grade metamorphic rock. We saw photos of several examples in Chapter 6 (Figures 6.118, 6.119, and 6.120). It forms because the albite component in plagioclase changes by solid-solid reaction into Na-pyroxene. Regional metamorphism occurs because both pressure and temperature increase with depth in Earth (Figure 8.3). Regional metamorphism of clay-rich parent rock results in an increase in grain size and preferred planar alignment of large visible platy grains. However, there are many kinds of metamorphic rocks, and some of them are more chemically reactive than others. Such areas are generally referred to as metamorphic core complexes. Some of the reactions give off water vapor, labeled as H2O. The table below lists the most important of these minerals, roughly in order of their appearance in response to increasing metamorphic grade. Sample is 28 cm across. 8.38 Garnet granulite, Piotr-Sosnowski, Wikimedia Common (This sometimes leads to confusion because builders and others use the same word to describe any polished slab of rock.). Ironstone is a general name we give to sedimentary rocks that contain more than 15% iron. Geologists use index minerals that form at certain temperatures and pressures to identify metamorphic grade. The specimen is 10 cm across. Oct 1, 2019 - Explore monica de leon's board "Metamorphic rocks" on Pinterest. These minerals are used to determine the grade and pressure of the metamorphic rocks that they occur in. For example, hornfels are dark colored fine-grained rocks lacking both lineation and foliation. During metamorphism, K-feldspar recrystallized to form very large pink crystals. At still higher grade, chlorite, epidote, and actinolite break down by dehydration reactions, producing a specific kind of rock called an amphibolite. In some places, next to large igneous intrusions, contact metamorphism occurs and extremely high temperatures may persist for short times before the intrusions cool. Metamorphic rock minerals which indicate the pressure and temperature range that a rock formed at are referred to as. The garnets in this granulite are only a few millimeters wide at most. The table below summarizes these relationships. Figure 8.34 shows a 9-cm wide sample of greenstone from Ely, Minnesota. Pure quartzites are generally white or light colored (like the one in Figure 8.49) but iron staining often adds a red or pinkish coloration. Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. Thin sheets of slate have historically been used for paving or roofing stone. Figure 8.36, for example, shows an 8-cm wide rock consisting only of coarse blue calcite. (of rock…. Many mineral grains contain fluid inclusions that have trapped samples of fluids that once flowed through them. So, the rate of temperature increase with depth is less than normal. Lawsonite has about the same composition as anorthite. 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. Geologists generally call metamorphosed carbonate rocks marbles, although this term is used in different ways by building contractors and others. Other examples are found in Precambrian shields, relatively flat-lying areas that may be thousands of kilometers across, that are the exposed roots of ancient mountains. The photo in Figure 8.25 above shows an example of slaty cleavage. 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. Note the presence of gneissic banding, most notably to the right of the marker pen. 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 shows a significant amount of deformation, recorded by the deformed sheets of biotite. Grade… 8.50 Quartzite, Gabriel Haute Maurienne, Wikimedia Commons Individual mineral grains are discernible by the naked eye. 8.49 Quartzite, James St. John, Wikimedia Commons (of rock) changed into a new form and structure by very great heat and pressure: 2. It occurs because of sudden pressure exerted by faults or meteorite impacts. The first photo in the block above (Figure 8.53) shows large, somewhat hexagonal, flakes of phlogopite with calcite behind. Grunerite (Figure 8.60) is an iron amphibole. Thus, unless metamorphic temperatures are very high, metamorphism of ultramafic rocks produces low-temperature minerals from high-temperature minerals, essentially retrograde metamorphism. I recall that gneiss is the highest grade because it is..."nice". Different parent-rock compositions produce different kinds of metamorphic rocks. The table seen here lists the most important classes considered by petrologists. Low-grade metamorphic rocks tend to be fine-grained (the newly formed metamorphic mineral grains that is). This photo (Figure 8.28) shows a typical schist. Greenstones may also contain Na-rich plagioclase (albite), quartz, carbonates, and zeolites. We call a stable mineral assemblage representative of a given set of pressure-temperature conditions a paragenesis. 