Metamorphic Rocks

Metamorphism: Is a process involving changes in the mineral content/composition and/or microstructure of a rock, dominantly in the solid state. This process is mainly due to an adjustment of the rock to physical conditions that differ from those under which the rock originally formed and that also differ from the physical conditions normally occurring at the surface of the Earth and in the zone of diagenesis. The process may coexist with partial melting and may also involve changes in the bulk chemical composition of the rock.

Main types of metamorphism:

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Fig.1: Main classification of metamorphism from the viewpoints of extent, setting and cause.



Regional metamorphism: is a type of metamorphism which occurs over an area of wide extent, that is, affecting a large rock volume, and is associated with large-scale tectonic processes, such as ocean-floor spreading, crustal thickening related to plate collision, deep basin subsidence, etc.

Local metamorphism: is a type of metamorphism of limited areal (volume) extent in which the metamorphism may be directly attributed to a localised cause, such as a magmatic intrusion, faulting or meteorite impact.

Orogenic metamorphism: is a type of metamorphism of regional extent related to the development of orogenic belts. The metamorphism may be associated with various phases of orogenic development and involve both compressional and extensional regimes. Dynamic and thermal effects are combined in varying proportions and timescales and a wide range of P-T conditions may occur. Burial metamorphism is a type of metamorphism, mostly of regional extent, which affects rocks deeply buried under a sedimentary-volcanic pile and is typically not associated with deformation or magmatism. The resultant rocks are partially or completely recrystallised and generally lack schistosity. It commonly involves from very low to medium metamorphic temperatures and low to medium P/T ratios.

Ocean-floor metamorphism: is a type of metamorphism of regional or local extent related to the steep geothermal gradient occurring near spreading centres in oceanic environments. The recrystallisation, which is mostly incomplete, encompasses a wide range of temperatures. The metamorphism is associated with circulating hot aqueous fluids (with related metasomatism) and typically shows an increasing temperature of metamorphism with depth.

Dislocation metamorphism: is a type of metamorphism of local extent, associated with fault zones or shear zones. Grain size reduction typically occurs in the rocks, and a range of rocks commonly referred to as mylonites and cataclasites is formed.

Impact metamorphism: is a type of metamorphism of local extent caused by the passage of a shock wave due to the impact of a planetary body (projectile or impactor) on a planetary surface (target). It includes melting and vaporisation of the target rock(s).

Contact metamorphism: is a type of metamorphism of local extent that affects the country rocks around magma bodies emplaced in a variety of environments from volcanic to upper mantle depths, in both continental and oceanic settings. It is essentially caused by the heat transfer from the intruded magma body into the country rocks. The range of metamorphic temperatures may be very wide. It may or may not be accompanied by significant deformation depending upon the dynamics of the intrusion.

Pyrometamorphism: is a type of contact metamorphism characterised by very high temperatures, at very low pressures, generated by a volcanic or subvolcanic body. It is most typically developed in xenoliths enclosed in such bodies. Pyrometamorphism may be accompanied by various degrees of partial melting (to form, for example, fritted rocks, buchites).

Hydrothermal metamorphism: is a type of metamorphism of local extent caused by hot H2Orich fluids. It is typically of local extent in that it may be related to a specific setting or cause (e.g. where an igneous intrusion mobilises H2O in the surrounding rocks). However, in a setting where igneous intrusion is repetitive (e.g. in ocean-floor spreading centres) the repetitive operation of circulating hot H2O fluids may give rise to regional effects as in some cases of ocean-floor metamorphism. Metasomatism is commonly associated with this type of metamorphism.

Combustion metamorphism: is a type of metamorphism of local extent produced by the spontaneous combustion of naturally occurring substances such as bituminous rocks, coal or oil. Lightning metamorphism is a type of metamorphism of local extent that is due to a strike of lightning. The resulting rock is commonly a fulgurite, an almost entirely glassy rock.

