Classification of Metamorphic rocks based on texture

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Metamorphic Rocks
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Metamorphic rocks

• 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

• 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 temperature
• High pressure
• Chemically active fluids

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Temperature

• 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. gt 1000 C and pressure gt
  1000s Mpa.
• Low-grade metamorphism
• Occurs at about 1000 C to 5000 C.
• High-grade metamorphism
• Occurs at gt 5000 C

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Pressure
DIRECT or Differential PRESSURE

• UNIFORM PRESSURE

- increases with depth upto some extent,
effective in the upper part of the crust.
- increases with depth due to increase in
overburden.
- acts in all direction and affects only on
solids resulting into deformation of shape and
change in mineral composition
- acts vertically downwards and affects the
volume of both liquid solids.
- high temperature is also associated with (due
to depth factor)
- high temperature is not always associated. to
depth factor)
- Lithostatic pressure- due to overburden
- Stress- due to tectonic forces
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min
max
inter
Differential Stress
Uniform Stress
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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).

Preferred orientation Of minerals
Random orientation Of minerals
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• 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)

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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.

• 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.

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Shale
Slate
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PHYLLITES

• usually associated with intermediate grade of
  metamorphism where the mineral 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 (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.

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Classification of Metamorphic rocks based on
texture/structures
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

• 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

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• SCHIST
• -parallel alignment of moderately coarse grains
  (fabricschistocity)
• -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 feldspars
• Non 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 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 mere mechanical
  breakdown of rocks 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, such as 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

• FOLIATED
• The common foliated rocks in the order of
  increasing grain size are
• SLATE PHYLLITE SCHIST GNEISS

• 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 electricity used 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 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

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• QUARTZITE
• SANDSTONE (composed of quartz/feldspars/feldspatho
  id 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.

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• 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