l>Types the Metamorphism
EENS 2120

Petrology

Prof. Stephen A. Nelson
Types the Metamorphism

Metamorphism is characterized as follows:

The mineralogical and also structural adjustment of heavy rocks to physical and also chemical conditions that have been enforced at depths listed below the close to surface area of weathering and diagenesis and also which differ from conditions under i beg your pardon the rocks in question originated.

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The indigenous "Metamorphism" comes from the Greek: meta = after, morph = form, so metamorphism means the after ~ form. In geology this describes the transforms in mineral assemblage and texture that result from subjecting a rock to conditions such pressures, temperatures, and chemical environments various from those under which the rock initially formed.

note that Diagenesis is additionally a readjust in type that occurs in sedimentary rocks. In geology, however, us restrict diagenetic procedures to those which happen at temperatures below 200oC and pressures below about 300 MPa (MPa represents Mega Pascals), this is indistinguishable to about 3 kilobars of push (1kb = 100 MPa). Metamorphism, as such occurs in ~ temperatures and pressures greater than 200oC and also 300 MPa. Rocks have the right to be based on these greater temperatures and pressures as they are hidden deeper in the Earth. Such burial usually takes place as a an outcome of tectonic procedures such as continental collisions or subduction. The top limit the metamorphism wake up at the pressure and also temperature wherein melting the the absent in question begins. Once melting begins, the procedure changes come an igneous procedure rather than a metamorphic process.

Grade of Metamorphism

As the temperature and/or pressure boosts on a human body of rock us say the absent undergoes prograde metamorphism or that the grade of metamorphism increases. Metamorphic grade is a general term for describing the loved one temperature and pressure problems under which metamorphic rocks form.

Low-grade metamorphism takes ar at temperature between about 200 to 320oC, and reasonably low pressure. Low grade metamorphic rocks room generally characterized by an abundance of hydrous minerals. With increasing grade that metamorphism, the hydrous minerals start to reaction with various other minerals and/or failure to less hydrous minerals.
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High-grade metamorphism takes place at temperatures higher than 320oC and relatively high pressure. Together grade of metamorphism increases, hydrous minerals come to be less hydrous, by shedding H2O, and also non-hydrous mineral become more common.

Types of Metamorphism

Contact Metamorphism call metamorphism occurs nearby to igneous intrusions and also results native high temperatures connected with the igneous intrusion.

Since just a tiny area surrounding the intrusion is heated by the magma, metamorphism is restricted to the zone neighboring the intrusion, referred to as a metamorphic or contact aureole. External of the contact aureole, the rocks room not impacted by the intrusive event. The great of metamorphism rises in every directions towards the intrusion. Because the temperature contrast between the bordering rock and also the force magma is bigger at shallow level in the tardy where press is low, call metamorphism is often referred to as high temperature, low push metamorphism. The rock produced is often a fine-grained rock that shows no foliation, dubbed a hornfels.
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Regional Metamorphism Regional metamorphism occurs over big areas and generally does not show any kind of relationship to igneous bodies. Most local metamorphism is accompanied by deformation under non-hydrostatic or differential stress conditions. Thus, local metamorphism usually outcomes in developing metamorphic rocks that space strongly foliated, such as slates, schists, and gniesses. The differential stress and anxiety usually results from tectonic forces that produce compressional stresses in the rocks, together as as soon as two continental masses collide. Thus, regionally metamorphosed rocks happen in the cores the fold/thrust mountain belts or in eroded hill ranges. Compressive stresses result in urgent of rock and also thickening that the crust, which has tendency to press rocks to deeper levels wherein they are subjected to higher temperatures and pressures.
Cataclastic Metamorphism Cataclastic metamorphism occurs together a an outcome of mechanically deformation, like as soon as two body of rock slide past one an additional along a fault zone. Warmth is generated by the friction the sliding follow me such a shear zone, and the rocks often tend to it is in mechanically deformed, being crushed and also pulverized, because of the shearing. Cataclastic metamorphism is not very common and is limited to a small zone along which the shearing occurred.
Hydrothermal Metamorphism Rocks the are changed at high temperatures and also moderate pressures by hydrothermal fluids space hydrothermally metamorphosed. This is typical in basaltic rocks that usually lack hydrous minerals. The hydrothermal metamorphism results in alteration to together Mg-Fe rich hydrous minerals as talc, chlorite, serpentine, actinolite, tremolite, zeolites, and clay minerals. Well-off ore deposits room often created as a an outcome of hydrothermal metamorphism.
Burial Metamorphism When sedimentary rocks are hidden to depth of number of kilometers, temperatures greater than 300oC may construct in the lack of differential stress. Brand-new minerals grow, but the absent does not show up to be metamorphosed. The main minerals created are frequently the Zeolites. Burial metamorphism overlaps, to part extent, through diagenesis, and also grades into local metamorphism together temperature and pressure increase. Shock Metamorphism (Impact Metamorphism) When one extraterrestrial body, such as a meteorite or comet effects with the earth or if over there is a very big volcanic explosion, ultrahigh pressures can be produced in the affected rock. These ultrahigh pressures can develop minerals that are just stable at very high pressure, such as the SiO2 polymorphs coesite and stishovite. In addition they can create textures recognized as shock lamellae in mineral grains, and such textures together shatter hat in the influenced rock.

