Petrology

Tulane University

Metamorphic Rocks- Classification, Field Gradients, & Facies

The mineralogical and structural adjustment of solid rocks to physical and chemical conditions that have been imposed at depths below the near surface zones of weathering and diagenesis and which differ from conditions under which the rocks in question originated.

The word "Metamorphism" comes from the Greek:  meta = change, morph = form, so metamorphism means to change form.  In geology this refers to the changes in mineral assemblage and texture that result from subjecting a rock to conditions such pressures, temperatures, and chemical environments different from those under which the rock originally formed.

• Note that Diagenesis is also a change in form that occurs in sedimentary rocks.  In geology, however, we restrict diagenetic processes to those which occur at temperatures below 200^oC and pressures below about 300 MPa (MPa stands for Mega Pascals), this is equivalent to about 3 kilobars of pressure (1kb = 100 MPa).
  
  
• Metamorphism, therefore occurs at temperatures and pressures higher than 200^oC and 300 MPa.  Rocks can be subjected to these higher temperatures and pressures as they are  buried deeper in the Earth.  Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction.
  
  
• The upper limit of metamorphism occurs at the pressure and temperature where melting of the rock in question begins.  Once melting begins, the process changes to an igneous process rather than a metamorphic process.
  
  

Grade of Metamorphism

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

• Low-grade metamorphism takes place at temperatures between about 200 to 320^oC, and relatively low pressure.  Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals.
  

• High-grade metamorphism takes place at temperatures greater than 320^oC and relatively high pressure.  As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H₂O, and non-hydrous minerals become more common.
  

Types of Metamorphism

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

Regional Metamorphism
Regional metamorphism occurs over large areas and generally does not show any relationship to igneous bodies.  Most regional metamorphism is accompanied by deformation under non-hydrostatic or differential stress conditions.  Thus, regional metamorphism usually results in forming metamorphic rocks that are strongly foliated, such as slates, schists, and gniesses.  The differential stress usually results from tectonic forces that produce compressional stresses in the rocks, such as when two continental masses collide. Thus, regionally metamorphosed rocks occur in the cores of fold/thrust mountain belts or in eroded mountain ranges.  Compressive stresses result in folding of  rock and thickening of the crust, which tends to push rocks to deeper levels where they are subjected to higher temperatures and pressures.

Cataclastic Metamorphism
Cataclastic metamorphism occurs as a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone.  Heat is generated by the friction of sliding along such a shear zone, and the rocks tend to be mechanically deformed, being crushed and pulverized, due to the shearing.  Cataclastic metamorphism is not very common and is restricted to a narrow zone along which the shearing occurred.

Hydrothermal Metamorphism
Rocks that are altered at high temperatures and moderate pressures by hydrothermal fluids are hydrothermally metamorphosed.  This is common in basaltic rocks that generally lack hydrous minerals.  The hydrothermal metamorphism results in alteration to such Mg-Fe rich hydrous minerals as talc, chlorite, serpentine, actinolite, tremolite, zeolites, and clay minerals. Rich ore deposits are often formed as a result of hydrothermal metamorphism.

Burial Metamorphism
When sedimentary rocks are buried to depths of several hundred meters, temperatures greater than 300^oC may develop in the absence of differential stress.  New minerals grow, but the rock does not appear to be metamorphosed.  The main minerals produced are often the Zeolites.  Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase.

Shock Metamorphism (Impact Metamorphism)
When an extraterrestrial body, such as a meteorite or comet impacts with the Earth or if there is a very large volcanic explosion, ultrahigh pressures can be generated in the impacted rock.  These ultrahigh pressures can produce minerals that are only stable at very high pressure, such as the SiO₂ polymorphs coesite and stishovite.  In addition they can produce textures known as shock lamellae in mineral grains, and such textures as shatter cones in the impacted rock. 

• The pressure and temperature reached during metamorphism
  
  
• The composition of any fluid phase present during metamorphism, and
  
  
• The bulk chemical composition of the rock.

• 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 over 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.  Just like in igneous rocks, the general term basic refers to low silica content.  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.
  
  
• Magnesian. Rocks that are rich in Mg with relatively less Fe, are termed magnesian.  Such rocks would contain Mg-rich minerals like serpentine, brucite, talc, dolomite, and tremolite.  In general, such rocks usually have an ultrabasic protolith, like peridotite, dunite, or pyroxenite.
  
  
• 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.
  
  
• Manganiferrous. Rocks that are characterized by the presence of Mn-rich minerals are termed manganiferrous.  They are characterized by such minerals as Stilpnomelane and spessartine.

1. Mineralogical - The most distinguishing minerals are used as a prefix to a textural term. Thus, 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 called a K-spar porphyroblastic schist.
   
   
2. Chemical - If the general chemical composition can be determined from the mineral assemblage, then a chemical name can be employed. For example a schist with a lot of quartz and feldspar and some garnet and muscovite would be called a garnet-muscovite quartzo-feldspathic schist. A schist consisting mostly of talc would be called a talc-magnesian schist.
   
3. Protolithic -  If a rock has undergone only slight metamorphism such that its original texture can still be observed then the rock is given a name based on its original name, with the prefix meta- applied. For example: metabasalt, metagraywacke, meta-andesite, metagranite.

• Amphibolites: These are medium to coarse grained, dark colored rocks whose principal minerals are hornblende and plagioclase. They result from metamorphism of basic igneous rocks.  Foliation is highly variable, but when present the term schist can be appended to the name (i.e. amphibolite schist).
  
• Marbles: These are rocks composed mostly of calcite, and less commonly of dolomite. They result from metamorphism of limestones and dolostones.  Some foliation may be present if the marble contains micas.
  
