METAMORPHIC METAMORPHIC ROCKSROCKS

Dr. P. Sarathbabu M.Sc. B.Ed. Ph.D.Department of GeologyAcharya Nagarjuna University

What type of rock is formed when change occurs from heat and pressure in the Earth?

Characteristics

Hardened sediment with layers (strata) of sandy or clayey stone; mostly brown to gray; may have fossils and water or wind marks

Metamorphic Rocks

• Metamorphic rocks are formed by the effect of heat and pressure on existing rocks.

• This can greatly affect the hardness, texture or layer patterns of the rocks.

Magma

metamorphicrock

forminghere

heat

Pressure from surface rocks

Metamorphic Rocks

Metamorphism

Is the process that occur in rocks due to the effects of

• High temperature• High pressure• Chemically active fluids

Uniform Stress

Differential Stress

min

inter

max

Granite Granite-Gneiss

Type of Metamorphism

• 1. Cataclastic MetamorphismThis type of metamorphism occurs mainly due to direct pressure

• Cataclastic metamorphism is mere mechanical breakdown of rocks without any new mineral formation, however, sometime due to intense shearing few new minerals are formed.

2.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

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

3.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, such as slates, schists, and gniesses.

Cataclastic ContactRegional

Types of Metamorphic RocksTypes of Metamorphic Rocks

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

SLATE – PHYLLITE – SCHIST – GNEISS

NON-FOLIATEDQuartzites and Marble

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). Foliation: planar fabric element

Random orientationOf minerals

Preferred orientationOf minerals

• Lineation: when prismatic or rod-like minerals (eg. Hornblende, tourmaline etc.) occurring in a rock orient themselves parallel to one another (perpendicular to direction of maxi. Pressure or stress)

Lineation: any linear fabric elements

SLATY CLEAVAGE- usually formed during the early stage of Low-grade Metamorphism due to lithostatic stress.

- New sheet-structure minerals tends to be parallel to the bedding planes during metamorphism.

Shale Slate

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.

GNEISSIC STRUCTURE

- usually associated with high-grade regional metamorphism (where differential 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.

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

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

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 permeable than 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.

MARBLE Latin word “Marmor”– Shining 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

Index Minerals

Metamorphic rocks1.GNEISS (Granite gneiss) Grain size : coarse grained Usual Colour : Grey Structure : Gneissose Texture : Banded texture. Composition : K-feldspar, Quartz are formed as light

colour bands, maffic minerals formed as dark bands and biotite,hornblende,pyroxene are occur in minor amounts

Occurrence: Its occurs as a product of high grade dynamothermal metamorphism and present of gneissos structure represent high grade metamorphism

GNEISS

Metamorphic rocks3. GNEISS Grain size : coarse grained Usual Colour : Grey Structure : Gneissose Texture : Banded texture. Composition : K-feldspar, Quartz are formed as light

colour bands, maffic minerals formed as dark bands and biotite,hornblende,pyroxene are occur in minor amounts

Occurrence: Its occurs as a product of medium-high grade metamorphism, gneiss are formed at low grade metamorphism (originated from regional metamorphic rocks)

Metamorphic rocks2.SCHIST (Mica Schist)

Grain size : Fine grained Usual Colour : Light white, Green Structure : Schistose, flaky Texture : fine grained with flakes Composition : Muscovite

Occurrence: Its occurs as dynamothermal metamorphism and present of muscovite indicate the low grade metamorphism

Schistose structure indicate that formed under dynamothermal metamorphism

schist

Metamorphic rocks6.SLATE Grain size : fine grained Usual Colour : Bluish black (melano) Structure : sheet like structure (laminated) Texture : fine grained with bedding planes.

Composition : Mica(muscovite,biotite), chlorite, Hornblend, qtz, feldspar

Occurrence: It’s a product of cataclastic metamorphism by directed pressure and its originated from dynamic metamorphism with the presence of hydroxyli minerals indicate low grade metamorphism.

1. Slate

Regional – composition change is gradational – texture is foliated

index mineralschloritemuscovitebiotitegarnetsillimanite

Slate

Phyllite

Schist

Gneiss

Incr

easi

ngm

etam

orph

ism

Increa

sing m

etamorp

hism

Metamorphic Rocks (7)Contact Metamorphic

Marble

Quartzite

Regional Metamorphic

Slate (L)

Phyllite (R)

Schist

GneissCataclastic Metamorphic

Mylonite

4.QUARTZITE Grain size : coarse grained Usual Colour : Brown Structure : Granulose Texture : Granoblastic.

Composition : SANDSTONE (composed of quartz/feldspars/feldspathoid minerals) when under go metamorphism result into Quartzite. Because metamorphism of Sst. Result disappearance of cementing material, bedding planes.

Occurrence: Its occurs as a product of thermal metamorphism.

Non-foliated metamorphic rocks

Quartzite

Quartzite

Metamorphic rocks5.MARBLE

Grain size : coarse grained Usual Colour : white, pink (leucocratic) Structure : Granulose Texture : Granoblastic. Composition : Calcite

Occurrence: Its occurs as a product of thermal metamorphism (low grade-high grade) of calcareous sediments.

MARBLE

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