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Term Paper on Metamorphic Rocks
Term Paper # 1. Definition of Metamorphic Rocks:
Metamorphic rocks are defined as those rocks which have formed through the operation of various types of metamorphic processes on the pre-existing igneous and sedimentary rocks involving changes in textures, structures and mineralogical compositions. The direction of change depends upon the type of the original rock and the type of metamorphic process that operates on the rock.
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Heat, pressure and chemically active fluids are the main agents involved in metamorphic processes. Plastic deformation, recrystallisation of mineral constituents and development of parallel orientation are typical characters of metamorphic rocks.
They are megascopically distinguished from other types of rocks by the development of features like cleavage, foliation, schistosity and granulation and by the presence of such minerals that are known to be end products of metamorphic recrystallisation.
Term Paper # 2. Mineralogical Composition:
Metamorphic rocks exhibit a great variation in their mineralogical composition that depends in most cases on:
(i) The composition of the parent rock;
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(ii) The type and degree of metamorphism undergone by the rock.
Many metamorphic rocks simply consist of reconstituted minerals of the parent rock. For instance marble, a metamorphic rock is made up of recrystallised calcite, (CaCO3) from sedimentary rock, limestone. Similarly, sandstone, another sedimentary rock made up mostly of quartz, (SiO2), may get metamorphosed to a rock quartzite made up mostly of recrystallised silica.
The hosts of new minerals formed due to metamorphism are essentially stable under the changed conditions of temperature, pressure and chemical environment.
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Two broad groups of minerals formed during metamorphism are:
i. Stress minerals and
ii. Anti-stress minerals.
i. Stress Minerals:
Those minerals, which are produced in the metamorphic rocks chiefly under the stress factor, are known as stress minerals. They are characterised by flaky, platy, lamellar, flattened and elongated forms. Examples of stress minerals are kyanite, staurolite, muscovite, chlorite and some amphiboles. These minerals are formed mostly under conditions of dynamothermal metamorphism.
ii. Anti-Stress Minerals:
These are metamorphic minerals produced primarily under the influence of temperature factor. Such minerals are generally of a regular equidimensional outline. Examples are sillimanite, olivine, cordierite and many pyroxenes. These minerals are quite unstable under high stress conditions. Plutonic metamorphism favours development of these minerals in metamorphic rocks.
Term Paper # 3. Textures of Metamorphic Rocks:
These can be broadly grouped under two headings:
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(a) Crystalloblastic Textures which include all those textures that have been newly imposed upon the rock during the process of metamorphism and are, therefore, essentially the product of metamorphism.
(b) Palimpsest (Relict) Textures that include textures which were present in the parent rock and have been retained by the rock despite metamorphic changes in other aspects.
Among the crystalloblastic textures, Porphyroblastic and Granoblastic types are most common. In the first case, the fine-grained ground mass of the metamorphic rock shows in it idioblastic crystals (that is, crystals with perfect outlines) of stronger minerals. In the granoblastic texture, the rock is made of equidimensional recrystallised minerals without there being any fine grained ground mass.
Palimpsest textures are similar in essential details as in the parent rock with little or no modifications taking place during metamorphism. These are described by using the term blasto— as a prefix to the name of the original texture retained by the rock. Thus, the porphyritic texture retained by an igneous rock even after having undergone metamorphism may be named as blastoporphyritic.
Term Paper # 4. Structure of Metamorphic Rocks:
Some of very common structures found in metamorphic rocks are discussed briefly as follows:
(i) Cataclastic Structure:
It is characterised by the development of extremely fine rock mass under the influence of severe crushing and shearing effects of stresses operating during metamorphism. The effect may be observed in rocks spread over wide areas. Rocks like crush breccia, mylonite and slate show this type of structure.
(ii) Schistose Structure:
The rock with schistose structure is made up of broadly parallel or sub- parallel layers or bands of flaky, platy or rod-like minerals making it very weak in the direction of parallelism. Rocks named as Schists typically show this structure
(iii) Gneissose Structure:
In this structure, bands or folia of platy and flaky minerals alternate with those of equidimensional and granular minerals. These bands are generally of contrasting colours, composition and textures. The gneissose texture is indicative of an advance degree of metamorphism under combined action of high temperature, high pressure and active participation of fluids.
(iv) Maculose Structure:
It is characterised by a spotted appearance of the rock that may be caused due to the formation of large-sized crystals (called the porphyroblasts) within an otherwise fine grained rock. This structure is typical product of thermal (contact) metamorphism and results generally due to incomplete recrystallisation of the constituent minerals.
Augen Structure is broadly similar to maculose type and is typical of metamorphic rocks in which the effects of crushing under pressure are clearly associated with recrystallisation. In this structure, some resistant minerals get deformed into thin, lenticular, lense like shapes giving a characteristic appearance to the rock.
(v) Granulose Structure:
It is a typical structure of metamorphic rocks like marble and quartzite and is characterised by an essentially granular character of the constituent minerals. The individual grains may be irregular in outline but are interlocking. Foliation is absent or negligible. Rocks with granular structure are termed as granulites.
Some of the above structures are developed on such a large scale in the metamorphic rocks that these become easily the most important distinguishing features of those rocks. These large-scale structural features are then specially referred as rock cleavage, schistosity and foliation. Each term deserves a clarifying statement.
Rock Cleavage:
It is an important and typical structural feature of metamorphic rocks and may be defined as the capacity of the rock to split along certain directions yielding parallel or sub-parallel smooth surfaces.
Two types of rock cleavage are distinguished theoretically:
(a) Flow Cleavage:
It is also called slaty cleavage because of its development in slates. This cleavage is developed during metamorphism by parallel arrangement of highly cleavable flaky and platy minerals like mica, talc and chlorite which make the bulk of the original shales or clays.
