l>Types of Metamorphism
EENS 2120

Petrology

Prof. Stephen A. Nelson
Types of Metamorphism

Metamorphism is identified as follows:

The mineralogical and structural adjustment of solid rocks to physical and also chemical problems that have actually been implemented at depths below the close to surface areas of weathering and also diagenesis and also which differ from conditions under which the rocks in question originated.

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The word "Metamorphism" originates from the Greek: meta = after, morph = create, so metamorphism implies the after develop. In geology this describes the alters in mineral assemblage and also texture that outcome from subjecting a rock to problems such pressures, temperatures, and also chemical environments various from those under which the rock initially formed.

Keep in mind that Diagenesis is additionally a readjust in create that occurs in sedimentary rocks. In geology, but, we restrict diagenetic processes to those which take place at temperatures listed below 200oC and also pressures listed below about 300 MPa (MPa stands for Mega Pascals), this is tantamount to around 3 kilobars of push (1kb = 100 MPa). Metamorphism, therefore occurs at temperatures and also pressures better than 200oC and also 300 MPa. Rocks have the right to be subjected to these higher temperatures and also pressures as they are hidden deeper in the Earth. Such funeral commonly takes area as an outcome of tectonic procedures such as continental collisions or subduction. The upper limit of metamorphism occurs at the push and temperature where melting of the rock in question starts. Once melting begins, the process transforms to an igneous procedure fairly than a metamorphic procedure.

Grade of Metamorphism

As the temperature and/or push boosts on a body of rock we say the rock undergoes prograde metamorphism or that the grade of metamorphism boosts. Metamorphic grade is a general term for describing the loved one temperature and push conditions under which metamorphic rocks develop.

Low-grade metamorphism takes area at temperatures between about 200 to 320oC, and also reasonably low pressure. Low grade metamorphic rocks are mostly characterized by an abundance of hydrous minerals. With enhancing grade of metamorphism, the hydrous minerals begin to react through various other minerals and/or break down to much less hydrous minerals.
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High-grade metamorphism takes location at temperatures greater than 320oC and reasonably high pressure. As grade of metamorphism increases, hydrous minerals end up being less hydrous, by shedding H2O, and also non-hydrous minerals come to be even more common.

Types of Metamorphism

Contact Metamorphism Contact metamorphism occurs nearby to igneous intrusions and also outcomes from high temperatures associated with the igneous intrusion.

Since only a little location bordering the intrusion is heated by the magma, metamorphism is minimal to the zone bordering the intrusion, referred to as a metamorphic or contact aureole. Outside of the call aureole, the rocks are not affected by the intrusive event. The grade of metamorphism boosts in all directions toward the intrusion. Due to the fact that the temperature contrast in between the bordering rock and also the intruded magma is larger at shenable levels in the crust wbelow pressure is low, contact metamorphism is frequently referred to as high temperature, low push metamorphism. The rock created is often a fine-grained rock that mirrors no foliation, referred to as a hornfels.
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Regional Metamorphism Regional metamorphism occurs over large areas and also mostly does not present any type of relationship to igneous bodies. Most regional metamorphism is accompanied by dedevelopment under non-hydrostatic or differential stress and anxiety problems. Thus, neighborhood metamorphism commonly outcomes in creating metamorphic rocks that are strongly foliated, such as slates, schists, and also gniesses. The differential tension commonly outcomes from tectonic pressures that develop compressional stresses in the rocks, such as as soon as two continental masses collide. Hence, regionally metamorphosed rocks take place in the cores of fold/thrust mountain belts or in eroded mountain ranges. Compressive stresses cause folding of rock and thickening of the crust, which has a tendency to push rocks to deeper levels wright here they are based on greater temperatures and also pressures.
Cataclastic Metamorphism Cataclastic metamorphism occurs as a result of mechanical deformation, like as soon as 2 bodies of rock slide previous one another alengthy a fault zone. Heat is produced by the friction of sliding along such a shear zone, and also the rocks tend to be mechanically dedeveloped, being cruburned and also pulverized, because of the shearing. Cataclastic metamorphism is not extremely prevalent and is restricted to a narrowhead zone along which the shearing developed.
Hydrothermal Metamorphism Rocks that are transformed at high temperatures and modeprice pressures by hydrothermal fluids are hydrothermally metamorphosed. This is prevalent in basaltic rocks that generally absence hydrous minerals. The hydrothermal metamorphism results in alteration to such Mg-Fe well-off hydrous minerals as talc, chlorite, serpentine, actinolite, tremolite, zeolites, and clay minerals. Rich ore deposits are regularly created as an outcome of hydrothermal metamorphism.
Burial Metamorphism When sedimentary rocks are buried to depths of numerous kilometers, temperatures greater than 300oC might build in the absence of differential tension. New minerals flourish, yet the rock does not show up to be metamorphosed. The major minerals produced are regularly the Zeolites. Burial metamorphism overlaps, to some degree, with diagenesis, and also qualities right into local metamorphism as temperature and pressure boost. Shock Metamorphism (Impact Metamorphism) When an extraterrestrial body, such as a meteorite or comet effects via the Planet or if there is an extremely huge volcanic explosion, ultrahigh pressures have the right to be produced in the influenced rock. These ultrahigh pressures have the right to create minerals that are only stable at extremely high press, such as the SiO2 polymorphs coesite and stishovite. In addition they have the right to create textures recognized as shock lamellae in mineral grains, and such textures as shatter cones in the influenced rock.

