Lecture TWO Lecture TWO Definition, Limits and Agents of Metamorphism

Metamorphism

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)  lower limit (150±50 °C)  higher limit (beginning of melting, °C)  higher limit (beginning of melting, °C) - Low limit depend on the original protolith  lower T (shale, organic matters)  lower T (shale, organic matters)  higher T (Igneous rocks and carbonates)  higher T (Igneous rocks and carbonates) Beginning of melting depend on: - Beginning of melting depend on:  protolith composition  protolith composition  the presence of aqueous fluids  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 (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)

2- Pressures: (leads to reducing grain size and deformation) Pressure is define as force/unit area - Unit of pressure (bar, kbar), 1 bar = atmosphere = 14.5 pound/inch 2 - pressures types  confining pressure - pressures types  confining pressure or lithostatic pressure (P lith ) or lithostatic pressure (P lith )  directive or deviatoric pressure  directive or deviatoric pressure  fluid pressure (P fluid )  fluid pressure (P fluid )  effective pressure (P e ) P e = P lith – P fluid P e = P lith – P fluid

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 = 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 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. 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 strain ellipsoid 11 33

Compression :  1 is dominant; therefore, folding or more homogenous flattening are caused. Compression :  1 is dominant; therefore, folding or more homogenous flattening are caused. 11 33

Shear motion occurs along planes at an angle to  1 and causing slip along parallel planes and rotation. Shear motion occurs along planes at an angle to  1 and causing slip along parallel planes and rotation. 11

Foliation is a common result, which allows us to estimate the orientation of  1 Foliation is a common result, which allows us to estimate the orientation of  1 -  1 >  2 =  3  foliation and no lineation -  1 =  2 >  s3  lineation and no foliation -  1 >  2 >  3  both foliation and lineation 11

3- Metamorphic fluids (leads to chemical changes) mostley are H 2 O and CO 2 types - include  Ascending fluids from Magma chamber - 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 + H 2 O  ms + chl  bt + grt + qtz + H 2 O  CaCO 3 + SiO 2  CaSiO 3 + CO 2  CaCO 3 + SiO 2  CaSiO 3 + CO 2   Fluids could preserved as inclusion in neoblasts in metamorphic rocks.