A rock that shows no foliation is called a hornfels if the grain size is small, and a granulite, if the grain size is large and individual minerals can be easily distinguished with a hand lens. 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. Just like in igneous rocks, minerals can only form if the necessary chemical constituents are present in the rock i.
Based on the mineral assemblage present in the rock one can often estimate the approximate bulk chemical composition 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 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.
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 rocks are calcium rich. They are usually derivatives of carbonate rocks. 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. 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. 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. 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.
Rocks that are characterized by the presence of Mn-rich minerals are termed manganiferrous. They are characterized by such minerals as Stilpnomelane and spessartine. These are: 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.
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.
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. 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. They result from metamorphism of limestones and dolostones.
Some foliation may be present if the marble contains micas. Eclogites usually do not show foliation. 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. These form by hydrothermal metamorphism of ultrabasic igneous rocks. Talc is an Mg-rich mineral, and thus soapstones from ultrabasic igneous protoliths, like peridotites, dunites, and pyroxenites, usually by hydrothermal alteration.
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. They are usually fine-grained, sometimes glassy, that are streaky or layered, with the layers and streaks having been drawn out by ductile shear. Foliated Nonfoliated textures Metamorphic Rock Textures Metamorphic rocks exhibit a variety of textures.
These can range from textures similar to the original protolith at low grades of metamorphism, to textures that are purely produced during metamorphism and leave the rock with little resemblance to the original protolith.
Most foliation is caused by the preferred orientation of phylosilicates, like clay minerals, micas, and chlorite. Preferred orientation develops as a result of non-hydrostatic or differential stress acting on the rock also called deviatoric stress. Foliation: -preferred orientation or location of minerals Stress and Preferred Orientation Pressure is defined as a force acting equally from all directions.
It is a type of stress, called hydrostatic stress or uniform stress. If the stress is not equal from all directions, then the stress is called a differential stress. Normally geologists talk about stress as compressional stress. Note that extensional stress would act along the direction of minimum principal stress. Minerals that crystallize or grow in the differential stress field may develop a preferred orientation.
Sheet silicates and minerals that have an elongated habit will grow with their sheets or direction of elongation orientated perpendicular to the direction of maximum stress. Rounded grains can become flattened in the direction of maximum compressional stress. Metamorphic petrologists and structural geologists refer to the original bedding surface as S0. The preferred orientation of these sheet silicates causes the rock to easily break planes parallel to the sheet silicates, causing a slatey cleavage. The foliation or surface produced by this deformation is referred to S1. Slate Eventually the rock develops a near planar foliation caused by the preferred orientation of sheet silicates mainly biotite and muscovite.
Quartz and feldspar grains, however show no preferred orientation. The irregular planar foliation at this stage is called schistosity These dark colored minerals tend to become segregated into distinct bands through the rock this process is called metamorphic differentiation , giving the rock a gneissic banding.
Because the dark colored minerals tend to form elongated crystals, rather than sheet- like crystals, they still have a preferred orientation with their long directions perpendicular to the maximum differential stress. The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks. Metamorphism and Deformation The result of compressional stress acting on rocks that behave in a ductile manner ductile behavior is favored by higher temperature, higher confining stress [pressure] and low strain rates is the folding of rocks. Original bedding is folded into a series of anticlines and synclines with fold axes perpendicular to the direction of maximum compressional stress.
Note that since the axial planes are oriented perpendicular to the maximum compressional stress direction, slatey cleavage or foliation should also develop along these directions. Thus, slatey cleavage or foliation is often seen to be parallel to the axial planes of folds, and is sometimes referred to axial plane cleavage or foliation.
Throughout the history of metamorphic petrology, several mechanisms have been proposed to explain metamorphic differentiation. In some rocks the compositional layering may not represent metamorphic differentiation at all, but instead could simply be the result of original bedding. For example, during the early stages of metamorphism and deformation of interbedded sandstones and shales the compositional layering could be preserved even if the maximum compressional stress direction were at an angle to the original bedding.
Here, it would be easy to determine that the compositional layers represented original bedding because the foliation would cut across the compositional layering. As shearing stretches the bedding, individual folded beds may be stretched out and broken to that the original folds are not easily seen. Original compositional layering a rock could also become transposed to a new orientation during metamorphism.
The diagram below shows how this could occur.
In the initial stages a new foliation begins to develop in the rock as a result of compressional stress at some angle to the original bedding. As the minerals that form this foliation grow, they begin to break up the original beds into small pods. As the pods are compressed and extended, partly by recrystallization, they could eventually intersect again to form new compositional bands parallel to the new foliation.
In fine grained metamorphic rocks small scale folds, called kink bands, often develop in the rock as the result of application of compressional stress. A new foliation begins to develop along the axial planes of the folds. Quartz and feldspar may dissolve as a result of pressure solution and be reprecipitated at the hinges of the folds where the pressure is lower.
As the new foliation begins to align itself perpendicular to s1, the end result would be alternating bands of micas or sheet silicates and quartz or feldspar, with layering parallel to the new foliation. Fluids present during metamorphism have the ability to dissolve minerals and transport ions from one place in the rock to another. Thus felsic minerals could be dissolved from one part of the rock and preferentially nucleate and grow in another part of the rock to produce discontinuous layers of alternating mafic and felsic compositions.
As discussed previously, migmatites are small pods and lenses that occur in high grade metamorphic terranes that may represent melts of the surrounding metamorphic rocks. Injection of the these melts into pods and layers in the rock could also produce the discontinuous banding often seen in high grade metamorphic rocks.
The process would be similar to that described in 4, above, except that it would involve partially melting the original rock to produce a felsic melt, which would then migrate and crystallize in pods and layers in the metamorphic rock. Further deformation of the rock could then stretch and fold such layers so that they may no longer by recognizable as migmatites. Progressive metamorphism of shale Stability ranges for Al2O5 minerals Index Minerals Metamorphic Grade Migmatite Schists with staurolite, biotite, muscovite, quartz, garnet, and plagioclase.
Schists and gneisses with sillimanite, biotite, muscovite, quartz, plagioclase, garnet, and perhaps staurolite.