FIGURE 4. Granulite facies rocks represent extreme temperature conditions. This results in abundant postcollisional volcanism that lies compositionally between calc-alkaline and strongly potassic compositions, exemplified by multiple collisions in the Mediterranean over the past 100 million years (Prelević et al., 2012). The basic idea states that, for a given pressure-temperature condition, the resulting mineralogy is controlled only by the chemical composition of the rock which does not change during metamorphism except for the loss of fluids. Andalusite and glaucophane are absent. Peak metamorphic conditions are about 1.1 gigapascals (GPa), or 37 km depth and about 440°C. (d) Late Oligocene–Middle Miocene regional sillimanite-grade metamorphism and partial melting to form migmatites and leucogranites, initiation of the Main Central Thrust and South Tibetan Detachment; channel flow. Granitic intrusions are present and ophiolites are rare. Note also in this figure how the isotherm rises within the thickened crust of the mountain belt. 3, Table II). From Understanding Earth, Press and Siever. To sum up, a metamorphic facies is the set of minerals found in a rock of a given composition. METAMORPHIC ROCKS | Facies and Zones. By continuing you agree to the use of cookies. This broadening may result from the effect of increasing temperature in weakening and making the mylonites less fabric softening. Metamorphic grade is generally low across the Tasman Orogenic Belt (Fig. Thus, the strength of the rock will decrease progressively as the mylonitic fabric develops and rotates toward the plane of shear, on which it will have the greatest resolved shear stress. Rocks, however, are poor conductors of heat. URL consultato il 17 luglio 2008 (archiviato dall'url originale il 30 aprile 2008). This spotlights an unrecognized bias in past studies, which have unconsciously supposed a deep, Andean-style subduction in which the slab proceeds to lower mantle depths. h�b``�a``ja�``�^� € ",�@���°���yo��aD쾮N�,o��Ӻ�7�|k��H�i�c���`� =@����Ң@,2�хA����7g7��^S^�X^�������8���g������ieZq�m�� �@$ ` �^' endstream endobj 305 0 obj <>>>/Metadata 217 0 R/Pages 301 0 R/Type/Catalog>> endobj 306 0 obj <>/ExtGState<>/Font<>/ProcSet[/PDF/Text/ImageC/ImageI]/XObject<>>>/Rotate 0/Type/Page>> endobj 307 0 obj <>stream High-grade implies high-temperature metamorphism consistent with amphibolite, granulite, and eclogite facies. Il termine facies è stato riferito per la prima volta ad uno specifico ambiente sedimentario, costituito da rocce tipiche, da parte del geologo svizzero Amanz Gressly nel 1838. An additional complication arises from the fact that in the presence of free H2O-fluid, mafic rocks begin to melt at higher temperatures. 8 Chapter 7: Metamorphism of Pelitic Rocks. Thus, isochemical conditions, the requisite basis for comparing metamorphic rocks from different grades, is no longer maintained in rocks that undergo partial melting. Greenschist is the name of a metamorphic facies, a set of typical minerals that form under specific conditions—in this case relatively cool temperatures at high pressures. (2003) measured U–Pb ion microprobe ages of 46.2 ± 0.7 Ma from zircon rims, interpreted as dating the timing of UHP metamorphism. These belts are subdivided upon the ratio of metamorphic pressure to metamorphic temperature. VIdeo Lecture: Metamorphism of Basic Rocks. A feedback mechanism is established whereby additional compression produces greater uplift and greater rates of erosion such that warm rock at depth is quickly exhumed toward the surface. Substantive können mit einem Artikel (Geschlechtswort) und i. The temperature and the corresponding pressure at which the highest temperature is attained are referred to as peak metamorphism. The eclogites then became encased within lower and middle crustal kyanite- and sillimanite-bearing gneisses of the main Himalayan metamorphic event (Figure 10(c) and 10(d)). Facies relationships were first described by Pentti Eskola in 1921. Reactions are less likely to occur if this driving force is increasingly dissipated during cooling. Here are the typical minerals in rocks that are derived from sediments. Metamorphic rocks are rocks whose characters have been changed from its original form by processes operating within the earth. (2001) discovered coesite in the Kaghan eclogites and determined UHP pressures of 27 kbar along the leading margin of the Indian Plate crust. Blueschists are easy to recognize because, unlike most rocks, they really are blue. b. Mineralogical & chemical composition. A subducting slab will remain cool as it is pushed deep into the Earth, even though surrounding rocks are much hotter. So, although this softening occurs with increase in shear strain, it should be more properly called fabric softening rather than strain softening. 