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Metamorphic rocks

When rocks are baked by heat of molten magma or

squeezed by the movements of huge tectonic plates orby the pressure of overlying thick succession of rocks

They are altered or changed beyond their recognition

i.e. change in Chemical composition, texture and

structure

Metamorphic rocks


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Metamorphism

Is the process that occur in rocks

due to the effects of

High temperatureHigh pressure

Chemically active fluids


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The source of temperature is either from magma or due tothe depth factor

Metamorphism usually result into change in min. comp. andtexture of rocks (Ig. and Sed.) which are subjected to temp.> 1000 C and pressure > 1000s Mpa.

Low-grade metamorphism:

Occurs at about 1000 C to 5000 C.

High-grade metamorphism:

Occurs at > 5000 C

Temperature


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UNIFORM PRESSURE

Pressure

- increases with depth due to

increase in overburden.

- acts vertically downwards and

affects the volume of both liquid

& solids.

- increases with depth upto someextent, effective in the upper part

of the crust.

DIRECT or DifferentialPRESSURE

- acts in all direction and affects only

on solids resulting into deformation

of shape and change in mineral

composition

- high temperature is also

associated with (due to depth

factor)

- high temperature is not alwaysassociated. to depth factor)

- Lithostatic pressure- due to

overburden- Stress- due to tectonic forces


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Uniform Stress

Differential Stress

min

inter

max


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Granite Granite-Gneiss


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STRUCTURES IN METAMORPHIC ROCKS

Foliation: when platy, lamellar or flaky minerals (eg.sheet silicate minerals the micas: biotite and muscovite,

chlorite, talc, and serpentine), occurring in rock orient

themselves parallel to one another (i.e. perpendicular to the

direction of maximum pressure or stress).

Random

orientation

Of minerals

Preferred

orientation

Of minerals


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Lineation: when prismatic or rod-likeminerals (eg. Hornblende, tourmaline etc.)occurring in a rock orient themselves parallel to

one another (perpendicular to direction of maxi.Pressure or stress)


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SLATY CLEAVAGE

-usually formed during the early stage of

Low-grade Metamorphism due to lithostaticstress.

- New sheet-structure minerals tends to be

parallel to the bedding planes during

metamorphism.

- however, further deep burial along

the continental margin;

compressional forces will cause

deformation (folding).

- hence, the sheet minerals as well asfoliation will no longer be parallel to

the bedding planes, such type of

foliation in fine grained rocks is

called slaty cleavage.


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Shale Slate


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PHYLLITES

- usually associated with intermediate grade of metamorphism; where themineral grains grows large in size as compare to that seen in slates

-This develops a pronounced foliation where the preferred oriented

minerals are seen.


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SCHISTOSE STRUCTURE

- usually formed during intermediate and high grade metamorphism

- Grain size increases and can be seen by naked eye; grains tends to enlarge

with increasing grade of metamorphism; the coarse grained sheet-structure

minerals show preferred orientation

- grain size is the main difference between the slaty structure and schistos

structure.


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GNEISSIC STRUCTURE

- usually associated with high-grade regional metamorphism (wheredifferential stress prevails I.e. tectonic forces)

- where the sheet silicates and other minerals like

quartz/feldspars/hornblende/pyroxene are segregated in distinct bands in

the rocks- known as gneissic banding.


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Classification of Metamorphic rocks based on

texture/structures

PHYLLITE

-similar to slate, but slightly coarser phyllosilicate grains-grains can be seen in hand specimen, giving silk appearance to cleavage

surfaces

-often cleavage planes less perfectly planar than slates

SLATE-strongly cleaved rock

-cleavage planes are developed due to orientation of fine phyllosilcate grains

eg. Muscovite, biotite, chlorite etc.

-individual grains too fine to be visible with naked eye

-overall dull appearance


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SCHIST-parallel alignment of moderately coarse grains (fabric=schistocity)

-grains are visible by eye

-mainly phyllosilicates and other minerals such as hornblende, kyanite etc.

