The texture produced when minerals within a metamorphic rock are layered parallel to each other

Abstract

Metamorphic rocks are the product of transformation or solid-state recrystallization of existing [protolith] igneous, sedimentary, and metamorphic rocks, by the change in physical and chemical conditions, principally temperature, pressure, and introduction of chemically active fluids. Metamorphism alters the mineral composition including the formation of new minerals [garnet, zoisite, kyanite, chlorite, biotite, sericite, staurolite, sillimanite, talc, and andalusite]. The sources of temperature are geothermic gradient, effect of magmatic body, and friction in rock masses of tectonic movements following prograde or retrograde mechanism. The pressure is caused by the weight of sediments or crust. Common textures are crystalline, granular, xenoblasts, idioblasts, granoblastic, and porphyrpblastic. Structures include gneissic, schistose, and slaty. Types of metamorphism include dynamic/kinetic, contact, regional, and plutonic. Dynamic metamorphism is due to mechanical deformations during tectonic movements forming mylonites, flazer, and augen gneisses. Contact metamorphism by the thermal effect of magma/lava generates skarns. Regional metamorphism, under extreme high temperature and uniform pressure over large areas of continental crust forms high-grade metamorphic rocks like slate, phyllite, amphibolites, varieties of schists, para- and ortho-gneisses, quartzite, and marble. Plutonic metamorphism occurs at high temperatures and strong pressure in deeper parts of the lithosphere producing granulites, eclogites, and migmatites. Metamorphic rocks are exceptionally appreciated as decorative and building stone due to crystalline texture, layering, brilliant colors, and excellent polishing capabilities.

Gem Minerals From Metamorphic Rocks

Metamorphic rocks are igneous, sedimentary, or metamorphic rocks that have been changed or altered in response to deep burial, intense heat and pressure without melting the rock or interaction with hot fluids. Common metamorphic rocks that may host gem minerals include schist, gneiss and marble. The type of gem mineral occurring in metamorphic rocks depends on the conditions of metamorphism and the composition of the original rock prior to metamorphism. Typical gem minerals of metamorphic origin include almandine and pyrope garnet, andalusite, kyanite, and iolite, the gem variety of cordierite.

Gem corundum can form naturally by the metamorphism of alumina-rich rocks under high temperature conditions. Corundum of metamorphic origin are formed in marble [e.g., the ruby deposits of Mogok, Myanmar], or by fluid-rock interaction [metasomatism] developed at the contact of desilicated pegmatites that have intruded mafic and ultramafic rocks [e.g., the sapphire deposits of Kashmir, India], skarn deposits, and shear zone–related deposits, represent metamorphic rocks that host gem corundum. Gem-quality spinel occurs in marbles and calc-silicate rocks that have undergone high-grade [high temperature] metamorphism and in skarns formed at the contact zones between calcium-rich rocks and magmatic intrusions.

Metamorphic environments characterized by high temperature and high pressure can produce jade or in rare cases diamonds. Most jadeite and nephrite jade deposits owe their origins to fluid interaction with their host serpentinite, a metamorphic rock mostly composed of serpentine group minerals [e.g., antigorite, lizardite, and chrysotile] produced by the hydrous alteration of ultramafic rocks. Both types of jade originate at convergent-margin tectonic settings either by the replacement of serpentinite at low-pressure and low- to moderate-temperature conditions or by the metasomatism of peridotite at high-pressure and low-temperature conditions. In some cases, nephrite jade may also form from the metasomatic replacement of dolomite by silica-rich aqueous fluids during the emplacement of felsic plutons [Harlow and Sorensen, 2005].

1.1.1.3 Metamorphic rocks

Metamorphic rocks are formed when rocks [sedimentary, igneous, and metamorphic] are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors deep within the earth or where tectonic plates meet. Metamorphism is the process of heating of the rocks at favorable temperature, pressure, and chemistry conditions without melting the rocks, but transforms them into denser, more compact rocks. During metamorphism, new minerals, textures, and structures like folds and foliations are formed either by rearrangement of mineral components or by reactions with fluids that enter the rocks.

