Environments of Metamorphism and Associated Textures
Transcript of Environments of Metamorphism and Associated Textures
EESC 3000
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Environments of Metamorphism and Associated Textures
EESC 3000
Metamorphic Lecture 1
Metamorphism
• “Change of Form”
• Solid-state changes
– Mineral assemblages
– Mineral textures
• Due to temperature and/or pressure change
Sources of Heat for Metamorphism
• Heat from Earth’s interior
• Geothermal gradient T/Depth
– Continental 25-30°C/km
– Volcanically active areas 30-50°C/km
– Oceanic trenches 5-10°C/km
Sources of Heat for Metamorphism
• Heat from magma
• Magma chambers add heat to surrounding rock
– Gabbroic magma ~1300°C
– Granitic magma ~700°C
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Pressure Associated with Metamorphism
• Lithostatic pressure: confining pressure created by overlying material
– Equal in all directions
– Evenly compresses volume of rock
– Basalt: 3 g/cm3 (3000 kg/m3)
– Granite: 2.7 g/cm3 (2700 kg/m3)
– Lithostatic pressure at 10 km 3 kbar = 0.3 GPa
Pressure Associated with Metamorphism
• Directed pressure: pressure is imposed in a particular direction due to a regional stress field.
• Affects shape and arrangement of minerals
• Varies with tectonic environment – Compressional environments: Horz > Vert Pressure
– Extensional environments: Vert > Horz Pressure
Three Variables Control Character of Metamorphism
• Depth of Burial
• Temperature (usually a function of depth) – Dominant control on mineral stability
• Lithostatic versus Directed Pressure – Controls textures
Types of Metamorphism
• Contact Metamorphism
– Thermal variation controls processes
• Regional Metamorphism
– Orogenic Metamorphism
• Combination of temperature and directed pressure
– Burial Metamorphism
• Combination of temperature and lithostatic pressure
• Fault-Zone Metamorphism
– Directed pressure controls processes
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Contact Metamorphism
• Adjacent to plutons
• Temperature contrast between magma and host rock
• Most evident in near-surface (low P) environments
Contact Metamorphism
• Steep thermal gradient
• Aureole width depends on: – Size of the pluton
– Cooling rate
– Time since intrusion
• Rapid relative to most geological processes
Development of Granoblastic Texture
• Pressure increases solubility
• Mineral dissolves/migrates
– High P to low P
Development of Poikiloblastic Texture
• Common in contact metamorphic rocks
• Due to rapid porphyroblast growth (rapid heat increase) – Crystal envelops non-reactive or
excess minerals
• High surface area -> High Surface Energy – Inclusions commonly rounded
– Reduced surface energy
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Development of Nodular/Spotted Texture
• Irregular, ovoid, poikiloblastic crystals
• Typical of andalusite and cordierite
• Due to rapid porphyroblast growth (rapid heat increase)
Progressive thermal
metamorphism of slate. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
Progressive Contact Metamorphism
Progressive thermal
metamorphism of slate. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
Progressive Contact Metamorphism
Progressive thermal
metamorphism of slate. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
Progressive Contact Metamorphism
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Regional Metamorphism
• Increase in temperature accompanied by increase in pressure
• Usually directed pressure
– Rock deforms
– Deformation increase with metamorphic grade
Orogenic Regional Metamorphism
• High T/Low P metamorphism – Associated with arc complex
– Heat added by magma
• High T/High P metamorphism – Associated with the fold and
thrust belts
– Little or no magma added
Orogenic Regional Metamorphism
• Low T/High P metamorphism
– Associated with oceanic trench environments
– Cold slab added
Development of Subgrains
• Minor degree of deformation causes lattice defects to migrate
• Local accumulation of lattice defects result in reorientation of the crystal lattice
– Undulose extinction
– Subgrain development
• Higher degree of directed pressure results in elongated subgrains
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Development of Foliations
Recrystallization Pressure Solution
Remobilization Rotation
Progressive syntectonic
metamorphism of a volcanic
graywacke, New Zealand. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco
Progressive Regional Metamorphism
Progressive syntectonic
metamorphism of a volcanic
graywacke, New Zealand. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco
Progressive Regional Metamorphism
Progressive syntectonic
metamorphism of a volcanic
graywacke, New Zealand. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco
Progressive Regional Metamorphism
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Progressive syntectonic
metamorphism of a volcanic
graywacke, New Zealand. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco
Progressive Regional Metamorphism