3 and Table II). These processes are important for arcs on the modern Earth, but will be especially applicable to the late Archean, during which crustal formation by the amalgamation of arcs may have been widespread (Condie, 1986). Under these conditions, the blueschist facies mineral assemblage will be destroyed in favor of an amphibolite facies mineral assemblage. Metamorphic facies are recognizable terranes or zones with an assemblage of key minerals that were in equilibrium under specific range of temperature and pressure during a metamorphic event. Abnormally high geothermal gradients and concomitant metamorphism result from a number of factors associated with subduction and continental collision. English: Diagram for metamorphic facies in the Earth's interior. Peak metamorphic conditions are about 0.7 GPa (21 km depth) and 640°C. With the major exception of the Valley and Ridge, nearly the entire US Appalachian Mountain belt was subjected to at least greenschist facies metamorphism, with more than half reaching high-grade amphibolite-granulite facies conditions. Integrated PTt path for the Tso Morari eclogites in Ladakh, NW India, showing all U–Pb age data. FIGURE 5. These areas are characterized by strongly curved subduction zones that result in the frequent occurrence of slab windows and tears that allow access of asthenospheric heat to the lithosphere. 5 and see below). My book Metamorphic Rocks and Metamorphic Belts (in Japanese) was published by Iwanami Shoten, Publishers, in Tokyo in 1965. Voci correlate. TABLE III. Granitic intrusions are very common, but ophiolites are absent. Metamorphic facies. Metamorphic facies. Some areas on the modern Earth, such as Indonesia, consist of a collection of small continental blocks and pieces of oceanic crust with restricted extent. Garnet growth in medium-pressure granulite facies metapelites from the central Damara Orogen: igneous versus metamorphic history P. Masberg Institut für Mineralogie, Petrologie und Kristallographie, Philipps-Universität Marburg 35032 Marburg, Germany FIGURE 20.7. Once shearing begins, the mylonitic fabric begins to be developed, which changes the ordering of the mineral phases. (c) Greenschist facies rocks will be composed primarily of slates, phyllites, and schists. ����K��J0;�^f��I)�"���攁H��`�4���@�1��Nl��S�30120�����L�?�o 4�U endstream endobj startxref 0 %%EOF 339 0 obj <>stream The blueschist metamorphic facies are characterized by the minerals jadeite, glaucophane, epidote, lawsonite, and garnet. It seems likely that following Late Cretaceous–Early Paleocene obduction of ophiolites onto the northern passive continental margin of India, the collision of India with Asia during the Early Eocene led to the final closing of Tethys at 50 Ma and the beginning of crustal thickening and regional metamorphism along the Indian Plate margin (Figure 10(a) and 10(b)). Figure 9. In the case of Path A, the mineral assemblage associated with peak metamorphism is that of the amphibolite facies. Metamorphic facies. Metamorphic facies by Dave Waters, su It is this high pressure–low temperature condition that produces blueschist and eclogite facies rocks. A, B, C subdivision of facies according to pressure. More commonly, only isotopic dating of minerals within the studied rock, or from related intrusive of extrusive rocks, can yield absolute ages. Chapter 26: Metamorphic Reactions. ), but those names don’t mean that the facies is limited to that one rock type. P-T scheme of metamorphic facies. 4, Table III). The low pressure/temperature belt is characterized by andalusite at lower temperatures and sillimanite at higher temperatures (Fig. Average velocities at temperature and pressure for metamorphosed basalt. Buoyant rebound of the subducted crustal slice requires some sort of slab break-off to trigger rapid exhumation. They take a long time to heat up, and once hot they take a long time to cool down. FIGURE 20.9. The easiest way to read Wikibooks is simply to open them in your web browser. (c) Greenschist facies rocks will be composed primarily of slates, phyllites, and schists. The type area for paired belts is in the Late Mesozoic metamorphic rocks of Japan (Fig. The concept was first defined in 1914 by a Metamorphism in the Cordillera, by contrast, is spotty, mainly because the mountain system is young and has not yet undergone deep erosion to expose metamorphic rock. Lower-pressure growth of titanite occurred at 46.4 and 44 Ma, implying extremely rapid exhumation through the mantle from > 100 km to c. 35 km depth, presumably during slab break-off and buoyant rebound of subducted Indian crust. The report is compiled on the basis of the explanatory text to the map of metamorphic facies of the ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL:, URL:, URL:, URL:, URL:, URL:, URL:, URL:, URL:, Encyclopedia of Physical Science and Technology (Third Edition), 2003, At conditions of the UHT facies, melt is produced even in the absence of H, Tectonic and Structural Framework of the Zagros Fold-Thrust Belt, Developments in Structural Geology and Tectonics. Note that basalt compressional and shear-wave velocities along a “normal” geotherm will increase with depth from 5.85 and 3.18 km/s at near-surface pressure and temperature conditions to 6.95 and 3.86 km/s at lower crustal granulite facies depths. Download as PDF. The metamorphic facies series in regional metamorphism may be classified