Sometimes this gives the appearance of bedding planes in the rock but actually in most cases it is developed cutting across the bedding planes. Traces of bedding planes may be found as relict structures in some slates. The slaty cleavage is the result of severe type of dynamic metamorphism of clays and shales.
(b) Fracture Cleavage:
This type of cleavage is developed due to parallel orientation of very fine fractures, often called micro-fractures that may be developed in minerals of a rock undergoing dynamic metamorphism.
It is often difficult to distinguish between two types of cleavage in hand specimen. The cleavage structure is of great importance in field mapping of metamorphic rocks as it provides very useful information about structural feature like folding.
Schistosity:
It is a structural feature of many metamorphic rocks called Schists and signifies that the rock is made up of parallel or sub-parallel bands or layers of platy and flaky minerals like mica, chlorite, tourmaline etc. Within these bands may be found enclosed grains or crystals of some other minerals like garnets and hornblende.
Schistosity results due to combined effect of heat and pressure during metamorphism involving both recrystallisation and reorientation of platy minerals as irregular layers or bands. In other words this structure is the product of combined action of thermal and dynamic metamorphism on a variety of rocks such as shales, rhyolites, granites and basalts.
Foliation:
It is a broader term explaining layered structure in metamorphic rocks and is often understood to include cleavage and schistosity. However, foliation in particular expresses segregation of minerals in an alternating manner- the granular minerals make one layer followed by another layer of flaky or platy minerals both downwards and upwards and the structure is repeated in three dimensions in the entire rock mass. It may be regarded as an advance stage of dynamothermal metamorphism in which the rock is further segregated into bands of alternating mineralogical composition.
Foliation generally results due to dynamothermal metamorphism in coarse-grained igneous rocks like granites and sandstones and metamorphic rocks like schists.
Term Paper # 5. Classification of Metamorphic Rocks:
Metamorphic rocks have been variously classified on the basis of texture and structure, degree of metamorphism, mineralogical composition and mode of origin etc.
A very general two-fold classification based on the presence or absence of layered structure or foliation as defined above is as follows:
(a) Foliated Rocks:
All metamorphic rocks showing development of conspicuous parallelism in their mineralogical and structural constitution falling under the general term foliation are grouped together as foliated rocks. The parallelism indicating features include slaty cleavage, schistosity and gneissose structures.
Typical rocks included in this group are slates, phyllites, schists and gneisses of great variety.
(b) Non-Foliated Rocks:
Included in this group are all those metamorphic rocks characterised with total or nearly total absence of foliation or parallelism of mineralogical constituents.
Typical examples of non-foliated rocks are quartzites, hornfels, marbles, amphibolites and soapstone etc.
Term Paper # 6. Rocks also Considered Metamorphic:
The following three rocks are generally classed as having doubtful origin. In many cases they show typically metamorphic features without there being sufficient evidence as to cause of the development of those features and also about their parent rocks.
(1) Migmatites:
These are essentially mixed or hybrid types of rocks that consist of intimately associated members of igneous (granitic rocks) and metamorphic (gneisses and schists) groups.
Two chief types of migmatites commonly recognized are:
(i) Border Zone Migmatites:
These occur along the contacts of massive igneous rocks and the country rocks. At these locations, migmatites form a sort of transition zone that separates the country rock from the congealed (cooled down) magmatic intrusion. It has characters of both the igneous rock and the country rock.
(ii) Regional Migmatites:
These are of widespread occurrence. A streaky or banded appearance is a typical structure of all the migmatites. The bands may be made of alternating igneous and metamorphic rocks. As regards their origin, there is as yet no close agreement among the petrologists.
The association of the granitic rocks inter-banded or intermingled with metamorphic rocks is explained in different ways, such as:
(a) Granites being the original rocks that have formed from magma got injected in the country rock which was metamorphosed in the process;
(b) The original country rock was of metamorphic type and granites were formed from it by a process of replacement or recrystallisation;
(c) The granitic magma has been produced locally by selective melting and then getting crystallized again.
It is possible that one of these modes of origin may be applicable in a particular migmatite mass under study.
(2) Eclogites:
These (metamorphic) rocks are characterized by distinct mineral assemblages consisting of:
(i) Omphacite – a green pyroxene, high in sodium and aluminium;
(ii) Pyrope – a brilliant reddish brown garnet.
Besides these two essential eclogite minerals, the rock may also contain diopside, enstatite, kyanite, and olivine and occasionally diamond.
An eclogite has a high density – 3.3 to 4.2 gm/cm2. Eclogites are rare in occurrence. They occur intercalated and associated with other metamorphic and igneous rocks.
Origin of eclogites is still an interesting problem for petrologists.
Many radically different modes of origin have been suggested for them; such as:
(i) These are considered igneous rocks crystallized from a different and distinct type of magma under conditions of high confining pressure.
(ii) These may be types of metamorphic rocks that have been subjected to partial melting and recrystallisation under different situations.
(3) Charnockites:
These are hypersthene bearing granitic rocks that are garnetiferous. T.H. Holland first used the term for hypersthene granite used as a tombstone of Job Charnock and later it was extended to a series of genetically related rocks occurring in Tamil Nadu (The Charnockite Series). The series shows a variation from acidic to ultramafic range.
The origin of charnockite is also a debatable topic. The rock shows granoblastic texture and quite a variable mineralogical composition. There is a broad consensus that charnockites are actually igneous rocks rich in hypersthene, which have been emplaced in sheets of variable thickness in folded basement rocks.