6.5: Metamorphic … Rocks metamorphosed at low temperature may change only very slowly, and some changes may not go to completion. When metamorphosed, dehydration reactions change clay minerals into new minerals containing less H2O. Eclogites, such as the one seen below in Figure 8.79, are mafic rocks metamorphosed at high pressure and moderate-to-high temperature. Figure 8.35 shows an outcrop of greenstone in Italy. Conversely, these diagrams allow us to estimate the pressure and temperature of formation for some rocks containing specific minerals. The light-colored crystals in Figure 8.80 are lawsonite. The photos below show minerals common in metamorphosed iron formations. Hydration reactions and carbonation reactions consume H2O and CO2, respectively. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. 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. 8.47 Cordierite with quartz, Density, Wikimedia Commons The weathered outcrop in Figure 8.72 is a typical occurrence. Heat transfer by convecting magmas can be much more significant and can warm huge regions of the crust. For example, many metamorphic rocks are deep in Earth where pressure and temperature are great. Magnesium oxides and hydroxides may also form. 8.12 Diamond in kimberlite, StrangerThanKindness, Wikimedia Commons a.gneiss. Directed stress, sometimes called differential pressure, is also a force applied to an object, but the force is not the same in all directions. Other common blueschist minerals include a colorless to green Na-pyroxene called jadeite, green or white lawsonite, and pale aragonite (the high-pressure polymorph of calcite). 8.20 Gneiss, slideserve.com Lithostatic pressure is the same in all directions (Figure 8.18), and thus can cause an object to become smaller without altering its overall shape. Schist has medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel). 8.4 Green Mountains schist, James St. John, Wikimedia Commons In principle, all rocks tend toward stable equilibrium. Some persist over a wide range of temperatures. In contrast with prograde reactions, retrograde reactions are often quite sluggish. 8.71 Pyke Hill, James St. John, Wikimedia Commons It has schistose foliation, … Schist is one of the most widespread rock types in the continental crust. At low grades, metasandstones typically appear massive and homogeneous, containing light-colored quartz and feldspar grains. The pyrite in this photo is somewhat tarnished. 8.72 Serpentinite, James St. John, Wikimedia Commons 8.81 Jadeite from Myanmar, rruff.info 8.45 Staurolite-muscovite schist, James St. John, Wikimedia Commons The term facies is an objective description of a rock. Thus, the zone names in this map. Metapelites (metamorphosed clay-rich sediments) of the Napier Complex, Enderby Land, Antarctica, are examples of extremely high-temperature metamorphism. 8.58 Hematite from near Marquette, Michigan, 8.60 Grunerite from near Marquette Michigan. Thermal energy is high for substances at high temperature and low for substances at low temperature. The quartz, K-feldspar, and plagioclase that make up most granites and intermediate igneous rocks are stable at all grades of metamorphism. Heat is thermal energy that can move (flow) from one place to another or from one substance – such as rock, magma, or water – to another. Because they are hard to study and frequently do not represent chemical equilibrium, many metamorphic petrologists prefer to study higher-grade rocks. At higher grade, biotite forms. 8.28 muscovite schist. 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. Al2SiO5 = Al2SiO5, andalusite = sillimanite (reaction 2) Metamorphic petrologists study metamorphic rocks to interpret those histories. Metapelites typically contain micas and may also contain staurolite, garnet, and other aluminous minerals. Thus the rade of metamorphism in these areas is generally low except in the vicinity of more highly deformed areas or close to the igneous intrusions, where minerals of a higher metamorphic grade are locally developed. 8.65 Greenstone, James St. John, Wikimedia Commons Gray glassy quartz, white plagioclase, and black biotite are also present. Thus the rade of metamorphism in these areas is generally low except in the vicinity of more highly deformed areas or close to the igneous intrusions, where minerals of a higher metamorphic grade are locally developed. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form. 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). 8.77 Magnesite forming from ultramafic rock near Turin, Italy. Many schists are medium-grade rocks. Schistose rocks are fissil… En savoir plus. This facies represents the lowest grade of metamorphism; it is often hard to distinguish zeolite-facies metamorphism from diagenesis. But any of the other original minerals may persist. The foliation is due to parallel alignment of very small – mostly microscopic – muscovite, chlorite, or other micas, sometimes with graphite. CONCLUSION : Metamorphic rocks are formed from the older rocks when they are subject to increasing T,P, & chemically active fluid. 8.34 Metamorphosed basalt from near Ely, Minnesota. This is Ostwald ripening in action (refer to the discussion in Chapter 4). The high-grade rock shown in Figure 8.10 contains conspicuous cm-sized red garnet, black hornblende, and white plagioclase feldspar. Figure 8.10 shows a different granulite; it contains hornblende and plagioclase besides large garnets. Phase diagrams like this permit prediction of the pressures and temperatures at which individual minerals and specific mineral assemblages will form. xxxMg2SiO4 + 2 CO2 = 2 MgCO3 + SiO2. What about four minerals together? 