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Lecture TWO Lecture TWO Definition, Limits and Agents of Metamorphism
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Metamorphism
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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, 650-1100 °C) higher limit (beginning of melting, 650-1100 °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
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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)
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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/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
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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
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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
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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
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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 11 33
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Compression : 1 is dominant; therefore, folding or more homogenous flattening are caused. Compression : 1 is dominant; therefore, folding or more homogenous flattening are caused. 11 33
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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. 11
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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 11
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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.
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