l>Types the Metamorphism
EENS 2120

Petrology

Prof. Stephen A. Nelson
Types the Metamorphism

Metamorphism is defined as follows:

The mineralogical and also structural mediate of solid rocks to physical and chemical conditions that have been enforced at depths below the near surface zones of weathering and diagenesis and which different from problems under i beg your pardon the rocks in question originated.

You are watching: Compare and contrast contact metamorphism and regional metamorphism

The native "Metamorphism" comes from the Greek: meta = after, morph = form, for this reason metamorphism way the after ~ form. In geology this describes the transforms in mineral assemblage and texture that an outcome from subjecting a absent to conditions such pressures, temperatures, and also chemical environments different from those under i beg your pardon the rock originally formed.

keep in mind that Diagenesis is likewise a change in form that occurs in sedimentary rocks. In geology, however, we restrict diagenetic processes to those which happen at temperatures listed below 200oC and also pressures below around 300 MPa (MPa means Mega Pascals), this is identical to about 3 kilobars of press (1kb = 100 MPa). Metamorphism, because of this occurs at temperatures and also pressures higher than 200oC and also 300 MPa. Rocks can be subjected to these greater temperatures and also pressures as they are buried deeper in the Earth. Such interment usually takes place as a result of tectonic procedures such as continent collisions or subduction. The top limit that metamorphism occurs at the pressure and also temperature whereby melting that the absent in question begins. When melting begins, the process changes to an igneous procedure rather than a metamorphic process.

Grade the Metamorphism

As the temperature and/or pressure rises on a human body of rock us say the rock undergoes prograde metamorphism or that the great of metamorphism increases. Metamorphic grade is a basic term for describing the relative temperature and also pressure conditions under i m sorry metamorphic rocks form.

Low-grade metamorphism takes location at temperature between around 200 to 320oC, and relatively low pressure. Low grade metamorphic rocks room generally defined by wealth of hydrous minerals. With boosting grade the metamorphism, the hydrous minerals begin to react with other minerals and/or failure to much less hydrous minerals.
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High-grade metamorphism takes location at temperatures better than 320oC and relatively high pressure. As grade of metamorphism increases, hydrous minerals come to be less hydrous, by losing H2O, and also non-hydrous mineral become more common.

Types that Metamorphism

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

Since only a small area neighboring the intrusion is cook by the magma, metamorphism is limited to the zone bordering the intrusion, called a metamorphic or contact aureole. Outside of the call aureole, the rocks space not affected by the intrusive event. The class of metamorphism boosts in all directions towards the intrusion. Since the temperature contrast between the neighboring rock and the coercion magma is larger at shallow levels in the late where push is low, contact metamorphism is frequently referred to as high temperature, low press metamorphism. The rock produced is regularly a fine-grained rock that shows no foliation, called a hornfels.
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Regional Metamorphism Regional metamorphism occurs over large areas and also generally does no show any type of relationship come igneous bodies. Most local metamorphism is add by deformation under non-hydrostatic or differential stress conditions. Thus, local metamorphism usually results in creating metamorphic rocks that room strongly foliated, such together slates, schists, and gniesses. The differential tension usually results from tectonic pressures that produce compressional stresses in the rocks, such as as soon as two continent 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 often tends to push rocks come deeper levels where they room subjected to greater temperatures and also pressures.
Cataclastic Metamorphism Cataclastic metamorphism occurs as a an outcome of mechanical deformation, like once two bodies of absent slide past one an additional along a fault zone. Warm is created by the friction of sliding follow me such a shear zone, and also the rocks have tendency to it is in mechanically deformed, being crushed and also pulverized, because of the shearing. Cataclastic metamorphism is not really common and also is restricted to a narrow zone follow me which the shearing occurred.
Hydrothermal Metamorphism Rocks the are altered at high temperatures and also moderate pressures by hydrothermal fluids room hydrothermally metamorphosed. This is typical in basaltic rocks that normally lack hydrous minerals. The hydrothermal metamorphism outcomes in change to together Mg-Fe rich hydrous minerals as 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 buried to depths of number of kilometers, temperatures better than 300oC may build in the lack of differential stress. New minerals grow, yet the rock does not show up to be metamorphosed. The main minerals produced are often the Zeolites. Interment metamorphism overlaps, to some extent, through diagenesis, and also grades into regional metamorphism together temperature and also pressure increase. Shock Metamorphism (Impact Metamorphism) When one extraterrestrial body, such together a meteorite or comet results with the earth or if there is a very big volcanic explosion, ultrahigh pressures deserve to be generated in the influenced rock. These ultrahigh pressure can produce minerals that are just stable at really high pressure, such together the SiO2 polymorphs coesite and also stishovite. In addition they can produce textures recognized as shock lamellae in mineral grains, and such textures together shatter cones in the influenced rock.

Classification of Metamorphic Rocks

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

Texture In metamorphic rocks separation, personal, instance minerals might or might not it is in bounded by decision faces. Those that are bounded by their very own crystal encounters are termed idioblastic. Those that present none that their own crystal deals with are termed xenoblastic. From examination of metamorphic rocks, it has been discovered that metamorphic minerals have the right to be detailed in a generalised sequence, known as the crystalloblastic series, listing minerals in order of their tendency to be idioblastic. In the series, every mineral has tendency to develop idioblastic surfaces against any mineral the occurs lower 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 series can, in a rather basic way, allow us to determine the beginning of a given rock. For example a rock that shows euhedral plagioclase crystals in contact with anhedral amphibole, likely had an igneous protolith, due to the fact that a metamorphic rock with the exact same minerals would be meant to present euhedral amphibole in call with anhedral plagioclase.

