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Lecture TWO Definition, Limits and Agents of Metamorphism

Lecture TWO Definition, Limits and Agents of Metamorphism. Metamorphism.  Metamorphism (Meta=change, Morph=form or character). So, metamorphism means to change form or character) .

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Lecture TWO Definition, Limits and Agents of Metamorphism

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  1. Lecture TWODefinition, Limits and Agents of Metamorphism

  2. 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 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 (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

  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 gradientGeothermal 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 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

  7. Pressure and fabric changes ►Lithostaticpressure = uniform stress (hydrostatic) ► Deviatoricstress = 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 ► Strainenergy 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. Shearmotion 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 >  2 =  3  foliation and no lineation -  1 =  2 >  s3  lineation and no foliation -  1 >  2 >  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.

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