GNEISS

-coarse grained rock (grain size several millimetres) and-foliated (planar fabric: either schistosity or compositional layering)

-tendency for different minerals to segregate into layers parallel to foliation

(gneissic layering): typically quartz and feldspar rich layers tend to separate

from micaceous layers.

Varieties:

--Orthogneiss: rocks formed from Igneous rocks

-- paragneiss: rocks formed from Sedimentary rocks -metasedimentary

gneisses


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QUARTIZITE

-it comprise equidimensional minerals viz. quartz and feldsparsNon foliated; show GRANULOSE STRUCTURE


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Type of Metamorphism

Cataclastic Metamorphism This type of metamorphism occurs mainly due to direct

pressure

eg. when two bodies of rock slide past one another along a faultzone. Heat is generated by the friction of sliding along the

zone, and the rocks tend to crushed and pulverized due to thesliding.

Cataclastic metamorphism is mere mechanical breakdown ofrocks without any new mineral formation, however, sometime

due to intense shearing few new minerals are formed.


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Contact Metamorphism- This type of metamorphism occurs locally adjacent to the igneous intrusion;

with high temp. and low stress

There is little change in bulk composition of the rock

Area surrounding the intrusion (Batholith) is heated by the magma;

metamorphism is restricted to a zone surrounding the intrusion, this zone is

know as METAMORPHIC AUREOLE.

The rocks formed are non-foliated fine-grained rocks called as

HORNFELS.


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Regional Metamorphism- metamorphism occurs covering larger area, which is subjected to intense

deformation under direct or differential stress.

Rocks formed under such environment are usually strongly foliated, suchas slates, schists, and gniesses.

The differential stresses result from tectonic forces,

eg. when two continental masses collide with one another resulting into

mountain building activity. Compressive stresses result in folding of the

rock


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Types of Metamorphic Rocks

FOLIATEDThe common foliated rocks in the order of increasing grain size are

SLATEPHYLLITESCHISTGNEISS

NON-FOLIATED

Quartzites and hornfels


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Importance of Metamorphic rocks- SLATES

Fine grained impermeable, cleavable and soft

Incompetent; cannot withstand great loads

But since they are impermeable and split easily; thin large sized slabs of

uniform thickness can be extracted for roofing purpose.

Economic importance: Since they are bad conductor of electricityused in

electrical industries for switch board base

GNEISS

Gneissic rocks are rich in SILICA i.e. predominantly Quartz and Feldspars

along with garnet, pyroxene, Hornblende etc.

Non-porous and impermeable nature increases the strength of the rock

Foliated character to some extend improves workability

Load perpendicular to foliated planes gives more stronger foundation


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If mineral assemblage is more or less similar to

Granite (with less % mafic minerals) then:

It is used as building stone

As aggregate for making concreteAs road metals etc.

SCHIST

Mainly composed of prismatic or platy minerals, which contributes indevelopment of Schistose Structure. Eg. Hornblende, tourmaline, sillimanite etc

(prismatic); chlorite, muscovite, biotite, talc, kyanite etc. (platy)

Cleavable nature of Schists is the main reason for their weakness; they are

incompetent


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QUARTZITE

SANDSTONE (composed of quartz/feldspars/feldspathoid minerals) when

under go metamorphism result into Quartzite.

Granulose texture/structure (Granoblastic) makes them most competent rock

amongst all other metamorphic rocks.

Because metamorphism of Sst. Result disappearance of cementing material,

bedding planes, fossil content etc.

Quartzites are compact, hard and strong; very less porous and less permeablethan the parent Sst.

Predominance of Quartz makes the rock very hard and suitable for road

metal; can be used as concrete aggregate etc.

Acts as strong foundation for any CE structure.


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MARBLE

Latin wordMarmorShining stone.

Calcareous metamorphic rock Though it shows granulose structure it is not as hard as Quartzite because of

its Calcareous composition; but can withstand reasonable load.

Due to its pleasant colour and brilliant appearance when polished it is

extensively used as building stone.

Calcite

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