At favorable temperature, pressure and chemistry conditions; sediments are metamorphosed into slates, phyllite/schist, to gneiss, granulite, and finally to eclogite—a process that is exemplified by pelitic rocks. Metamorphic rocks can also be classified based on their facies: zeolite—[Prehnite–pumpellyite/greenschist/blue schist]—amphibolites—gneiss—granulite—eclogite; and depending on the state of temperature and pressure, hornfels, sanidinite, and skarns can be formed [Fig. 1.6].

If metamorphic rocks are buried much deeper in the crust [Fig. 1.6], they melt and form igneous rocks. Igneous and metamorphic rocks, when exposed to erosion, form sediments that when deposited in basins followed by lithification, form sedimentary rocks again—hence the rock cycle as was indicated in Fig. 1.2.

Metamorphic Rocks, Ophiolite and Arc Volcanics 136–141°E

Metamorphic rocks [Derewo Metamorphics] extend for 550 km along the northern front of the central range. They are faulted against Mesozoic sediments of the Papuan Basin. The sediments are folded and weakly metamorphosed close to the contact. The rocks of the Derewo Metamorphics are mostly greenschist facies phyllite and schist, but on the north side include a discrete mass of higher grade metamorphics [amphibolite, granulite, blueschist, eclogite]. The higher-grade rocks are close to the contact with a major ophiolite, and are probably part of its metamorphic aureole. Ages determined for the metamorphic rocks indicate that metamorphic events [emplacement of ophiolite?] occurred in the Late Cretaceous [68 Ma] and Eocene [44 Ma].

The ophiolite comprises ultramafic and gabbroic rocks and is exposed for a strike length of 440 km along the northern flank of the main range. Other discrete bodies of ophiolite [April ultramafics] occur on the PNG side of the border within the Sepik Complex at 142-143°E [Fig. 3]. Along the north coast, on the Indonesian side of the border, there are other discrete bodies of ophiolite and related metamorphic rocks between 138 and 142°E.

How to Classify and Name a Metamorphic Rock

Metamorphic rocks originate, if not already of metamorphic origin, from sedimentary and igneous rocks. Therefore, their bulk chemical composition is extremely variable, and because they can have formed under pressures and temperatures ranging between those existing some kilometres beneath the Earth's surface and those existing when rocks start to melt, under the influence of varying fluid compositions or metasomatic processes, the mineralogical composition is far more variable than that of igneous or sedimentary rocks. This contrasts with the small number of well-known and most frequently used names for metamorphic rocks.

The definitions of these names unfortunately are not based on the same features or properties. Some definitions use purely mineralogical, others structural or a combination of structural and mineralogical properties, others chemical parameters or genetic considerations, or otherwise [Tables 1a, 1b, and 2]. This means that only some of the terms can be used to derive a coherent classification system encompassing all the possible varieties of metamorphic rock types, because such a system requires that each subdivision in that system use only one criterion for each subdivision. If it is not built up in this manner, it will contain many gaps, and will not be able to offer names for rocks falling within such gaps.

Table 1a. Definitions of the main specific rock names which have to be given preference over the equivalent structural root names schist, gneiss, or granofels

Table 1b. List of minor specific rock names names which need not be given preference over the equivalent structural root names schist, gneiss, or granofels, because they only poorly define the modal composition of a rock and should therefore only be applied when it is appropriate to do so. The list only presents some examples of minor specific rock names and is not complete

Table 2. Definitions of the three structural root names, and of the structural terms used in these definitions

Abstract

Metamorphic rocks—rocks that undergo transformations in response to changes in pressure [P] and temperature [T]—are a primary source of information for investigating tectonic processes. In their major and accessory mineral assemblages, compositions and microstructures, they provide a temporal record of metamorphism that informs us about our planet's dynamic history. On the contemporary Earth, distinct plate tectonic settings are characterized by differences in heat flow that are encoded in metamorphic rocks as differences in thermobaric ratio [T/P]. Using a dataset of more than 550 metamorphic localities around the world ranging in age from the Eoarchean to Cenozoic eras, we assess the P, T, T/P and age of metamorphism for evidence of secular change. Based on thermobaric ratios, metamorphic rocks may be classified into three natural groups: high T/P, intermediate T/P and low T/P metamorphism. Plots of T, P and, most informatively, T/P against age, and the age distribution of metamorphism, show a cyclicity in Earth's geodynamics since 3 Ga that is related to the assembly and breakup of supercratons/supercontinents. The thermobaric ratios of high and intermediate T/P metamorphism before c. 2.5 Ga can be described as a single population, which suggests a transition from sporadic unstable subduction to widespread stable subduction as a stagnant lid regime transformed to a global plate tectonics regime. Since the Neoarchean, the distribution of thermobaric ratios of metamorphism has gradually become wider and more distinctly bimodal, consistent with the evolution of plate tectonics due to secular cooling of the mantle. Any interpretation of the geodynamic regime operating on the early Earth [older than 3 Ga] based on the record of metamorphism requires more data than are currently available.