Category of Metamorphic Rocks

Group of metamorphic rocks is based on mineral assemblage, texture, protolith, and also mass chemical composition of the rock. Each of these will certainly be questioned subsequently, then we will certainly summarize just how metamorphic rocks are classified.

Texture In metamorphic rocks individual minerals may or might not be bounded by crystal encounters. Those that are bounded by their very own crystal encounters are termed idioblastic. Those that display namong their very own crystal faces are termed xenoblastic. From examination of metamorphic rocks, it has actually been uncovered that metamorphic minerals deserve to be noted in a generalised sequence, known as the crystalloblastic series, listing minerals in order of their tendency to be idioblastic. In the series, each mineral often tends to develop idioblastic surfaces versus any mineral that occurs lower in the series. This series is listed below:

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

This series have the right to, in a quite general means, allow us to recognize the origin of a offered rock. For instance a rock that shows euhedral plagioclase crystals in call through anhedral amphibole, likely had an igneous protolith, given that a metamorphic rock through the exact same minerals would certainly be intended to display euhedral amphibole in call through anhedral plagioclase.

Anvarious other aspect of the crystalloblastic series is that minerals high on the list tfinish to form porphyroblasts (the metamorphic tantamount of phenocrysts), although K-feldspar (a mineral that occurs reduced in the list) may additionally form porphyroblasts. Porphyroblasts are regularly riddled with inclusions of other minerals that were enveloped in the time of expansion of the porphyroblast. These are shelp to have a poikioblastic texture.

Many metamorphic textures involve foliation. Foliation is primarily brought about by a desired orientation of sheet silicates. If a rock has a slatey cleavage as its foliation, it is termed a slate, if it has actually a phyllitic foliation, it is termed a phyllite, if it has actually a shistose foliation, it is termed a schist. A rock that shows a banded texture without a distinct foliation is termed a gneiss. All of these can be porphyroblastic (i.e. can contain porhyroblasts).

A rock that reflects no foliation is called a hornfels if the grain dimension is small, and a granulite, if the grain size is large and also individual minerals have the right to be conveniently distinguiburned with a hand also lens.

Protolith Protolith refers to the original rock, before metamorphism. In low grade metamorphic rocks, original textures are regularly preserved permitting one to determine the likely protolith. As the grade of metamorphism rises, original textures are reput via metamorphic textures and also various other clues, such as bulk chemical composition of the rock, are offered to recognize the protolith.

Bulk Chemical Composition The mineral assemblage that establishes in a metamorphic rock is dependent on The pressure and temperature reached throughout metamorphism The complace of any type of liquid phase existing during metamorphism, and also The mass chemical complace of the rock. Similar to in igneous rocks, minerals can only form if the essential chemical constituents are existing in the rock (i.e. the principle of silica saturation and also alumina saturation applies to metamorphic rocks as well). Based on the mineral assemblage present in the rock one can regularly estimate the approximate bulk chemical complace of the rock. Some terms that describe this general bulk chemical composition are as follows:

