l>Types of Metamorphism
EENS 2120

Petrology

Prof. Stephen A. Nelson
Types of Metamorphism

Metamorphism is characterized as follows:

The mineralogical and also structural mediate of heavy rocks come physical and also chemical conditions that have been enforced at depths listed below the close to surface area of weathering and diagenesis and which differ from problems under i m sorry the rocks in inquiry originated.

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The word "Metamorphism" originates from the Greek: meta = after, morph = form, for this reason metamorphism means the after ~ form. In geology this refers to the alters in mineral assemblage and texture that an outcome from subjecting a rock to problems such pressures, temperatures, and chemical environments various from those under i beg your pardon the rock originally formed.

note that Diagenesis is also a adjust in type that occurs in sedimentary rocks. In geology, however, we restrict diagenetic procedures to those which take place at temperatures below 200oC and also pressures below about 300 MPa (MPa stands for Mega Pascals), this is equivalent to around 3 kilobars of pressure (1kb = 100 MPa). Metamorphism, therefore occurs in ~ temperatures and also pressures higher than 200oC and also 300 MPa. Rocks deserve to be subjected to these higher temperatures and also pressures as they are hidden deeper in the Earth. Such funeral usually takes place as a an outcome of tectonic processes such as continent collisions or subduction. The top limit the metamorphism wake up at the pressure and temperature whereby melting of the absent in inquiry begins. Once melting begins, the process changes come an igneous procedure rather 보다 a metamorphic process.

Grade that Metamorphism

As the temperature and/or pressure rises on a body of rock we say the absent undergoes prograde metamorphism or that the grade of metamorphism increases. Metamorphic grade is a general term for describing the relative temperature and also pressure conditions under which metamorphic rocks form.

Low-grade metamorphism takes ar at temperature between around 200 to 320oC, and reasonably low pressure. Short grade metamorphic rocks space generally characterized by an abundance of hydrous minerals. With boosting grade of metamorphism, the hydrous minerals start to reaction with various other minerals and/or breakdown to much less hydrous minerals.
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High-grade metamorphism takes location at temperatures better than 320oC and reasonably high pressure. Together grade the metamorphism increases, hydrous minerals become less hydrous, by shedding H2O, and also non-hydrous mineral become an ext common.

Types that Metamorphism

Contact Metamorphism contact metamorphism occurs surrounding to igneous intrusions and also results from high temperatures linked with the igneous intrusion.

Since just a little area bordering the intrusion is cook by the magma, metamorphism is restricted to the zone surrounding the intrusion, dubbed a metamorphic or contact aureole. Outside of the contact aureole, the rocks are not influenced by the intrusive event. The class of metamorphism boosts in every directions toward the intrusion. Due to the fact that the temperature contrast between the bordering rock and the force magma is bigger at shallow levels in the crust where pressure is low, contact metamorphism is frequently referred to as high temperature, low pressure metamorphism. The rock created is regularly a fine-grained absent that shows no foliation, dubbed a hornfels.
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Regional Metamorphism Regional metamorphism wake up over big areas and also generally does not show any type of relationship to igneous bodies. Most local metamorphism is add by deformation under non-hydrostatic or differential tension conditions. Thus, regional metamorphism usually results in developing metamorphic rocks that are strongly foliated, such as slates, schists, and gniesses. The differential stress usually outcomes from tectonic pressures that create compressional emphasize in the rocks, such as when two continental masses collide. Thus, regionally metamorphosed rocks occur in the cores that fold/thrust hill belts or in eroded mountain ranges. Compressive stresses an outcome in urgent of rock and thickening of the crust, which tends to push rocks to deeper levels where they are subjected to greater temperatures and pressures.
Cataclastic Metamorphism Cataclastic metamorphism occurs as a result of mechanically deformation, like when two body of rock slide previous one another along a fault zone. Warm is created by the friction that sliding follow me such a shear zone, and also the rocks have tendency to it is in mechanically deformed, being crushed and also pulverized, as result 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 moderate pressures by hydrothermal fluids room hydrothermally metamorphosed. This is usual in basaltic rocks that generally lack hydrous minerals. The hydrothermal metamorphism outcomes in alteration to such Mg-Fe affluent hydrous minerals together talc, chlorite, serpentine, actinolite, tremolite, zeolites, and also clay minerals. Wealthy ore deposits space often formed as a an outcome of hydrothermal metamorphism.
Burial Metamorphism When sedimentary rocks are hidden to depths of numerous kilometers, temperatures higher than 300oC may construct in the lack of differential stress. Brand-new minerals grow, but the rock does not appear to it is in metamorphosed. The key minerals produced are frequently the Zeolites. Interment metamorphism overlaps, to part extent, through diagenesis, and grades into regional metamorphism as temperature and also pressure increase. Shock Metamorphism (Impact Metamorphism) When an extraterrestrial body, such together a meteorite or comet impacts with the earth or if over there is a very large volcanic explosion, ultrahigh pressures deserve to be created in the impacted rock. These ultrahigh pressures can create minerals the are just stable at an extremely high pressure, such together the SiO2 polymorphs coesite and also stishovite. In enhancement they can produce textures well-known as shock lamellae in mineral grains, and also such textures as shatter cones in the affected rock.

