Environments of Metamorphism and Associated Textures

Transcription

1 Metamorphism Environments of Metamorphism and Associated Textures EESC 3000 Metamorphic Lecture 1 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 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 Volcanically active areas C/km Oceanic trenches 5-10 C/km 1

2 Pressure Associated with Metamorphism Lithostatic pressure: confining pressure created by overlying material Equal in all directions Evenly compresses volume of rock Basalt: 3 g/cm 3 (3000 kg/m 3 ) Granite: 2.7 g/cm 3 (2700 kg/m 3 ) 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 2

3 Contact Metamorphism Contact Metamorphism Adjacent to plutons Steep thermal gradient Temperature contrast between magma and host rock Most evident in near-surface (low P) environments Aureole width depends on: Size of the pluton Cooling rate Time since intrusion Rapid relative to most geological processes Development of Granoblastic Texture Development of Poikiloblastic Texture Common in contact metamorphic rocks Due to rapid porphyroblast growth (rapid heat increase) Crystal envelops non-reactive or excess minerals Pressure increases solubility Mineral dissolves/migrates High P to low P High surface area -> High Surface Energy Inclusions commonly rounded Reduced surface energy 3

4 Development of Nodular/Spotted Texture Progressive Contact Metamorphism Irregular, ovoid, poikiloblastic crystals Typical of andalusite and cordierite Due to rapid porphyroblast growth (rapid heat increase) Progressive thermal metamorphism of slate. From. Progressive Contact Metamorphism Progressive Contact Metamorphism Progressive thermal metamorphism of slate. From. Progressive thermal metamorphism of slate. From. 4

5 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 5

6 Development of Foliations Recrystallization Pressure Solution Remobilization Rotation 6

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