Another aspect of the crystalloblastic series is that minerals high on the list often tend to kind porphyroblasts (the metamorphic indistinguishable of phenocrysts), return K-feldspar (a mineral the occurs lower in the list) might also type porphyroblasts. Porphyroblasts are regularly riddled v inclusions of other minerals that were enveloped throughout growth that the porphyroblast. These are said to have a poikioblastic texture.

Most metamorphic structure involve foliation. Foliation is normally caused by a desired orientation of paper 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 a shistose foliation, the is termed a schist. A rock that mirrors a banded texture there is no a distinctive foliation is termed a gneiss. Every one of these might be porphyroblastic (i.e. Could contain porhyroblasts).

A absent that reflects no foliation is called a hornfels if the grain size is small, and also a granulite, if the grain dimension is large and separation, personal, instance minerals have the right to be easily distinguished with a hand lens.

Protolith Protolith refers to the initial rock, before metamorphism. In low grade metamorphic rocks, original textures are regularly preserved permitting one to recognize the most likely protolith. Together the grade of metamorphism increases, initial textures are replaced with metamorphic textures and other clues, together as bulk chemical ingredient of the rock, are used to determine the protolith.

Bulk chemistry Composition The mineral assemblage that creates in a metamorphic rock is dependency on The pressure and temperature reached throughout metamorphism The ingredient of any type of fluid phase current during metamorphism, and The bulk chemical composition of the rock. similar to in igneous rocks, minerals can only kind if the important chemical ingredient are existing in the rock (i.e. The principle of silica saturation and also alumina saturation uses to metamorphic rocks together well). Based on the mineral assemblage existing in the rock one can frequently estimate the approximate mass chemical composition of the rock. Part terms that describe this general bulk chemical composition space as follows:

Pelitic. These rocks room derivatives that aluminous sedimentary rocks favor shales and also mudrocks. Because of their high concentration of alumina castle are well-known by wealth of aluminous minerals, choose clay minerals, micas, kyanite, sillimanite, andalusite, and garnet. Quartzo-Feldspathic. Rocks that originally consisted of mostly quartz and also feldspar like granitic rocks and also arkosic sandstones will also contain an abundance of quartz and also feldspar as metamorphic rocks, since these minerals room stable end a wide range of temperature and also pressure. Those that exhibit mostly quartz and feldspar with just minor quantities of aluminous minerals are termed quartzo-feldspathic. Calcareous. Calcareous rocks space calcium rich. They space usually derivatives of lead carbonate rocks, although lock contain various other minerals that an outcome from reaction of the carbonates with connected siliceous detrital minerals that were existing in the rock. At low qualities of metamorphism calcareous rocks are known by their abundance of carbonate minerals prefer calcite and dolomite. With raising grade of metamorphism these are replaced by minerals like brucite, phlogopite (Mg-rich biotite), chlorite, and tremolite. At even higher grades anhydrous minerals favor diopside, forsterite, wollastonite, grossularite, and also calcic plagioclase. Basic. Just like in igneous rocks, the general term an easy refers to short silica content. An easy metamorphic rocks are normally derivatives of an easy igneous rocks choose basalts and gabbros. Castle have wealth of Fe-Mg minerals choose biotite, chlorite, and hornblende, and calcic minerals like plagioclase and also epidote. Magnesian. Rocks that space rich in Mg with relatively less Fe, room termed magnesian. Together rocks would certainly contain Mg-rich minerals choose serpentine, brucite, talc, dolomite, and tremolite. In general, together rocks usually have actually an ultrabasic protolith, prefer peridotite, dunite, or pyroxenite. Ferriginous. Rocks that space rich in Fe with little Mg room termed ferriginous. Together rocks could be derivatives that Fe-rich cherts or ironstones. They are characterized by wealth of Fe-rich minerals choose greenalite (Fe-rich serpentine), minnesotaite (Fe-rich talc), ferroactinolite, ferrocummingtonite, hematite, and magnetite at short grades, and also ferrosilite, fayalite, ferrohedenbergite, and also almandine garnet at higher grades. Manganiferrous. Rocks the are defined by the existence of Mn-rich minerals room termed manganiferrous. They are characterized by such minerals together Stilpnomelane and also spessartine. Classification category of metamorphic rocks counts on what is visible in the rock and its degree of metamorphism. Note that category is generally loosened and valuable such that names deserve to be adjusted to define the absent in the most satisfactory means that conveys the crucial characteristics. Three kinds of criteria are generally employed. These are:

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

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

Amphibolites: these are medium to outlet grained, dark fancy rocks whose primary minerals space hornblende and plagioclase. They an outcome from metamorphism of simple igneous rocks. Foliation is extremely variable, however when existing the hatchet schist have the right to be appended come the surname (i.e. Amphibolite schist). Marbles: These space rocks composed largely of calcite, and less generally of dolomite. They result from metamorphism of limestones and dolostones. Part foliation may be present if the marble has micas. Eclogites: these are tool to rough circuit grained consisting mainly of garnet and also green clinopyroxene referred to as omphacite, that result from high great metamorphism of simple igneous rocks. Eclogites usually execute not present foliation. Quartzites: Quartz arenites and chert both are composed greatly of SiO2. Because quartz is secure over a wide range of pressures and also temperatures, metamorphism the quartz arenites and cherts will an outcome only in the recrystallization the quartz developing a difficult rock with interlocking crystals the quartz. Such a absent is called a quartzite. Serpentinites: Serpentinites are rocks that consist greatly of serpentine. These form by hydrothermal metamorphism of ultrabasic igneous rocks. Soapstones: Soapstones room rocks the contain an abundance of talc, which provides the rock a greasy feel, comparable to that of soap. Talc is one Mg-rich mineral, and also thus soapstones indigenous ultrabasic igneous protoliths, prefer peridotites, dunites, and also pyroxenites, usually by hydrothermal alteration.

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