Metamorphic Rocks and Metamorphism

Metamorphic rocks are defined as rocks derived from a primary rock [protolith] by chemical and physical processes that change the minerals and the structure of rock. The protolith can be of sedimentary, metamorphic or igneous origin. The type, modal abundance and composition of the minerals may change. The processes typically involve a low-density fluid phase but no silicate melt. The collective name for all processes is metamorphism. The similar processes of sediment diagenesis are normally separated from metamorphism by an artificial boundary at 200 °C. Rock deformation such as brittle fracturing or ductile folding is typically associated with metamorphism.

Fig. 1 shows a slab of a representative metamorphic rock. It would be named garnet-hornblende schist or gneiss. It has a fine-grained greenish matrix consisting of chlorite, white mica, amphibole, plagioclase and quartz. The texture suggests that coarse garnet and hornblende formed from matrix minerals notably from the chlorite. The green matrix has been discolored near Grt and Hbl because they grow on the expense of chlorite. Consequently the “food” for the Grt and Hbl producing reactions must have been transported across the discolored white zone increasing in thickness with the progress of the reaction. The reaction has apparently been “frozen in,” which means that it did not run to completion because some regions of the rock still contain the green chlorite. The structure of the Grt-Hbl schist shows that metamorphism produces new minerals from old minerals. The structure shown in Fig. 1 suggests that the transformation processes may slow down and stop because the forces or imbalances driving the reactions fade away. If the reactions would have continued, all chlorite would have disappeared and a homogeneously coarse-grained rock with the minerals Grt, Hbl, Pl and Qz would have resulted. This group of minerals is the expected solid phase assemblage corresponding to the suggested equilibrium assemblage of the rock. However, the Grt-Hbl schist is a typical metamorphic rock with more than one successive assemblage preserved, with stalled reactions linking the assemblages, disequilibrium textures, transport limited reaction progress and preserved general chemical and mechanical imbalances. Fortunately the common metamorphic rock is thus far more interesting than the rare rocks that may have reached complete mechanical and chemical equilibrium.

Chemical reactions in rocks follow the rules of chemical thermodynamics. The Gibbs free energy is a thermodynamic potential and a function that depends on temperature and pressure in closed systems. The rules of thermodynamics request that any rock arranges itself into a group of minerals that represent the lowest possible Gibbs free energy. Chemical reactions in rocks are driven by the effort to reach a state of minimum Gibbs free energy. Therefore, changing T and P acting on a volume of rock necessarily requires an adjustment of the rock to the new T-P conditions. T changes from adding or removing thermal energy from the volume of rock by variations in heat flow. Changes of isotropic P result from vertical movements of the volume of rock in the rock column. If the rock exchanges material with its surroundings [open systems] or if anisotropic pressure is acting on the rock additional imbalances may alter the mineral assemblage and structure of metamorphic rocks. In the following sections, various causes of metamorphism are further illustrated using specific field examples.

Metamorphic Rocks

Metamorphic rocks are formed from magmatic and sedimentary rocks by means of their profound alteration and transformation under the influence of high temperature, pressure, hot solutions, and gaseous components. This involves a complex process of recrystallization of minerals and rocks, changes of chemical composition, breakdown of old structures and formation of new ones, etc. Metamorphic rocks themselves can also be recrystallized anew if they meet with the corresponding thermodynamic conditions. One of the characteristic features of such rocks is their schistose structure, with pronounced parallel arrangement of components.

The following seven basic groups of rocks are singled out: 1] phyllites; 2] schists; 3] gneisses; 4] amphibolites and amphibolitic rocks; 5] marbles; 6] quartzes; and 7] other massive metamorphic rocks.

Phyllites are rocks of low crystallinity and well expressed schistose texture formed by metamorphosis of claystones. The leading mineral of these rocks is sericite, followed by quartz. The rocks are very subject to mechanical disintegration and pass over easily into a friable mass.