Classification the Metamorphic Rocks

Classification of metamorphic rocks is based upon mineral assemblage, texture, protolith, and also bulk chemical composition the the rock. Each of these will certainly be disputed in turn, climate we will certainly summarize exactly how metamorphic rocks room classified.

Texture In metamorphic rocks individual minerals may or might not be bounded by decision faces. Those that space bounded by their own crystal deals with are termed idioblastic. Those that show none of their very own crystal faces are termed xenoblastic. From examination of metamorphic rocks, it has actually been found that metamorphic minerals have the right to be noted in a generalised sequence, known as the crystalloblastic series, listing minerals in order of their propensity to be idioblastic. In the series, each mineral tends to construct idioblastic surfaces versus any mineral that occurs reduced in the series. This collection is detailed 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, permit us to determine the beginning of a given rock. For instance a absent that reflects euhedral plagioclase crystals in call with anhedral amphibole, likely had actually an igneous protolith, because a metamorphic rock through the very same minerals would be supposed to display euhedral amphibole in call with anhedral plagioclase.

Another aspect of the crystalloblastic series is the minerals high top top the list often tend to form porphyroblasts (the metamorphic identical of phenocrysts), although K-feldspar (a mineral that occurs lower in the list) may also form porphyroblasts. Porphyroblasts are frequently riddled with inclusions of other minerals that were enveloped throughout growth that the porphyroblast. This are claimed to have a poikioblastic texture.

Most metamorphic structure involve foliation. Foliation is normally caused through a preferred orientation of paper silicates. If a rock has a slatey cleavage as its foliation, that is termed a slate, if it has a phyllitic foliation, it is termed a phyllite, if it has actually a shistose foliation, it is termed a schist. A absent that mirrors a banded texture without a unique foliation is termed a gneiss. Every one of these might be porphyroblastic (i.e. Might contain porhyroblasts).

A rock that shows no foliation is called a hornfels if the grain dimension is small, and a granulite, if the grain dimension is large and separation, personal, instance minerals deserve to be easily differentiated with a hand lens.

Protolith Protolith refers to the original rock, prior to metamorphism. In short grade metamorphic rocks, initial textures are regularly preserved allowing one to recognize the likely protolith. Together the great of metamorphism increases, initial textures are changed with metamorphic textures and other clues, together as mass chemical ingredient of the rock, are offered to determine the protolith.

Bulk chemistry Composition The mineral assemblage that develops in a metamorphic absent is dependent on The pressure and also temperature reached throughout metamorphism The ingredient of any fluid phase current during metamorphism, and The mass chemical ingredient of the rock. as with in igneous rocks, minerals have the right to only type if the important chemical constituents are existing in the absent (i.e. The concept of silica saturation and alumina saturation uses to metamorphic rocks together well). Based upon the mineral assemblage current in the absent one can frequently estimate the approximate bulk chemical composition of the rock. Part terms that explain this general mass chemical composition are as follows:

