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Geology GEOLOGY Subject : GEOLOGY (For under graduate student.) Paper No. : Paper – 02 Introduction to Geology – 02 Topic No. & Title : 49 Metamorphic Rocks Academic Script INTRODUCTION It is not uncommon to find a gradual transition from typically sedimentary or igneous rocks to rocks whose mineralogy and structure can neither be ascribed to sedimentation or magmatism. Such transition clearly indicate that igneous and sedimentary rocks are subjected to physic-chemical conditions different from those under which they were originally formed. Vanhise (1904) introduced the term metamorphism. The word "Metamorphism" comes from the Greek: meta = change, morph = form, so metamorphism means to change form. Metamorphic Rocks are formed by the transformation of pre-existing rocks under the influence of high temperatures and pressures and chemically active fluids. 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 Degree Celsius 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 200oC 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 metamorphicprocess. 1 Geology AGENTS OF METAMORPHISM Metamorphism is the mineralogical, textural, structural adjustment in the solid state of the preexisting rock due to change in physical and chemical conditions. This process leads to transformation of existing rocks due to recrystallisation of preexisting rocks. This transformation occurs due to marked changes in the physical nature of the surroundings such as temperature, pressure, etc. Metamorphism is mainly due to the operation of three agents: 1. The chemical activity of the liquids and gases within the rocks 2. The temperature 3. The pressure •Liquids, particularly water on and in the earth, are often effective agents of alteration of rock material, which is dissolved and subsequently crystallizes in new mineral combinations. Various gases and vapours, especially those which escape from molten magma into surrounding rocks, often bring about important chemical changes and rearrangement of mineral matter in rocks. •Heat is an important metamorphic agency. It renders liquids and gases much more active chemically. It helps to alter the composition of many minerals and thus to bring new ones into existence. A sedimentary rock mass may be metamorphosed by heat along the contact with a magma which is intrusive into it. A rock mass may be heated not only by a hot igneous body, but also by deep burial within the generally heated crust of the earth. Some heat may also result from friction between adjacent parts of the crust moving past one another. •At any point within the crust of the earth, pressure may be resolved into two kinds: (a) Lateral pressure & (b) Downward pressure Lateral pressure is a very important agency in the metamorphism of igneous as well as sedimentary rocks. The crust of the earth is in many places subjected to tremendous stresses and compressive forces causing rocks to be bent, sheared, fractured and often locally crumpled into mountain ranges. Mineral grains or rocks fragments may thus be crushed or flattened; rearrangement of minerals may take place and the rock structure and texture may be greatly changed. Another important factor in the transformation of rocks is the downward pressure exerted upon deeply buried rocks, particularly sediments, aided by the heat of the earth's interior and often by water and other liquids. 2 Geology TYPES OF METAMORPHISM Metamorphism is broadly classified into 1. Contact or thermal & 2. Regional Contact or Thermal Metamorphism Contact or thermal metamorphism occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion. Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to the zone surrounding the intrusion, called a metamorphic or contact aureole. Outside of the contact aureole, the rocks are not affected by the intrusive event. The grade of metamorphism increases in all directions toward the intrusion. Because the temperature contrast between the surrounding rock and the intruded magma is larger at shallow levels in the crust where pressure is low, contact metamorphism is often referred to as high temperature, low pressure metamorphism. The rock produced is often a finegrained rock that shows no foliation, called a hornfels. 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. 3 Geology 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 degree Celsius 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 different kinds of metamorphism are thus due to various combinations of the four agencies: heat, directed pressure or stress, uniform pressure or hydrostatic pressure and chemically active fluids. Chemically active fluids are essential to all kinds of mineral change in metamorphic rocks. ROLE OF TEMPERATURE & PRESSURE (a) Heat Predominant- Heat is the dominating factor in the metamorphism which ensures in the proximity in igneous masses. Nevertheless, the factor of pressure comes in through the crowding aside of the country rock, its volume expansion due to heating and the downwardly-acting weight of the superincumbent mass, although pressure effects are always subordinate to those of heat. In general, the country rock is soaked with gaseous and liquid emanation form the magma, and these greatly facilitate the mineral transformations which take place. (b) Directed Pressure Predominant - Natural pressure within the crust is resolvable into 4 Geology (1) hydrostatic pressure & (2) stress, the latter operating in a definite direction. In co-operation with heat, stress is the principal factor in regional metamorphism. With little or no heat, directed pressure produces crushing and granulation through the enforced movement of rock masses upon one another. (c) Directed