Classification the Metamorphic Rocks

Classification the metamorphic rocks is based on mineral assemblage, texture, protolith, and bulk chemical composition the the rock. Every of these will certainly be discussed in turn, climate we will certainly summarize how metamorphic rocks space classified.

Texture In metamorphic rocks individual minerals may or may not it is in bounded by decision faces. Those that space bounded through their very own crystal deals with are termed idioblastic. Those that show none the their very own crystal faces are termed xenoblastic. From check of metamorphic rocks, it has actually been uncovered that metamorphic minerals deserve to be provided in a generalised sequence, recognized as the crystalloblastic series, listing minerals in bespeak of their tendency to be idioblastic. In the series, every mineral has tendency to develop idioblastic surfaces against any mineral that occurs reduced in the series. This collection is provided below:

rutile, sphene, magnetite tourmaline kyanite, staurolite, garnet, andalusite epidote, zoisite, lawsonite, forsterite pyroxenes, amphiboles, wollastonite micas, chlorites, talc, stilpnomelane, prehnite dolomite, calcite scapolite, cordierite, feldspars quartz

This collection can, in a rather basic way, enable us to identify the beginning of a given rock. For instance a absent that shows euhedral plagioclase crystals in call with anhedral amphibole, likely had actually an igneous protolith, due to the fact that a metamorphic rock with the very same minerals would certainly be intended to show euhedral amphibole in call with anhedral plagioclase.

Another element of the crystalloblastic collection is the minerals high top top the list tend to form porphyroblasts (the metamorphic tantamount of phenocrysts), back K-feldspar (a mineral that occurs reduced in the list) may also kind porphyroblasts. Porphyroblasts are frequently riddled v inclusions of various other minerals that were enveloped throughout growth of the porphyroblast. These are claimed to have a poikioblastic texture.

Most metamorphic textures involve foliation. Foliation is typically caused through a preferred orientation of sheet silicates. If a rock has a slatey cleavage together its foliation, it is termed a slate, if it has actually a phyllitic foliation, that is termed a phyllite, if it has actually a shistose foliation, it is termed a schist. A rock that mirrors a banded texture there is no a distinctive foliation is termed a gneiss. All of these might be porphyroblastic (i.e. Can contain porhyroblasts).

A rock that shows no foliation is dubbed a hornfels if the grain size is small, and also a granulite, if the grain size is big and individual minerals deserve to be easily identified with a hand lens.

Protolith Protolith describes the original rock, prior to metamorphism. In short grade metamorphic rocks, original textures are regularly preserved permitting one to recognize the most likely protolith. As the grade of metamorphism increases, initial textures are replaced with metamorphic textures and other clues, together as bulk chemical ingredient of the rock, are offered to identify the protolith.

Bulk chemistry Composition The mineral assemblage that creates in a metamorphic rock is dependency on The pressure and temperature reached during metamorphism The ingredient of any kind of fluid phase present during metamorphism, and also The mass chemical composition of the rock. as with in igneous rocks, minerals can only type if the vital chemical ingredient are current in the absent (i.e. The principle of silica saturation and also alumina saturation uses to metamorphic rocks as well). Based upon the mineral assemblage current in the absent one can regularly estimate the approximate mass chemical ingredient of the rock. Some terms that explain this general bulk chemical composition are as follows:

