Lecture 6_Metamorphic rocks.pdf

Evolution of Earth and life ECS 1213 Lecture – 6 Metamorphism and metamorphic rocks  What is metamorphism? -Metamorphism represents processes by which rocks undergo solid-state changes in mineralogy, texture, or both to reach equilibrium with its cha...

Evolution of Earth and life ECS 1213 Lecture – 6 Metamorphism and metamorphic rocks  What is metamorphism? -Metamorphism represents processes by which rocks undergo solid-state changes in mineralogy, texture, or both to reach equilibrium with its changing environment - Changes in any/all of the following: ~ Mineralogy ~ Texture ~ Chemical composition -Changes generally occur in a zone below the Earth’s crust and extending up to the upper mantle. -Identification of parent rock is often impossible. What is not metamorphism? - Any process that involves complete melting.  Agents of metamorphism: Heat -Heat provides the energy to drive the re-crystallization process -Two major change caused by heat Formation of big crystals: Formation of new minerals: Some fine grained material tend to join Some minerals are not stable at higher together to form larger grains when temperature --- they would try to change exposed to heat. to a more stable mineral. Formation of larger crystals Formation of new minerals What is the source of heat?  Source of heat Earth’s internal heat comes from - Continuous radioactive decay - Thermal energy generated during the formation Geothermal gradient: Rate at which the temperature increases with depth - Geothermal gradient varies in different parts of the world - Tectonics plays an important role in determining the nature of the gradient. A B Depth Depth Temperature Temperature Temperature Temperature changes at a changes at a much faster rate much slower rate with depth. with depth. A B  Agents of metamorphism: Pressure -Pressure increases with depth - There are two main types of pressure that operate on the rocks 1. Confining pressure: Non-directional 2. Differential stress: Directional 1. 2 1.. Near the surface at low temp. rocks are brittle – tend to fracture -- minerals tend to get crushed into smaller grain in differential stress At depths at high temp. rocks are ductile -- tend to flow 2. --mineral grains tend to flatten in differential stress  Agents of metamorphism: Fluids Fluids that play important role in metamorphism is mainly composed of - Water - Dissolved volatiles (CO2, SO2) -These fluids help in recrystalizing minerals Where do we get the water from?? - Minerals often carry water in their structure (mica, clay etc) -Temperature causes dehydration of the minerals -Those expelled water molecules could play Mica important role in ion transport. Silica tetrahedra sheet structure _ with (OH) group -The water could also come from rocks when subjected to extreme heat.  Causes of metamorphic texture - Two main types of texture: Foliated and non-foliated -Mineral grains are often oriented in a preferred direction. This is called foliation (leaflike) - Foliation could form in any of the following ways: 1. Rotation of platy and/or elongated minerals METAMORPHISM 2. Recrystallization of minerals to form crystals in new preferred direction. METAMORPHISM 3. Changing the shape of existing mineral in a preferred manner. METAMORPHISM Development of textures in metamorphic rocks  Metamorphic texture 1. Foliated: Shows preferred arrangement a. Rock / Slaty cleavage: Closely spaced surfaces which split easily. Product of low grade metamorphism. Example: Slate b. Schistosity: Layered structure with high mica content with a sparkly appearance Medium-high grade metamorphism. Example: Mica schist c. Gneissic texture: Layers of alternating dark and light bands. High grade metamorphism. Example: Granite gneiss 2. Non-foliated texture a. When there is no preferred orientation. a. Granoblastic (Marble) b b. Porphyroblastic (Garnet porphyroblast).  Some common foliated metamorphic rocks Slate Phyllite Schist Naming convention: Example: Granite gneiss Garnet-biotite schist Gneiss  Non-foliated metamprphic rock Quartzite: -Made up of mineral quartz (SiO2) -Very hard - Scratches glass - Parent rock: generally a sedimentary rock (sandstone) Marble: -Made up of mineral calcite (CaCO2) - Soft - Fizzes in reaction with acid - Parent rock: generally a sedimentary rock (limestone)  A. B D C Identify the metamorphic textures.  Types of metamorphism Regional metamorphism Metamorphism along faults Impact/shock Hydrothermal Contact metamorphism metamorphism metamorphism Burial http://www.uky.edu/AS/Geology/howell/goodies/ metamorphism elearning/module02swf.swf Metamorphic grade: textural variation Metamorphic grade: textural variation  Metamorphic grade: Mineralogical variation - Increasing temperature affects the mineral composition. - Every mineral has specific pressure- temperature ranges where they are stable. - Index mineral and isograd Example: Silimanite isograd records the following reaction: Muscovite + Quartz  Orthoclase Feldspar + Silimanite + Water KAl3Si3O10(OH) SiO2 KAlSi3O8 Al2SiO5 H2O Temperature: 600C, Pressure 5kbar  Metamorphic grade: Pressure-Temperature combination -Increasing pressure changes the texture. - Increasing temperature affects the mineral composition. - These are important tool to detect plate-tectonic setting