Metamorphicrocks 160104104711

Dr. V. R Ghodake
Sinhgad College of Engineering,
Vadgaon(Bk), Pune.
Email- vyanky.g@gmail.com
Mobile- 9764484757

They are altered or changed beyond their recognition,
i.e. change in Chemical composition, texture and
structure
When rocks are baked by heat of molten magma or
squeezed by the movements of huge tectonic plates or
by the pressure of overlying thick succession of rocks

Metamorphism
Agents of Metamorphism

The source of temperature is either from magma or due
to the depth factor
Metamorphism usually result into change in min. comp. and
texture of rocks (Ig. and Sed.) which are subjected to temp.
 Low-grade metamorphism:
Occurs within 1000 C to 5000 C.
 High-grade metamorphism:
Occurs at > 5000 C
Temperature

•It is present at great depth.
•The pressure is due to overlying rocks
•If depth increases the uniform pressure also increases
with respect to temperature.
•It is present at shallow depth.
•The pressure operates during folding movements
•If the depth increases the directed pressure decreases

UNIFORM PRESSURE
Types of Pressure
increases with depth due to
increase in overburden.
acts vertically downwards and
affects the volume of both liquid
& solids.
increases with depth up to some
extent, effective in the upper part of
the crust.
DIRECT or Differential PRESSURE
acts in all direction and affects
only on solids resulting into
deformation of shape and change
in mineral composition
High temperature is also
associated due to depth factor
high temperature is not always
associated. to (depth factor)
Lithostatic pressure- due to
overburden
Stress- due to tectonic forces

Uniform Stress
Directed Stress
min
inter
max

Effects of Metamorphism
Mineralogical: - change minerals to reflect new P-T
(equilibrium) conditions.
Recrystallization: change of existing crystal size and
shape to form layers (e.g., shale to schist), interlocking
crystals (e.g., sandstone to quartzite).
Recombination: Recombination of elements in pre-
existing minerals to form stable new ones.
Addition/subtraction of ions common when fluid is
present.

Types of Metamorphism 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.

Types of Metamorphism: Contact
Thermal, local, around intrusions. Size of aureole
depends on:
Size of intrusion
Heat (composition)
Fluid content of
magma
Fluid content of
country rock
Country rock type

Types of Metamorphism
Cataclastic Metamorphism
 This type of metamorphism occurs mainly due to direct
pressure
 e.g. when two bodies of rock slide past one another along a
fault zone. Heat is generated by the friction of sliding along
the zone, and the rocks tend to crushed and pulverized due to
the sliding.
 Cataclastic metamorphism is mechanical breakdown of rocks
without any new mineral formation, however, sometime due
to intense shearing few new minerals are formed.

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, schist's, and
gneisses.
 The differential stresses result from
tectonic forces,
 e.g. when two continental masses
collide with one another resulting
into mountain building activity.
Compressive stresses result in
folding of the rock
Regional Metamorphism

Types of Metamorphism:
Produces non-foliated, granoblastic rocks:
Hornfels (if clastic - shale)
Quartzite (if sandstone) Marble (if carbonate)

Types of Metamorphism: Regional
Associated with Mountain buildings
Also known as Dynamothermal - produces both foliated and
non-foliated metamorphic rocks.
Heat & directed pressure on rocks buried deep within the
Earth - Prograde or Retrograde.
Associated with mountain belts - affects very large areas.
Prograde metamorphic reactions liberate a fluid.
Retrograde is difficult – pore spaces decrease during
prograde so getting fluid back in is not easy.

Meteorite impact – sudden and intense deformation.

High-pressure polymorph of quartz – coesite – can form.
Impact melt can form.
Lots of rock fragmentation & mineral deformation.
Moon – no
atmosphere so
lots of meteorite
impacts (micro
and macro!).
Produces regolith,
rock flour, impact
melt, breccias.

Water in Metamorphism
Provides transport mechanism and can promote reactions.
Hydrothermal metamorphism: hot water streams
add/remove ions. May promote ore formation.

Metasomatism: addition of ions from external source.

Metamorphic aureole is greater around granitic plutons than around
gabbroic plutons, even though the magma temperature is lower.

Metamorphic Grade:
Degree of parent rock
alteration, mostly
dependent on
increasing temperature
for increasing grade
Prograde: SLATE -
phyllite-schist-gneiss-
migmatite (melting).
SLATE: oriented clay
minerals allows the
rock to be easily
cleaved.

Prograde: slate-
PHYLLITE-schist-
gneiss-migmatite
(melting).
PHYLLITE: growth of
microscopic micas to give
strong foliation – rock is
“shiny”.

Prograde: slate-phyllite-SCHIST-gneiss-migmatite.
SCHIST: strongly foliated with visible micas & feldspars.

Prograde: slate-
phyllite-schist-
GNEISS-
GNEISS: strongly banded
rock with dark bands of
micas & pyroxenes and
light bands of feldspars
and quartz.

Prograde: slate-
phyllite-schist-
gneiss-
MIGMATITE.
MIGMATITE:
strongly veined
rock as it has
partially melted.

STRUCTURES IN METAMORPHIC ROCKS
 Foliation: when platy, lamellar or flaky minerals (e.g..
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

- 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.
- However, further deep burial
along the continental margin;
compressional forces will cause
deformation (folding).
hence, the sheet minerals as
well as foliation will no longer be
parallel to the bedding planes,
such type of foliation in fine
grained rocks is called slaty
cleavage.

- 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
schistose 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 phyllosilicate grains
e.g. 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 phyllosilicate and other minerals such as hornblende, kyanite etc.
GNEISS
Coarse grained rock (grain size several millimeters) 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:
--Orthogenesis: rocks formed from Igneous rocks
-- paragneiss: rocks formed from Sedimentary rocks -metasedimentary
gneisses

-it comprise equidimensional minerals viz. quartz and feldspars
Non foliated; show GRANULOSE STRUCTURE

Types of Metamorphic Rocks
The common foliated rocks in the order of increasing grain size
are
SLATE – PHYLLITE – SCHIST – GNEISS
Quartzite's and hornfels

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 electricity–
used in electrical industries for switch board base
Slate

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
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 concrete
As road metals etc.

SCHIST
Mainly composed of prismatic or platy minerals, which contributes in
development 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
GARNET-MICA SCHIST
Schist
Biotite Schist

 The minerals that compose gneiss
are the same as granite.
 Feldspar is the most important
mineral that makes up gneiss along
with mica and quartz. Gneiss can be
formed from a sedimentary rock
such as sandstone or shale, or it can
be formed from the metamorphism
of the igneous rock granite. Gneiss
can be used by man as paving and
building stone.

Q U A R T Z I T E
 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.
 Quartzite's are compact, hard and strong; very
less porous and less permeable than the parent
sandstone.
 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 C.E. 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.
Marble

Metamorphicrocks 160104104711

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