Transcript Lecture notes on Metamorphic Petrology

Lecture TWO
Definition, Limits and
Agents of Metamorphism
Metamorphism
Metamorphism (Meta=change, Morph=form 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
transformation is known as metasomatism.
changes
during
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 (150±50 °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
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 °C
Source 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
- Precambrian Shields
- Collisionl orogens
- Active arc-margin
- Extensional orogens
- Mid-ocean ridges
(10 °C/km)
(12-20 °C/km)
(25-30 °C/km)
(30-35 °C/km)
(40-50 °C/km)
(~ 60 °C/km)
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
Pressures:
- Limits of pressure
 lower limit (a few of bars, at Earth’s 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
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
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
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.
original shape
1
3
strain
ellipsoid
Compression: 1 is dominant; therefore, folding or more
homogenous flattening are caused.
3
1
Shear motion occurs along planes at an angle to
slip along parallel planes and rotation.
1
 1 and causing
Foliation is a common result, which allows us to estimate the
orientation of  1
1
- 1 >  2 =  3  foliation and no lineation
-  1 =  2 >  s3  lineation and no foliation
-  1 >  2 >  3  both foliation and lineation
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.