Pelitic. This rocks are derivatives of aluminous sedimentary rocks favor shales and also mudrocks. Due to the fact that of their high concentration of alumina lock are recognized by an abundance of aluminous minerals, like clay minerals, micas, kyanite, sillimanite, andalusite, and garnet. Quartzo-Feldspathic. Rocks that originally included mostly quartz and also feldspar choose granitic rocks and also arkosic sandstones will likewise contain wealth of quartz and also feldspar together metamorphic rocks, since these minerals space stable end a wide variety of temperature and also pressure. Those that exhibit mostly quartz and also feldspar with just minor quantities of aluminous minerals room termed quartzo-feldspathic. Calcareous. Calcareous rocks are calcium rich. They space usually derivatives of lead carbonate rocks, although lock contain various other minerals that an outcome from reaction the the carbonates with associated siliceous detrital minerals the were present in the rock. In ~ low qualities of metamorphism calcareous rocks are known by your abundance of carbonate minerals favor calcite and also dolomite. With enhancing grade the metamorphism these are changed by minerals prefer brucite, phlogopite (Mg-rich biotite), chlorite, and also tremolite. In ~ even higher grades anhydrous minerals prefer diopside, forsterite, wollastonite, grossularite, and also calcic plagioclase. Basic. Just favor in igneous rocks, the basic term simple refers to low silica content. Simple metamorphic rocks are generally derivatives of an easy igneous rocks favor basalts and also gabbros. Lock have an abundance of Fe-Mg minerals choose biotite, chlorite, and also hornblende, and calcic minerals like plagioclase and also epidote. Magnesian. Rocks that are rich in Mg with fairly less Fe, space termed magnesian. Such rocks would certainly contain Mg-rich minerals choose serpentine, brucite, talc, dolomite, and tremolite. In general, together rocks usually have actually an ultrabasic protolith, favor peridotite, dunite, or pyroxenite. Ferriginous. Rocks that room rich in Fe with small Mg space termed ferriginous. Together rocks might be derivatives the Fe-rich cherts or ironstones. They are identified by an abundance of Fe-rich minerals choose greenalite (Fe-rich serpentine), minnesotaite (Fe-rich talc), ferroactinolite, ferrocummingtonite, hematite, and magnetite at low grades, and ferrosilite, fayalite, ferrohedenbergite, and almandine garnet at greater grades. Manganiferrous. Rocks that are identified by the presence of Mn-rich minerals space termed manganiferrous. Lock are identified by such minerals together Stilpnomelane and spessartine. Classification group of metamorphic rocks relies on what is visible in the rock and its degree of metamorphism. Note that classification is generally loosened and practical such that names have the right to be adjusted to describe the rock in the most satisfactory means that conveys the vital characteristics. 3 kinds the criteria are generally employed. These are:

Mineralogical - The most separating minerals are used as a prefix to a textural term. Thus, a schist include biotite, garnet, quartz, and feldspar, would be referred to as a biotite-garnet schist. A gneiss comprise hornblende, pyroxene, quartz, and also feldspar would certainly be called a hornblende-pyroxene gneiss. A schist containing porphyroblasts that K-feldspar would be dubbed a K-spar porphyroblastic schist. chemical - If the general chemical composition deserve to be determined from the mineral assemblage, climate a chemical name deserve to be employed. For example a schist through a lot of quartz and feldspar and some garnet and also muscovite would certainly be referred to as a garnet-muscovite quartzo-feldspathic schist. A schist consisting largely of talc would be called a talc-magnesian schist. Protolithic - If a rock has undergone only slight metamorphism such the its initial texture can still it is in observed climate the absent is given a name based upon its original name, with the prefix meta- applied. For example: metabasalt, metagraywacke, meta-andesite, metagranite.

In addition to these conventions, particular non-foliated rocks with details chemical compositions and/or mineral assemblages space given certain names. These space as follows:

Amphibolites: this are medium to coarse grained, dark colored rocks whose principal minerals are hornblende and plagioclase. They an outcome from metamorphism of an easy igneous rocks. Foliation is highly variable, but when present the hatchet schist have the right to be appended to the surname (i.e. Amphibolite schist). Marbles: These space rocks composed greatly of calcite, and also less generally of dolomite. They result from metamorphism that limestones and also dolostones. Part foliation may be present if the marble consists of micas. Eclogites: this are tool to coarse grained consisting greatly of garnet and green clinopyroxene called omphacite, that an outcome from high great metamorphism of basic igneous rocks. Eclogites usually do not display foliation. Quartzites: Quartz arenites and chert both space composed mostly of SiO2. Due to the fact that quartz is steady over a wide selection of pressures and temperatures, metamorphism that quartz arenites and cherts will an outcome only in the recrystallization of quartz creating a difficult rock with interlocking crystals of quartz. Such a rock is dubbed a quartzite. Serpentinites: Serpentinites space rocks that consist greatly of serpentine. These form by hydrothermal metamorphism the ultrabasic igneous rocks. Soapstones: Soapstones are rocks that contain wealth of talc, which gives the absent a greasy feel, comparable to the of soap. Talc is an Mg-rich mineral, and thus soapstones native ultrabasic igneous protoliths, prefer peridotites, dunites, and pyroxenites, usually by hydrothermal alteration.

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Skarns: Skarns space rocks that originate from call metamorphism that limestones or dolostones, and also show proof of having actually exchanged constituents through the intruding magma. Thus, skarns are normally composed of minerals favor calcite and dolomite, native the original carbonate rock, yet contain abundant calcium and also magnesium silicate minerals prefer andradite, grossularite, epidote, vesuvianite, diopside, and also wollastonite that type by reaction of the initial carbonate minerals through silica native the magma. The chemical exchange is the takes place is dubbed metasomatism. Mylonites: Mylonites room cataclastic metamorphic rocks that are created along shear area deep in the crust. They space usually fine-grained, periodically glassy, that space streaky or layered, through the layers and also streaks having actually been drawn out through ductile shear.