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 Above gneiss, when the rock actually starts to melt, it is called a migmatite. Common quartzites are white or gray, but minor components may add color. Metamorphism often begins with the formation of zeolites, or of prehnite. Schist (pronounced / ʃ ɪ s t / SHIST) is a medium-grade metamorphic rock. The metamorphism of limestone or dolostone composed only of carbonate minerals produces few mineralogical changes. Phyllites, which form at higher metamorphic grades than slates, sparkle because clay minerals have metamorphosed to produce small grains of mica. As temperature increases, minerals containing H2O or CO2 become increasingly unstable, causing dehydration or decarbonation, and the release of H2O or CO2 as intergranular fluid. In either case, the metamorphism involves hot, generally water-rich fluids that flow through cracks and along grain boundaries. (of rock…. And, sometimes, water flowing through vast regions of the crust alters rocks far from mountain belts. 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. 8.29 Biotite-quartz gneiss , James St. John, Wikimedia Commons When weathered or metamorphosed at low temperature, the original minerals often react to create low-temperature minerals. Low-grade metamorphic rocks form at low temperatures, generally between 150 and 450 °C. Many hornfels form at low pressure from contact metamorphism of a mudstone or shale. Temperature and pressure are the most important causes of metamorphism. Most form at high pressures. And, metabasites commonly contain plagioclase, pyroxenes, and amphiboles. However, many granites contain mafic minerals, most commonly biotite and hornblende. Schists are higher- grade rocks than phyllites, and most form when phyllites are further metamorphosed. Slate forms when shale, which consists of clay minerals, is put under pressure with temperatures of a few hundred degrees or so. Lizardite (Figure 8.73), named for occurrences in the Lizard Complex, Cornwall, and chrysotile (Figure 8.74) are less common and form exclusively at low pressures or at Earth’s surface. Yet, petrologists do occasionally find rocks that were metamorphosed at pressures greater than 10 kbar or temperatures greater than 850 °C. The main and most widely spread metamorphic rocks from the group of low-grade schist metamorphism are argillaceous rocks namely slate, phyllites and schists as shown in Table 6.1. 8.62 Siderite with calcite, John Betts Fine Minerals Thus, we see that the more minerals present, the more restricted the conditions of formation. Actinolite, seen in Figure 8.59, is a calcium-iron amphibole. This is why most phase diagrams only have reactions labeled. 8.66 Greenschist, James St. John, Wikimedia Commons 8.51 Kyanite quartzite, Siim Sepp Some petrologists do not consider these minerals to be metamorphic minerals, while others do. 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. Iron formations are similar to ironstones but are mainly Precambrian (ironstones are Phanerozoic). Al2SiO5 = Al2SiO5, kyanite = sillimanite (reaction 3) Minor minerals at all grades include many that are present in mafic igneous rocks. (Mg,Fe)3Al2Si3O12. About 7.5 cm across. 8.82 Glaucophane with fuchsite, Didier Descouens, Wikimedia Commons Many nonfoliated metamorphic rocks are dominated by a single mineral. Schist. In subduction zones (right column, Figure 8.17), generally cooler temperatures are present. The preexisting, or parent rocks, are called protoliths. The green jadeite, in Figure 8.81, is an Na-rich pyroxene that is only stable at high pressure. As temperature rises, the zeolite facies gives way to the prehnite-pumpellyite facies, the greenschist facies, the amphibolite facies, and the granulite facies. If andalusite accompanies the kaolinite and quartz, pressure is restricted to less than 1 kbar. Schist exhibits schistosity, which is formed by the alignment of platy medium- to coarse-grained minerals formed under moderate- ... Phyllite is a low- to moderate-grade metamorphic rock that contains aligned platy mica minerals and has slaty cleavage. And, the absence of fluids means that some low-grade minerals cannot form. 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. The formation of contact aureoles frequently involves metasomatism, a change in rock composition due to flowing metamorphic fluids. Quartz usually dominates, and the amounts of other minerals depend on how much clay was in the protolith. When we stretch a rubber band, we are also applying directed stress. But most metamorphic reactions involve more than two minerals, and many involve H2O or CO2. Topics similar to or like Schist. It seems that conditions of low T and high P exist only in special places. The results are often the fracturing and granulation of rocks and sometimes the creation of high-pressure minerals such as coesite or stishovite, both polymorphs of quartz. Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals. In this region, metamorphic grade increases from southeast to northwest.