Pressure & Heat - The combination of directed pressure and heat, operating at some depth in the crust, is one of the most powerful in producing metamorphism. It leads to more or less complete recrystallisation of the rocks combined with the production of new structures. Under these conditions direction pressure lowers the melting points of minerals locally and temporarily and thus facilitates diffusion and recrystallisation of material. The main direction of pressure and movement is tangential to the earth's surface. (d) Uniform Pressure and Heat: This combination of factors dominates conditions of metamorphism at depth in which directed pressure diminished and finally ceases to operate because of the increasing plasticity of the rocks. Mineral transformations are carried to completion, but new structures are not greatly in evidence. Those minerals are formed which help the rock material to best adapt itself to reduced volume. TEXTURE OF METAMORPHIC ROCKS The fabric of metamorphic rocks is a result of mechanical deformation and mineralogical reconstitution of the pre-existing rocks. Texture is a term that describes the size, shape and orientation of the grains constituting a rock, as well as the relationship between these grains. Metamorphic textures can be grouped into three main groups: A -Relict textures (palimpsest textures): are textures inherited from the original rock type, and which have survived metamorphism. B - Typomorphic textures: textures characteristic of metamorphism C - Superimposed textures: textures characteristic of a post- metamorphic event, e.g. alteration, weathering, ... etc. Other smaller groups as “reaction textures”, “polydeformational textures”, etc. may also be typomorphic or replacement, but are grouped separately because they have some genetic connotation. A- Relict Textures There are several types of relict textures. Relict textures in metamorphic rocks are indicated by applying the prefix "blasto" to the original textural name. Relict textures 5 Geology are best preserved in low-grade rocks. Examples of such textures include: Porphyritic, ophitic, intergranular, amygdaloidal, spherulitic, variolitic, pisolitic and oolitic. B- Typomorphic textures Textures characteristic of thermal metamorphism: When thermal metamorphism is not associated with any deformation, the mineral grains are randomly oriented, resulting in either granoblastic or hornfelsic textures. Please note that the granoblastic texture can also develop in regionally metamorphosed rocks. The following are some of the types of granoblastic textures: 1- Granoblastic polygonal: where the equidimensional grains may have well developed crystal faces resulting in straight grain boundaries, and where triple junctions are common. 2- Granoblastic interlobate: where the grain boundaries are somewhat irregular 3- Granoblastic amoeboid: where all the grains have irregular outlines, and all the minerals are anhedral. 4- Granoblastic decussate: where the interlocking randomly oriented crystals are somewhat elongate, prismatic or subidioblastic. Usually applied to rocks with one or two mineral species only. Triple junctions are common. d- Snowball: Similar to rotational texture, but where the inclusions define a spiral shaped trail, which may have developed from the "rolling over" of the poikiloblasts. e- Helicitic: Where the poikiloblasts overgrow the pre-existing foliation. This texture therefore indicates post-tectonic crystallization of the poikiloblasts. C- Replacement textures (superimposed in part!) Mesh texture: develops in serpentinites, where the needle shaped serpentine minerals occur in aggregates interwoven like a mesh. Hour-glass texture: Also in serpentinites, where the serpentine minerals replace the granular olivine crystals giving rise to hour-glass like appearances. Bastite texture: A third texture that occurs in serpentinites, where Opx crystals were completely replaced by aggregates of serpentine minerals retaining the prismatic shape of the original Opx. Pseudomorphic replacement textures: (i) single-crystal (ii) multicrystal (iii) multi-phase, multi-crystal D- Reaction textures Epitaxial overgrowth: Epitaxial overgrowth is characterized by optical continuity between the mineral and its overgrowth. Both the mineral and the overgrowth must 6 Geology belong to the same structural group, and may possibly be the same mineral. This type of overgrowth is controlled fully by the the matrix mineral. CLASSIFICATION OF METAMORPHIC ROCKS Classification of metamorphic rocks depends on what is visible in the rock and its degree of metamorphism. Note that classification is generally loose and practical such that names can be adapted to describe the rock in the most satisfactory way that conveys the important characteristics. Three kinds of criteria are normally employed. 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. 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. In addition to these conventions, certain non-foliated rocks with specific chemical compositions and/or mineral assemblages are given specific names. These are as follows: • 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 7 Geology 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. CONCLUSION Metamorphic rocks constitutes a very important part of the study of the different types of rocks. The study of metamorphic rocks in all its variety enables one to trace the evolution of the Earth’s crust right from its primitive stage up to the present . The chemistry of the metamorphosed sedimentary rocks reveals their parentage, the mineral assemblage of metamorphic rocks present a fossillised record of the P-T regimes that prevailed in different parts of the crust at different times in the past. 8