Lecture TWO Definition, Limits and Agents of Metamorphism

1
Lecture TWO Definition, Limits and Agents of
Metamorphism
2
Metamorphism

• ?Metamorphism (Metachange, Morphform or
  character). So, metamorphism means to change form
  or character).
• It is define as a subsolidus process leading to
  change in mineralogy, structures (textures)
  and/or chemical composition of an igneous,
  sedimentary, or prior metamorphic rocks. These
  changes were made due to subjection of these
  rocks to physicochemical conditions (P, T, active
  chemical fluids) higher than those occurring in
  the zone of weathering, cementation and
  diagenesis
• ? Features of Metamorphism
• It principally formed in solid state and before
  melting,
• Metamorphism can be considered to be
  isochemical, except perhaps for removal or
  addition of volatiles (H2O, CO2),
• The process of extensive chemical changes during
  transformation is known as metasomatism.

3
Factor and limits of metamorphism

• Factor of metamorphism include three variables
• ? Temperature ? Pressure ? chemical active
  fluids
• 1- Temperatures (leads to increase in grain
  size)
• Limits of temperatures
• - Limits of Temperature
• ? lower limit (15050 C)
• ? higher limit (beginning of melting, 650-1100
  C)
• - Low limit depend on the original protolith
• ? lower T (shale, organic matters)
• ? higher T (Igneous rocks and
  carbonates)
• - Beginning of melting depend on
• ? protolith composition
• ? the presence of aqueous fluids

4
Example - At 5 kbar and presence of aqueous
fluid - granites begin to melt at 660 C
- basalts begin to melt at
800 - At 5 kbar and dry conditions
- granites begin to melt at 1000
C - basalts begin to melt at 1120
CSource of Temperature for metamorphism -
heat flowing into the base of the crust from the
mantle - heat brought into the crust by rising
magma bodies - heat generated from radioactive
decay - the effect of rapid uplift and erosion
- heat related to burial effect and geothermal
gradient Geothermal gradient (rate of
increasing temperature with depth, mean 25
C/km) - Subduction zone (10 C/km) -
Precambrian Shields (12-20 C/km) - Collisionl
orogens (25-30 C/km) - Active
arc-margin (30-35 C/km) - Extensional
orogens (40-50 C/km) - Mid-ocean ridges
( 60 C/km)
5
2- Pressures (leads to reducing grain size and
deformation) - --- Pressure is define as
force/unit area - Unit of pressure (bar, kbar), 1
bar 0.987 atmosphere 14.5 pound/inch2 -
pressures types ? confining pressure
or lithostatic pressure
(Plith) ? directive or deviatoric
pressure ? fluid pressure (Pfluid) ?
effective pressure (Pe) Pe Plith Pfluid
6
Pressures - Limits of pressure ? lower limit (a
few of bars, at Earths surface) ? Higher limits
(30-40 in the collisional orogen or up to 100
kbar in the ultrahigh pressure metamorphism) -
Source of pressure ? burial influence of an
overlying rock column ? Plate tectonic and
movement of plate segments - Geobaric gradient
(change of pressure with depth ) ? average
0.285 kbar/km or 1kbar/3km
7
Pressure and fabric changes

• ?Lithostatic pressure uniform stress
  (hydrostatic)
• ? Deviatoric stress unequal pressure in
  different directions. Deviatoric stress can be
  resolved into three mutually perpendicular stress
  (?) components
• i) ?1 is the maximum principal stress
• ii) ?2 is an intermediate principal stress
• iii) ?3 is the minimum principal stress
• In hydrostatic situations all three are equal

8
Pressure and fabric changes, Cont.

• ? Stress is an applied force acting on a rock
  (over a particular cross-sectional area)
• ? Strain is the response of the rock to an
  applied stress ( yielding or deformation)
• ? Deviatoric stress affects the textures and
  structures, but not the equilibrium mineral
  assemblage
• ? Strain energy may overcome kinetic barriers to
  reactions
• Deviatoric stresses come in three principal
  types
• Tension
• Compression
• Shear

9
Tension ?3 is negative, and the resulting
strain is extension, or pulling apart. Tension
fractures may open normal to the extension
direction and become filled with mineral
precipitates.
strain ellipsoid
original shape
s1
s3
10
Compression ?1 is dominant therefore,
folding or more homogenous flattening are caused.
s3
s1
11
Shear motion occurs along planes at an angle
to ? 1 and causing slip along parallel planes and
rotation.
s1
12

• Foliation is a common result, which allows
  us to estimate the orientation of ? 1

s1
- ?1 gt ? 2 ? 3 ? foliation and no lineation - ?
1 ? 2 gt ? s3 ? lineation and no foliation - ?
1 gt ? 2 gt ? 3 ? both foliation and lineation
13

• 3- Metamorphic fluids (leads to chemical changes)
• mostley are H2O and CO2 types
• - include ? Ascending fluids from Magma chamber
• ? Descending fluids of the meteoric water
• - Proofs of importance of fluids in metamorphism
• ? most metamorphic minerals are hydrous, so water
  should be present
• ?most of metamorphic reactions involves
  dehydration of decarbonation
• ms chl ? bt grt qtz H2O ?
• CaCO3 SiO2 ? CaSiO3 CO2 ?
• Fluids could preserved as inclusion in neoblasts
  in metamorphic rocks.