into the following categories according to an order of increasing rock pressure: (1) andalusite-sillimanite type, (2) low-pressure intermediate group, (3) kyanite-sillimanite type, (4) high-pressure intermediate group, and (5) jadeite-glaucophane type. Seismic facies. A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar pressures and temperatures. Metamorfismo; Roccia metamorfica; Ultima modifica il 23 ago 2020 alle 15:35. Nikolas I. Christensen, Darrell Stanley, in International Geophysics, 2003. These may be the source of not only the potassium, but also characteristic trace elements such as a high Th/La ratio (Tommasini et al., 2011; Wang et al., 2016a,b). Path B reaches blueschist facies and is then immediately returned to the surface. Figure 20.8 is a schematic diagram that shows the depth at which 500°C is reached across a subduction zone boundary. This was based on the succession of metamorphic facies and the associated geologic setting for a given metamorphic belt, e.g., the Barrovian metamorphism in part of the Scottish Highlands. The leading margin of India was dragged down the subduction zone and metamorphosed to coesite–eclogite conditions during the Late Paleocene–Early Eocene. Although there has been some experimental work on the formation of mylonites in analog materials (Kawamoto and Shimamoto, 1998), there has been little discussion of the mechanism for this softening in the literature. The increase in temperature is the primary driving force for reaction. (c) Late Eocene–Early Oligocene crustal thickening stage to form regional Barrovian kyanite metamorphism. The concept implies that by simply looking at a rock and noting its mineral assemblage, a geologist can determine the temperature and pressure range under which the rock was metamorphosed. D. Fettes e J. Desmons 2007 “Metamorphic Rocks: A Classification and Glossary of Terms” Cambridge University Press. The five rocks are blueschist, greenschist, amphibolite, granulite, and eclogite. Metamorphic facies: minerals depend on compo The characteristic set of minerals in a meta-pelitic greenschist is going to be different from that in an equivalent-grade meta-basalt. 3, Table II). Metamorphic facies. The plate tectonic model was not available when Miyashiro proposed the concept of paired metamorphic belts. Facies metamorfica: insieme delle associazioni mineralogiche metamorfiche, associate The three pressure–temperature regimes are given in Table II. Qualitatively, the reaction sequence in natural rocks is often visible in the form of growth features, compositional zoning, or breakdown textures. The time-dependent decays of radioactive isotopes in the systems Rb–Sr, K–Ar, U–Th–Pb, Nd–Sm, or Lu–Hf are commonly used in geochronology. Metamorphic facies are named for rocks that form under specific conditions (e.g., eclogite facies, amphibolite facies etc. Amphibolite facies rocks will be mostly schists and gneisses, while high grade metamorphic rocks will be dominated by gneisses. The shear zone is widening by adding new mylonite from its boundaries as net shear increases. Thickening of relatively low-density continental rock will cause the crust to rise isostatically, producing mountains. When compiling the map it was in many cases Fig. and export them together (again, in PDF format). (PDF) METAMORPHISM, METAMORPHIC FACIES AND MINERAL PARAGENESIS | Nelson Nwobi - Metamorphic rocks are rocks whose characters have been changed from its original form by processes operating within the earth. There is, however, an abundance of blueschist and some eclogite facies rocks in the Coast Ranges as well as isolated blueschist rocks in the Klamath Mountains and Sierra Nevada. H2O and other volatiles dissolve in the silicate melt or may be expelled from the melt. The belts form and then are placed in contact along a major fault, the Median Tectonic line (after Yardley, 1989, and after Miyashiro, 1994). 1. That is, these will be found in slate, schist and gneiss. Amphibolite facies rocks will be mostly schists and gneisses, while high grade metamorphic rocks will be dominated by gneisses. The facies concept still provides us with the most convenient framework for the discussion of metamorphic rocks. Reproduced from St-Onge MR, Rayner N, Palin RM, Searle MP, and Waters DJ (2013) Integrated pressure–temperature–time constraints for the Tso Morari dome (Northwest India): Implications for the burial and exhumation path of UHP units in the Western Himalaya. The widespread distribution of these facies indicates that this is the temperature-pressure range under which most mountain-building occurs (Figure 20.7). The most common metamorphic facies in mountain belts are the greenschist and the higher-temperature amphibolite facies. Evidence for subduction in the Zagros comes from three main sources: (1) windows in the Hajiabad area at the SE end of the Sanandaj–Sirjan zone, (2) the ∼29 shreds of Mesozoic ophiolites incorporated within the Zagros orogen, and (3) blue amphiboles in the mafic inclusions carried to the surface in the diapirs of Hormoz salt found at the northeast end of the simply folded belt. Diagnostic