Remember the Rock Cycle Metamorphic rocks: What is metamorphism? think Baking (+ Pressure) Firing clay in a kiln to make pottery Sintering clay and sand to make bricks Definition: Metamorphism = changes in composition, texture and mineralogy of pre-existing rock in the solid state in response to changing P and T (>200_C) 1
Agents of Metamorphism: Heat = baking (favors high T minerals) Pressure = Squeezing (favors denser minerals) Fluids (H 2O, CO 2)! Very important, w/out fluids, elements would have to diffuse through solids (very slow) rather than diffuse through and be advected by fluid. Fluids thus change chemical environment Fluids are an excellent transport media on all scales Metamorphic Grade Grade is a simple classification of the intensity of metamorphic conditions Based on the changes in mineralogy and texture in response to increasing metamorphic conditions. Low, Medium, High See rock examples: Metamorphic Grade: Changes in Mineralogy Chemical reactions in solid state! Controlled by T., P. and protolith composition Canʼt get blood from a stone Example: above - same composition different minerals Protolith = Mudstone (clay, qtz, feldspar) High grade metamorphism of mudstone = qtz, plagioclase, garnet, amphibole, silimanite 2
Index Minerals: Minerals that indicate P and T of metamorphism! Geologists use these to understand conditions of metamorphism! i.e. geologists argue backward from the minerals observed too the metamorphic conditions required to create the minerals from a presumed protolith. Reconstructing geologic history of rocks! Types of Metamorphism Thermal, Regional & High-Pressure Thermal Metamorphism= High T, ʻLowʼ P Bring Heat to protolith Rock = hornfels (equant grains - no foliation) Contact aureole Heating drives rocks off geotherm 3
Changes in metamorphic rocks with increasing Grade (Thermal Metamorphism produces rocks with no foliation) Increasing Metamorphic Conditions (Grade) => Grade: Low Grade=> Medium Grade => High Grade Process Rotation Recrystallization Recrystallization and growth Growth & Chemical Differentiation Fabric Slaty Cleavage Incipient Mineral Layering (Schistosity) Well developed Mineral Layering (Schistosity) Segregation of compositional layers (Gneissic layering) Rock Name Slate Phylllite Schist Gneiss (no fabric) argillite Hornfels, quartzite Marble Hornfels, quartzite Marble Skarn Granofels Granulites Note: layering is dependant on composition of protolith: e.g. if no micas then no schist Mountain building events Drives rocks to great depth. Tectonic stresses produce foliated rocks: Slatephyllite-schist-gneiss Exposure of these rocks implies uplift and erosion of mountains! Regional Metamorphism: High P & T Burial takes rocks down geotherm Develop fabric: e.g. foliation Re-crystallization in divergent stress field causes minerals to grow in parallel orientation Changes with prograde metamorphism Grain size increases & fabric develops as P and T increase Develop foliation 4
Changes in metamorphic rocks with increasing Grade (Stress form tectonics produces foliation) Increasing Metamorphic Conditions (Grade) => Grade: Low Grade=> Medium Grade => High Grade Process Rotation Recrystallization Recrystallization and growth Growth & Chemical Differentiation Fabric Slaty Cleavage Incipient Mineral Layering (Schistosity) Well developed Mineral Layering (Schistosity) Segregation of compositional layers (Gneissic layering) Rock Name Slate Phylllite Schist Gneiss (no fabric) argillite Hornfels, quartzite Marble Hornfels, quartzite Marble Skarn Granofels Granulites Note: layering is dependant on composition of protolith: e.g. if no micas then no schist How do metamorphic rocks become layered (foliated)? A. Mineral grains rotate under stress to align. B. Crystals grow under stress such that they align. C. Both A and B D. None of the above Very strange, not well explained until 1980s Form in subduction zones Blueschist & eclogite Find these on surface requires rapid exhumation! High Pressure Metamorphism: High P, ʻlowʼ T Subduction takes rocks down deep but they donʼt heat up (below geotherm). Why? 5
Summary: Types of Metamorphism Slate Phyllite 6
Schist Gneiss Return to rock cycle 7
Review Questions What is metamorphism? How are fluids important to metamorphic reactions? How is a high grade metamorphic rock different from a low grade metamorphic rock? Why donʼt thermally metamorphosed rocks show foliations? At what tectonic setting do the different types of metamorphism occur? How does each type of metamorphism relate to the geothermal gradient? 8