Depending on whether metamorphism is accompanied by increasing or decreasing temperature two types can be distinguished:

Prograde (= progressive): As the pressure and temperature increase, a rock of a given chemical composition is expected to undergo a continuous series of chemical reactions between its constituent minerals and any fluid phase present to produce a series of new mineral assemblages that are stable at the higher pressures and temperatures.
Retrograde (= retrogressive): metamorphism is a metamorphism giving rise to the formation of minerals which are typical of a lower grade (i.e. lower temperature) than the former phase assemblage.
Isograd is a surface across the rock sequence, represented by a line on a map, defined by the appearance or disappearance of a mineral, a specific mineral composition or a mineral association, produced as a result of a specific reaction, for example, the ‘staurolite-in’ isograd defined by the reaction: garnet+chlorite+muscovite=staurolite+biotite+quartz+H2O. Isograds represent mineral reactions not rock chemical composition.

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Fig.2: P-T diagram showing various metamorphic facies and several geothermal gradients. 1) Geothermal gradient in contact zones. 2)-3) Average geothermal gradient of the continental crust. 4) Geothermal gradient in subduction zones.



Metamorphic facies

The concept of metamorphic facies was first proposed by Eskola (1915) who later gave the following definition: A metamorphic facies is "a group of rocks characterised by a definite set of minerals which, under the conditions obtaining during their formation, were at perfect equilibrium with each other. The quantitative and qualitative mineral composition in the rocks of a given facies varies gradually in correspondence with variation in the chemical bulk composition of the rocks".


The Subcommission proposes the following definition of facies, which follows Eskola's writings and the commentaries of other workers."A metamorphic facies is a set of metamorphic mineral assemblages, repeatedly associated in time and space and showing a regular relationship between mineral composition and bulk chemical composition, such that different metamorphic facies (sets of mineral assemblages) appear to be related to different metamorphic conditions, in particular temperature and pressure, although other variables, such as PH2O may also be important."

Eskola distinguished eight facies, namely: greenschist facies (f.), epidote-amphibolite f., amphibolite f., pyroxene-hornfels f., sanidinite f., granulite f., glaucophane-schist f. and eclogite facies. Coombs et al. (1959), building on a suggestion of Eskola's, added a zeolite facies and a prehnite-pumpellyite zone. More recently various authors have recognised distinctions in the assemblages containing prehnite and pumpellyite, and erected three facies or subfacies based on the assemblages prehnite-pumpellyite, prehnite-actinolite and pumpellyite-actinolite. These facies or subfacies, involving prehnite and pumpellyite, may be collectively referred to as the subgreenschist facies.

Facies boundaries are defined by the appearance or disappearance of a mineral or group of minerals, and not a specific P and T. The boundaries betweeen the different facies are therefore transitional in many cases, as the compositions of the minerals and/or fluids in question vary due to bulk rock chemical control (and other factors as well). Such variations in turn affect the P-T location of the boundary reactions.

Zeolite facies: Defined by the occurrence of zeolites in mafic rocks, but not identifiable in metapelites.

The Subgreenschist facies:
(a) Prehnite-pumpellyite facies: Defined by the occurrence of prehnite and pumpellyite in metabasites. Defined by the occurrence of illite, chlorite and smectite in metapelites.
(b) Pumpellyite-Actinolite facies: Defined by the occurrence of pumpellyite + actinolite in mafic rocks.
(c) Prehnite -Actinolite facies: Defined by the occurrence of prehnite + actinolite in mafic rocks.

Blueschist facies:

(a) Lawsonite-albite subfacies/Lawsonite blueschist subfacies: Defined by the occurrence of lawsonite + albite + chlorite +/- pumpellyite or actinolite or glaucophane in mafic rocks. Defined by the occurrence of carpholite + chlorite or phengite + paragonite in metapelites.
(b) Epidote-blueschist subfacies: Defined by the occurrence of glaucophane + epidotes + garnet/chlorite + phengite +/- actinolite in metabasites. Defined by the occurrence of chloritoid + paragonite + chlorite + phengite in metapelites. Characterized by the lack of biotite in metasediments and metabasites (phengite instead of Bt).