The schist group includes rocks of medium to high crystallinity with well expressed schistosity that is predominantly formed by metamorphosis of claystones. They are named for the mineral or minerals dominant in them: mica schists [if mica is dominant], muscovite schists, and chlorite schists, talc schists, etc. They readily undergo mechanical disintegration.

Gneisses are highly metamorphosed medium – to coarse – grained rocks composed mainly of quartz, alkali feldspar, and mica. Mica, amphibolic, pyrone, and other gneisses are distinguished, depending on the colored mineral present in them. Weathering of gneisses occurs in the same way as in granites, but due to facilitated circulation along the surface of their schistosity, the process is accelerated in relation to granitic rocks.

Amphibolites are rocks of high crystallinity built mainly of amphibole and plagioclase with or without quartz. Although they are massive in texture, there also exist partially schistose rocks of this group.

Marbles are rocks of massive structure formed by metamorphosis of limestones and dolomites. They are composed of calcite, more rarely of dolomite or calcite and dolomite together. Of accessory minerals, muscovite has somewhat higher content than others. Like limestone, marble is very subject to the karst process.

Quarzites are metamorphic rocks in which more than 80% of the rock mass consists of quartz. They are formed by metamorphosis of quartz sand and sandstones.

Serpentinites are singled out in addition to many kinds of regional metamorphic rocks. They represent an expanded mass formed by metamorphic transformation [serpentinization] of peridotites. The rocks are built of the mineral serpentine and as accessory ingredients can contain chlorite, talc, chromite, and magnetite. They are green to dark green in color.

A certain association of minerals or metamorphic facies is characteristic for every condition of pressure and temperature prevailing during the metamorphic process. For example, we note the zeolite facies, the blueschist facies, the greenschist facies, the amphibolite facies, and the granulite facies, which consistently reflect the degree of regional metamorphism under conditions of pressure and temperature increase.

Any rock, metamorphic rock as well, with concentration of an element or compound perceptibly greater than the average content acts as a potential source of pollution of soil, water, flora, or fauna. Whether it will act as a source of pollution depends on whether that element or mineral is present in exceptionally great quantities and in a form that can be assimilated [by plants for instance].

Mercury can be cited as an example of variability in the distribution of elements in the lithosphere. An interesting case is that of ultramafic rocks in California whose original composition included mercury – rich sedimentary rocks. The content of mercury in soil at a certain distance from these rocks is ten times greater than mercury content in other soil. If the soil is still closer, it can contain mercury in amounts obtainable on the premises of mines in the basic rocks.

1.3 Metamorphic Rocks

“Metamorphic rocks are the result of metamorphism. Metamorphism is the solid-state conversion of igneous and sedimentary rocks under the pressure–temperature regime of the crust” [Huckenholz, 1982]. During this process, the original mineral assemblages [magmatic or sedimentary] are converted into new assemblages corresponding to the thermodynamic conditions over a geologic time.

Through the different metamorphic processes [regional metamorphism, contact metamorphism, cataclastic metamorphism, etc.], the great variety of original rocks and their composition results in a broad spectrum of metamorphic rock types. Typical members of these metamorphic rock types are phyllites, schists, gneisses, skarns, marbles, felses, quartzites, serpentinites, and amphibolites.

As a result of the metamorphic process, many rocks show a typical structure with parallel-oriented elements like mineral axes, fractures, and fissures. This results in anisotropy of certain physical properties.

1.3 Metamorphic Rocks

“Metamorphic rocks are the result of metamorphism. Metamorphism is the solid-state conversion of igneous and sedimentary rocks under the pressure–temperature regime of the crust” [Huckenholz, 1982]. During this process the original mineral assemblages [magmatic or sedimentary] are converted into new assemblages corresponding to the thermodynamic conditions over a geologic time.

Through the different metamorphic processes [regional metamorphism, contact metamorphism, cataclastic metamorphism, etc.], the great variety of original rocks and their composition result in a broad spectrum of metamorphic rock types. Typical members of these metamorphic rock types are phyllites, schists, gneisses, skarns, marbles, felses, quartzites, serpentinites, and amphibolites.

As a result of the metamorphic process, many rocks show a typical structure with parallel-oriented elements like mineral axes and/or fractures and fissures. This results in anisotropy of certain physical properties.