Pelitic. These rocks space derivatives the aluminous sedimentary rocks favor shales and also mudrocks. Due to the fact that of their high concentration of alumina lock are well-known by an abundance of aluminous minerals, like clay minerals, micas, kyanite, sillimanite, andalusite, and also garnet. Quartzo-Feldspathic. Rocks that originally consisted of mostly quartz and feldspar like granitic rocks and arkosic sandstones will additionally contain an abundance 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 are termed quartzo-feldspathic. Calcareous. Calcareous rocks are calcium rich. They room usually derivatives of lead carbonate rocks, although lock contain various other minerals that result from reaction of the carbonates with connected siliceous detrital minerals the were current in the rock. In ~ low grades of metamorphism calcareous rocks are well-known by their abundance of lead carbonate minerals like calcite and dolomite. With enhancing grade the metamorphism this are changed by minerals prefer brucite, phlogopite (Mg-rich biotite), chlorite, and also tremolite. At even greater grades anhydrous minerals favor diopside, forsterite, wollastonite, grossularite, and also calcic plagioclase. Basic. Just choose in igneous rocks, the basic term an easy refers to low silica content. Basic metamorphic rocks are normally derivatives of an easy igneous rocks choose basalts and gabbros. Lock have an abundance of Fe-Mg minerals choose biotite, chlorite, and also hornblende, as well as calcic minerals favor plagioclase and also epidote. Magnesian. Rocks that room rich in Mg with reasonably less Fe, space termed magnesian. Together rocks would contain Mg-rich minerals prefer serpentine, brucite, talc, dolomite, and also tremolite. In general, such rocks usually have an ultrabasic protolith, choose peridotite, dunite, or pyroxenite. Ferriginous. Rocks that room rich in Fe with little Mg space termed ferriginous. Together rocks can 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 also magnetite at low grades, and ferrosilite, fayalite, ferrohedenbergite, and almandine garnet at higher grades. Manganiferrous. Rocks that are identified by the existence of Mn-rich minerals room termed manganiferrous. Castle are identified by such minerals as Stilpnomelane and spessartine. Classification group of metamorphic rocks relies on what is visible in the rock and also its degree of metamorphism. Note that classification is generally loosened and useful such the names have the right to be adapted to explain the rock in the many satisfactory means that conveys the vital characteristics. Three kinds that criteria are generally employed. This are:

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

In enhancement to this conventions, specific non-foliated rocks with specific chemical compositions and/or mineral assemblages are given details names. These space as follows:

Amphibolites: this are tool to rough circuit grained, dark fancy rocks whose major minerals are hornblende and also plagioclase. They result from metamorphism of basic igneous rocks. Foliation is extremely variable, yet when current the ax schist can be appended come the surname (i.e. Amphibolite schist). Marbles: These are rocks composed largely of calcite, and less typically of dolomite. They result from metamorphism of limestones and dolostones. Some foliation may be present if the marble includes micas. Eclogites: these are tool to rough circuit grained consisting largely of garnet and also green clinopyroxene dubbed omphacite, that result from high class metamorphism of basic igneous rocks. Eclogites usually carry out not show foliation. Quartzites: Quartz arenites and chert both room composed mainly of SiO2. Due to the fact that quartz is stable over a wide selection of pressures and temperatures, metamorphism the quartz arenites and also cherts will result only in the recrystallization the quartz creating a difficult rock through interlocking crystals the quartz. Together a absent is called a quartzite. Serpentinites: Serpentinites room rocks the consist mostly of serpentine. These form by hydrothermal metamorphism the ultrabasic igneous rocks. Soapstones: Soapstones are rocks that contain wealth of talc, which provides the absent a greasy feel, comparable to that of soap. Talc is an Mg-rich mineral, and thus soapstones indigenous ultrabasic igneous protoliths, choose peridotites, dunites, and pyroxenites, generally by hydrothermal alteration.

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Skarns: Skarns are rocks that originate from call metamorphism that limestones or dolostones, and also show evidence of having exchanged constituents with the intruding magma. Thus, skarns are normally composed that minerals like calcite and also dolomite, indigenous the original carbonate rock, yet contain abundant calcium and also magnesium silicate minerals choose andradite, grossularite, epidote, vesuvianite, diopside, and also wollastonite that kind by reaction of the initial carbonate minerals v silica native the magma. The chemical exchange is the takes ar is called metasomatism. Mylonites: Mylonites space cataclastic metamorphic rocks the are developed along shear area deep in the crust. They room usually fine-grained, sometimes glassy, that are streaky or layered, through the layers and streaks having actually been attracted out by ductile shear.