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






Progressive thermal






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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



Freeman. San Francisco

Progressive Regional Metamorphism















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