Eclogite facies: Defined by the occurrence of garnet + omphacite in mafic rocks. No albite (epidote or grossular garnet are the stable Ca Al silicates). Defined by the occurrence of talc + kyanite + phengite +/- yoderite in metapelites. Eclogites can be subdivided into three groups:

Type A: Eclogites that form in the mantle and are brought up to the surface with kimberlites in diatremes. Garnets in these eclogites are rich in pyrope.
Type B: Eclogites that form in the lower crust and are associated with gneiss terrains (granulites and high grade amphibolites). Garnets in this group are rich in grossular and almandine.
Type C: Eclogites that form at relatively low temperatures in what are now known as subduction zones. These eclogites contain almandine rich garnet and are associated with blueschists.

Greenschist facies: Defined by the occurrence of actinolite + chlorite + albite +/ epidote in mafic rocks. Defined by the occurrence of chlorite + albite + biotite + muscovite +/- andalusite/kyanite +/- chloritoid in metapelites.

Epidote amphibolite facies: Defined by the occurrence of epidote + amphibole (actinolite/hornblende) + plagioclase +/- chlorite/garnet in mafic rocks. Defined by the occurrence of biotite + garnet (almandine) + plagioclase + chlorite + muscovite +/- chloritoid in metapelites.

Amphibolite facies: Defined by the occurrence of amphibole (hornblende) + plagioclase (andesine) +/- garnet +/- epidote +/- diopside in mafic rocks. Defined by the occurrence of garnet + staurolite + muscovite + biotite +/ andalusite/kyanite/sillimanite in metapelites.

Granulite facies: Defined by the occurrence of plagioclase + hypersthene + diopside + garnet + spinel in mafic rocks. Defined by the occurrence of perthite + plagioclase + sillimanite + garnet +/- hypersthene in metapelites. cordierite + garnet are characteristic of low P granulites.

Sanidinite facies: Defined by the occurrence of plagioclase + hypersthene + augite + tridymite in mafic rocks. Defined by the occurrence of sanidine + Cordierite + spinel + hypersthene + sillimanite in metapelites.


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Facies, Protolith and Mineral assemblage for different protolith.



Protolith:

Protolith refers to the original rock, prior to metamorphism. In low grade metamorphic rocks, original textures are often preserved allowing one to determine the likely protolith. As the grade of metamorphism increases, original textures are replaced with metamorphic textures and other clues, such as bulk chemical composition of the rock, are used to determine the protolith:

Pelitic: These rocks are derivatives of aluminous sedimentary rocks like shales and mudrocks. Because of their high concentrations of alumina they are recognized by an abundance of aluminous minerals, like clay minerals, micas, kyanite, sillimanite, andalusite, and garnet.

Quartzo-Feldspathic: Rocks that originally contained mostly quartz and feldspar like granitic rocks and arkosic sandstones will also contain an abundance of quartz and feldspar as metamorphic rocks, since these minerals are stable of a wide range of temperature and pressure. Those that exhibit mostly quartz and feldspar with only minor amounts of aluminous minerals are termed quartzo-feldspathic.

Calcareous: Calcareous rocks are calcium rich. They are usually derivatives of carbonate rocks, although they contain other minerals that result from reaction of the carbonates with associated siliceous detrital minerals that were present in the rock. At low grades of metamorphism calcareous rocks are recognized by their abundance of carbonate minerals like calcite and dolomite. With increasing grade of metamorphism these are replaced by minerals like brucite, phlogopite (Mg-rich biotite), chlorite, and tremolite. At even higher grades anhydrous minerals like diopside, forsterite, wollastonite, grossularite, and calcic plagioclase.

Basic/Ultrabasic: Basic metamorphic rocks are generally derivatives of basic igneous rocks like basalts and gabbros. They have an abundance of Fe-Mg minerals like biotite, chlorite, and hornblende, as well as calcic minerals like plagioclase and epidote.

Ferriginous: Rocks that are rich in Fe with little Mg are termed ferriginous. Such rocks could be derivatives of Fe-rich cherts or ironstones. They are characterized by an abundance of Fe-rich minerals like greenalite (Fe-rich serpentine), minnesotaite (Fe-rich talc), ferroactinolite, ferrocummingtonite, hematite, and magnetite at low grades, and ferrosilite, fayalite, ferrohedenbergite, and almandine garnet at higher grades.