Pelitic. These rocks are derivatives of aluminous sedimentary rocks like shales and also mudrocks. Because of their high concentrations of alumina they are well-known by an abundance of aluminous minerals, prefer clay minerals, micas, kyanite, sillimanite, andaluwebsite, and also garnet. Quartzo-Feldspathic. Rocks that initially consisted of mainly quartz and feldspar favor granitic rocks and also arkosic sandstones will additionally contain a wealth of quartz and also feldspar as metamorphic rocks, considering that these minerals are steady over a broad range of temperature and push. Those that exhilittle greatly quartz and also feldspar with only minor amounts of aluminous minerals are termed quartzo-feldspathic. Calcareous. Calcareous rocks are calcium well-off. They are generally derivatives of carbonate rocks, although they contain other minerals that result from reactivity of the carbonates with linked siliceous detrital minerals that were current in the rock. At low qualities of metamorphism calcareous rocks are well-known by their abundance of carbonate minerals choose calpoint out and also dolomite. With boosting grade of metamorphism these are reput by minerals like brumention, phlogopite (Mg-affluent biotite), chlorite, and also tremolite. At even greater qualities anhydrous minerals choose diopside, forsterite, wollastonite, grossularite, and also calcic plagioclase. Basic. Similar to in igneous rocks, the general term standard describes low silica content. Basic metamorphic rocks are mainly derivatives of standard igneous rocks favor basalts and also gabbros. They have a wealth of Fe-Mg minerals prefer biotite, chlorite, and hornblende, as well as calcic minerals prefer plagioclase and epidote. Magnesian. Rocks that are well-off in Mg with reasonably less Fe, are termed magnesian. Such rocks would contain Mg-wealthy minerals choose serpentine, brupoint out, talc, dolomite, and also tremolite. In basic, such rocks usually have an ultrastandard protolith, prefer peridotite, dunite, or pyroxenite. Ferriginous. Rocks that are well-off in Fe via little Mg are termed ferriginous. Such rocks might be derivatives of Fe-well-off cherts or ironstones. They are characterized by an abundance of Fe-affluent minerals favor greenalite (Fe-rich serpentine), minnesotaite (Fe-affluent talc), ferroactinolite, ferrocummingtonite, hematite, and also magnetite at low qualities, and ferrosilite, fayalite, ferrohedenbergite, and almandine garnet at higher grades. Manganiferrous. Rocks that are defined by the existence of Mn-well-off minerals are termed manganiferrous. They are defined by such minerals as Stilpnomelane and spessartine. Classification Classification of metamorphic rocks counts on what is visible in the rock and also its degree of metamorphism. Keep in mind that classification is mainly loose and also valuable such that names have the right to be adapted to define the rock in the many satisfactory method that conveys the vital attributes. Three kinds of criteria are typically employed. These are:

Mineralogical - The the majority of separating minerals are offered as a preresolve to a textural term. Hence, a schist containing biotite, garnet, quartz, and feldspar, would be called a biotite-garnet schist. A gneiss containing hornblende, pyroxene, quartz, and feldspar would be called a hornblende-pyroxene gneiss. A schist containing porphyroblasts of K-feldspar would be referred to as a K-spar porphyroblastic schist. Chemical - If the general chemical composition have the right to be figured out from the mineral assemblage, then a chemical name can be employed. For example a schist with many quartz and feldspar and also some garnet and also muscovite would be called a garnet-muscovite quartzo-feldspathic schist. A schist consisting largely of talc would be referred to as a talc-magnesian schist. Protolithic - If a rock has actually undertaken only slight metamorphism such that its original texture can still be observed then the rock is given a name based upon its original name, via the preresolve meta- used. For example: metabasalt, metagraywacke, meta-andesite, metagranite.

In enhancement to these conventions, certain non-foliated rocks via specific chemical compositions and/or mineral assemblperiods are given certain names. These are as follows:

Amphibolites: These are medium to coarse grained, dark colored rocks whose primary minerals are hornblende and plagioclase. They outcome from metamorphism of fundamental igneous rocks. Foliation is very variable, but when existing the term schist deserve to be appended to the name (i.e. amphibolite schist). Marbles: These are rocks composed greatly of calcite, and much less generally of dolomite. They outcome from metamorphism of limestones and dolostones. Some foliation may be present if the marble has micas. Eclogites: These are medium to coarse grained consisting mostly of garnet and green clinopyroxene called omphamention, that result from high grade metamorphism of fundamental igneous rocks. Eclogites commonly perform not present foliation. Quartzites: Quartz arenites and also chert both are created mostly of SiO2. Due to the fact that quartz is secure over a wide array of pressures and also temperatures, metamorphism of quartz arenites and also cherts will certainly result only in the recrystallization of quartz forming a hard rock via interlocking crystals of quartz. Such a rock is called a quartzite. Serpentinites: Serpentinites are rocks that consist greatly of serpentine. These form by hydrothermal metamorphism of ultrafundamental igneous rocks. Soapstones: Soapstones are rocks that contain an abundance of talc, which gives the rock a grbasic feel, similar to that of soap. Talc is an Mg-wealthy mineral, and also therefore soapstones from ultrabasic igneous protoliths, prefer peridotites, dunites, and pyroxenites, usually by hydrothermal alteration.

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Skarns: Skarns are rocks that originate from call metamorphism of limestones or dolostones, and also show proof of having exreadjusted constituents with the intruding magma. Thus, skarns are mostly created of minerals prefer calmention and dolomite, from the original carbonate rock, yet contain abundant calcium and magnesium silicate minerals prefer andradite, grossularite, epidote, vesuvianite, diopside, and also wollastonite that create by reactivity of the original carbonate minerals through silica from the magma. The chemical exadjust is that takes area is called metasomatism. Mylonites: Mylonites are cataclastic metamorphic rocks that are produced alengthy shear zones deep in the crust. They are typically fine-grained, occasionally glassy, that are streaky or layered, through the layers and streaks having been drawn out by ductile shear.