mineral assemblages. These factors include crustal thickening, the concentration of heat sources such as radioactive elements, the introduction of rising magma bodies, and the presence below the crust of hot mantle rock. Mit Flexionstabellen der verschiedenen Fälle und Zeiten Aussprache und … A possible pressure-temperature path that preserves blueschist facies metamorphism is shown as Path B in Figure 20.9. • Explain how metamorphic facies and index minerals are used to characterize metamorphism in a region. One has to be aware of these complications when working in migmatite areas. The role of water and heat in metamorphism is considered in Chapter VI (Fyfe and Verhoogen). A now vanished ocean basin must have at one time separated now juxtaposed terranes on either side of the blueschist-eclogite facies rocks. This rock could have been sliced off the subducting slab soon after burial, incorporated into the accretionary prism, and almost immediately carried to the surface by thrust faults. He also suggested the concept of paired metamorphic belts, that is, that metamorphic belts of different pressure–temperatures regimes are related in time and space. The absence of migmatites (partially melted rocks) in many granulite facies terrains is strong evidence of low water pressure during metamorphism. Generally, in order to quantitatively determine a PTt path, it will be necessary to extract age information using a number of different methods on different minerals. This gradient, however, is not sustained but decreases to no more than about 16°C/km at a depth of 40 km. Metamorphism in both the Cordilleran and Appalachian Mountain belts is complicated by multiple heating events that in some cases are superimposed on one another. This is a form of strain delocalization, which implies some sort of hardening mechanism. • Explain why fluids are important for metamorphism and describe what happens during metasomatism. Each specific assemblage in a facies reflects a specific protolith composition! A set of metamorphic mineral assemblages indicative of a certain P-T range.! Video Lecture: Medium PT Metamorphism of Pelites. Greenschist Facies Amphibolite Facies Granulite Facies Contact Metamorphism Metamorphic Facies These keywords were added by machine and not by the authors. Without scale numbers, names and/or units, to make clickable templates. Definition (amerikanisch) metamorphic, facies: Thesaurus, Synonyme, Antonyme metamorphic, facies: Etymology metamorphic: metamorphe Fazies. This process is experimental and the keywords may be updated as the learning algorithm improves. From: Encyclopedia of Physical Science and Technology (Third Edition), 2003, K. Bucher, in Encyclopedia of Geology, 2005. The concept was first defined in 1914 by a … See also. Consider the case of a shear zone in the greenschist metamorphic facies, that is, in the temperature range of approximately 300–450 °C. 1.10), which also means that the metamorphic facies and mineral assemblages will differ. “metamorphic facies” therefore, may be defined as a group of metamorphic rocks that have formed under the same set of phsico-chemical conditions and is characterized by a definite set of minerals. Metamorphic petrologists studying contact metamorphism early in the 20th century introduced the idea of metamorphic facies (part of a rock or group of rocks that differs from the whole formation) to correlate metamorphic events. A common description for the intensity of metamorphism is the metamorphic facies concept (not to be confused with sedimentary facies; the two are entirely different). Cross-section showing the inferred postions of low P/T metamorphism and high P/T metamorphism in a plate tectonic setting. About this page. Fluids are driven from the rock during metamorphism. C.J. The facies figures have been prepared with this premise. Searle, in Treatise on Geophysics (Second Edition), 2015. A few years later, Mr D. Lynch-Blosse of George Allen & Unwin Ltd contacted me to explore the possibility of translating it into English. Under those conditions, quartz can flow plastically and feldspar cannot. For the Tso Morari eclogites in Ladakh, P–T conditions of 550 ± 50 °C and 20 ± 3 kbar were initially documented by de Sigoyer et al. Earlier studies have shown that velocities generally increase systematically with progressive metamorphism (e.g., Christensen, 1996). Schematic cross-section of an island arc illustrating isotherm depression along the outer belt and elevation along the inner axis of the volcanic arc. Under most circumstances, a rock will retain the metamorphic assemblage that forms at the highest temperature attained, regardless of the pressure. 304 0 obj <> endobj 327 0 obj <>/Filter/FlateDecode/ID[]/Index[304 36]/Info 303 0 R/Length 115/Prev 1569182/Root 305 0 R/Size 340/Type/XRef/W[1 3 1]>>stream (a) Zeolite, blueschist, ecologite facies The high-pressure blueschist and eclogite facies assemblages have average velocities higher than the mafic granulite rocks. Another descriptor for metamorphism is grade, which is a measure of metamorphic pressure-temperature (and fluid) intensity.