• Eclogites: These are medium to coarse grained consisting mostly of garnet and green clinopyroxene called omphacite, that result from high grade metamorphism of basic igneous rocks. Eclogites usually do not show foliation.
  
  Quartzites: Quartz arenites and chert both are composed mostly of SiO₂.  Since quartz is stable over a wide range of pressures and temperatures, metamorphism of quartz arenites and cherts will result only in the recrystallization of quartz forming a hard rock with interlocking crystals of quartz.   Such a rock is called a quartzite.
  
  Serpentinites:  Serpentinites are rocks that consist mostly of serpentine.  These form by hydrothermal metamorphism of ultrabasic igneous rocks.
  
  Soapstones: Soapstones are rocks that contain an abundance of talc, which gives the rock a greasy feel, similar to that of soap.   Talc is an Mg-rich mineral, and thus soapstones from ultrabasic igneous protoliths, like peridotites, dunites, and pyroxenites, usually by hydrothermal alteration.
  
  Skarns: Skarns are rocks that originate from contact metamorphism of limestones or dolostones, and show evidence of having exchanged constituents with the intruding magma.  Thus, skarns are generally composed of minerals like calcite and dolomite, from the original carbonate rock, but contain abundant calcium and magnesium silicate minerals like andradite, grossularite, epidote, vesuvianite, diopside, and wollastonite that form by reaction of the original carbonate minerals with silica from the magma.  The chemical exchange is that takes place   is called metasomatism.
  
• Mylonites: Mylonites are cataclastic metamorphic rocks that are produced along shear zones deep in the crust.  They are usually fine-grained, sometimes glassy, that are streaky or layered, with the layers and streaks having been drawn out by ductile shear.

Metamorphic Facies

In general, metamorphic rocks do not drastically change chemical composition during metamorphism, except in the special case where metasomatism is involved (such as in the production of skarns, as discussed above).  The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism.  Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to.  This pressure and temperature environment is referred to as Metamorphic Facies.  (This is similar to the concept of  sedimentary facies, in that a sedimentary facies is also a set of environmental conditions present during deposition).  The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism.
 

Estimating Pressure and Temperature of Metamorphism

Using combinations of reactions that have likely taken place during metamorphism, petrologists have been able over the years to determine the pressure and temperature of metamorphism in a variety of rocks, and in so doing have been able to place constraints on the fields of temperature and pressure for the various metamorphic facies.  Some of these reactions that have been determined experimentally, are shown in the diagram below with reaction boundaries superimposed over the facies diagram.

• A low geothermal gradient of around 10^o/km would cause prograde metamorphism to occur along a sequence of facies from zeolite to blueschist to eclogite.  Such a progression is termed a facies series, and in general terms this would be called a high pressure facies series, as shown in the diagram below. Such a facies series would be expected in areas near subduction zones where cool lithosphere is pushed to higher pressure.
  

• A geothermal gradient of around 30^o/km, expected in areas undergoing an orogenic event, would produce a succession of facies from zeolite to prehnite pumpellyite to greenschist to amphibolite to granulite.  Note that in pelitic rocks of this series, the Al₂SiO₅ minerals would change from kyanite to sillimanite somewhere in the amphibolite facies. This facies series is termed the Medium pressure series or Barrovian facies series.
  

Note that a slightly higher geothermal gradient would produce the same succession of facies, but pelitic rocks would show a change in the Al₂SiO₅ minerals from kyanite to andalusite to sillimanite.  This facies series is called the Low-pressure series or Buchan facies series.


• Along very high geothermal gradients, such as might be expected in the vicinity of intruding magmas the succession of facies would increase from the albite-epidote hornfels facies to the hornblende hornfels facies to pyroxene hornfels and sanidinite facies, the facies of contact metamorphism.  This facies series is called the hornfels facies series or the contact facies series.

Metamorphic Field Gradients

As rocks are pushed deeper into the Earth as a result of tectonism, they will encounter four stages during which temperature and pressure change in the rock in different ways.

• During the burial stage the effects of increasing pressure are encountered by the rock immediately as depth of burial increases.  But increasing temperature will take time, because heat has to be conducted into the rock in order to increase the temperature to that of the surroundings.  So, the rock will follow a path resembling a low geothermal gradient. 

• When the rocks are nearly buried to their maximum depth, heat conducted from below will cause an increase in temperature and the rock will undergo a heating stage at a pressure equivalent to the maximum pressure encountered by the rock. Along this path the rock will encounter the temperature and pressure of the regional geothermal gradient and will also encounter the maximum pressure of metamorphism. 

• As rocks heat up, they will decrease in density, and thus there will be an isostatic response to the heating, wherein the rocks will begin to rise resulting in uplift of the overlying rock. Uplift alone will not cause a decrease in pressure, but erosion of the uplifted region will remove material and eventually pressure will start to decrease. During this uplift stage, the rocks will continue to gain temperature because they are expanding and they are carrying heat with them.  At some point during this uplift stage, the maximum temperature will be encountered.

• The unroofing stage occurs when erosion rates become high enough that pressure starts to decrease.  As rocks move to a lower pressure environment heat will conducted into their surroundings and temperature will decrease to that present on the surface. 
  

Since chemical equilibrium will be controlled more by  temperature than by pressure, the mineral assemblages  will reflect those stable at the maximum temperature, but not necessarily the maximum pressure. Thus, the geothermal gradient deduced from a series of rocks in any area will reflect a thermal gradient somewhat higher than the true geothermal gradient.  To distinguish between the true geothermal gradient and that deduced from the facies series, the deduced geothermal gradient is called the metamorphic field gradient.