Metamorphism. Sources of Heat for Metamorphism. Sources of Heat for Metamorphism. Environments of Metamorphism. and Associated Textures

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1 Environments of Metamorphism and Associated Textures GEOL Metamorphic Lecture 1 Sources of Heat for Metamorphism Heat from Earth s interior Geothermal gradient is the increase in temperature with depth Typical continental geothermal gradient is C/km Volcanically active areas have geothermal gradients of C/km Oceanic trenches have geothermal gradients as low as 5-10 C/km Metamorphism Literally translates to change of form In geology it refers to solid-state changes in mineral assemblages of a rock, and/or the texture of these minerals Due to changes in temperature and/or pressure Sources of Heat for Metamorphism Heat from magma Emplacement of magma chambers will add heat to the immediately surrounding rock Gabbroic magma ~1300 C Granitic magma ~700 C 1

2 Controls textures Lithostatic versus Directed Pressure P and T control mineral stability Depth of Burial Temperature (usually a function of depth) Essentially Three Environment-Based Variables that Control the Character of Metamorphism The lithostatic pressure at a 10 km depth is 3 kbar = 0.3 GPa Granite: 2.7 g/cm3 (2700 kg/m3) Fault-Zone Metamorphism Directed pressure controls processes (GEOL 41.1) Regional Metamorphism Orogenic Metamorphism Combination of temperature and directed pressure Burial Metamorphism Combination of temperature and lithostatic pressure Contact Metamorphism Thermal variation controls processes Types of Metamorphism Compressional environments: Horz > Vert Pressure Extensional environments: Vert > Horz Pressure Directed pressure varies with tectonic environment Directed pressure affects the shape and arrangement of the minerals Directed pressure: pressure is imposed in a particular direction due to a regional stress field. Lithostatic pressure: the confining pressure created by the material that sits above a particular location. Lithostatic pressure is equal in all directions and compresses the volume of rock. Basalt: 3 g/cm3 (3000 kg/m3) Pressure Associated with Metamorphism Pressure Associated with Metamorphism 2

3 Contact Metamorphism Occurs adjacent to igneous intrusions Temperature contrast between magma chamber and host rock Most evident in lowpressure (near-surface) environments Development of Granoblastic Texture Pressure increases solubility Mineral dissolves along, or migrates from, high pressure points to low pressure areas Contact Metamorphism Steep thermal gradient in host rock around the intrusion Size of aureole depends on the size of the pluton, the rate at which it cools, and time since intrusion Rapid relative to most geological processes Development of Poikiloblastic Texture Common feature in contact metamorphic rocks Due to rapid porphyroblast growth (rapid heat increase in contact aureoles) Crystal envelops non-reactive or excess minerals High-energy porphyroblast due to increased surface area of inclusions Inclusions are commonly rounded to reduce surface energy 3

4 Progressive thermal metamorphism of slate. From. Progressive Contact Metamorphism Due to rapid porphyroblast growth (rapid heat increase in contact aureoles) Typically andalusite or cordierite Anhedral, ovoid, poikoblastic crystals Development of Nodular/Spotted Texture Progressive thermal metamorphism of slate. From. Progressive Contact Metamorphism Progressive thermal metamorphism of slate. From. Progressive Contact Metamorphism 4

5 Low T/High P metamorphism associated with oceanic trench environments Orogenic Regional Metamorphism Usually there is directed pressure, so rock deformation increases with metamorphic grade Increase in temperature is accompanied by an increase in pressure Regional Metamorphism Higher degree of directed pressure will result in elongated subgrains Undulose extinction Subgrain development Local accumulation of lattice defects result in reorientation of the crystal lattice Minor degree of deformation causes lattice defects to migrate Development of Subgrains High T/High P metamorphism associated with the fold and thrust belt Contact metamorphism at shallow depth High T/Low P metamorphism associated with arc complex Orogenic Regional Metamorphism 5

6 Development of Foliations Recrystallization Pressure Solution Remobilization Rotation Progressive Regional Metamorphism Progressive Regional Metamorphism Progressive Regional Metamorphism 6

7 Progressive Regional Metamorphism 7