GY111 Physical Geology Lectures on Metamorphic Rocks.
Metamorphism
• Causes of Metamorphism– Elevated T & P– Fluids (H2O, CO2,
CH4, etc.)– Directed Stress
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Presentation Notes
Causes of Metamorphism: 1. Elevated Temp. & Press. 2. Presence of Pore Fluids (H2O, CO2, CH4, etc.). 3. Directed Stress – compressive stress not equal in all directions.
Types of Metamorphism
• Regional: occur along convergent plate boundaries.
• Contact: occurs along margin of a magma intrusion.
• Seafloor/Hydrothermal: associated with circulating hydrothermal fluids- mostly at divergent ocean ridge systems.
• Shock: meteorite impact.
Presenter
Presentation Notes
Types of Metamorphism: 1. Regional: occur along convergent plate boundaries. 2. Contact: occurs along margin of a magma intrusion at relatively shallow levels in the crust. 3. Seafloor/Hydrothermal: associated with circulating hydrothermal fluids – mostly at divergent ocean ridge systems. 4. Shock: meteorite impact.
Contact Metamorphic Rocks
• Fine-grained because of relatively short time frame for recrystallization.
formed by intrusion of silicate magma into limestone or dolostone.
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Presentation Notes
Contact Metamorphic Rocks 1. Fine-grained because of relatively short time frame for recrystallization. 2. Develop low-pressure metamorphic minerals (i.e. Andalusite). 3. Hornfels: generic dark contact metamorphic rock. 4. Felsite: light-colored contact metamorphic rock. 5. Skarn: Ca-silicate rich contact metamorphic rock formed by intrusion of silicate magma into limestone or dolostone.
Types of Metamorphism: Tectonic Environments
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Presentation Notes
Tectonic Environments and Types of Metamorphism: Regional Metamorphism: convergent plate boundaries with active subduction at depth under volcanic arc. Regional High-P Metamorphism: convergent plate boundaries with active subduction at depth near trench and subducted slab. Contact Metamorphism: any environment where magma can come into contact with relatively cool crustal rocks. Seafloor/Hydrothermal Metamorphism: common at oceanic divergent boundaries where seafloor metamorphism is driven by convecting hydrothermal fluids. Hydrothermal metamorphism is possible where magma may heat H2O rich fluids and those fluids are able to circulate through crustal rocks. Shock Metamorphism: occurs where extremely violent energy is imparted into the lithosphere either by meteorite impact or very intense seismic events.
Metamorphic Textures
• Cleavage: tendency of a rock to break along smooth even planes
• Foliation: preferred alignment of platy grains (i.e. mica) or banding (i.e. gneiss or marble)
• Lineation: preferred alignment of elongated minerals (i.e. amphibole)
Presenter
Presentation Notes
Metamorphic Textures: 1. Cleavage: tendency of a rock to break along smooth even planes. 2. Foliation: preferred alignment of platy grains (i.e. mica) or banding (i.e. gneiss or marble). 3. Lineation: preferred alignment of elongated minerals (i.e. amphibole).
Metamorphic Texture: Foliation
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Presentation Notes
Foliation: Preferred alignment of mica normally defines the foliation. Fine-grained rocks like slate and phyllite show foliation only at microscopic scale. The microscopic foliation will cause the rock to have cleavage. Granoblastic rocks lack mica and therefore cannot display foliation or cleavage.
• All regional metamorphic rocks contain a foliation- in low grade (Low T) rocks the grains are microscopic so you can’t “see” the foliation
• Cleavage in rocks is the tendency to split along smooth planes. Rocks with microscopic foliation tend to have excellent rock cleavage
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Presentation Notes
Foliation vs. Cleavage 1. All regional metamorphic rocks contain a foliation- in low grade (Low T) rocks the grains are microscopic so you can’t “see” the foliation. 2. Cleavage in rocks is the tendency to split along smooth planes. Rocks with microscopic foliation tend to have excellent rock cleavage.
Granoblastic Metamorphic Rocks
• Granoblastic metamorphic textures are produced when the constituent grains of the rock are equidimensional- i.e. the grains have the same diameter in any direction.
• Granoblastic rocks therefore do not develop foliation
Granoblastic Metamorphic Rocks 1. Granoblastic metamorphic textures are produced when the constituent grains of the rock are equidimensional- i.e. the grains have the same diameter in any direction. 2. Granoblastic rocks therefore do not develop foliation. 3. Examples: marble, quartzite, greenstone, amphibolite*, hornfels, granulite. Note: some varieties of amphibolite contain enough amphibole and/or biotite to produce foliation.
Granulites
• Granulites, as their name implies, have a granular texture composed of pyroxene, plagioclase and garnet
• Granulites form at the highest grades of metamorphism when portions of the protolith melt and exit the rock leaving behind a “restite” that is devoid of H2O or other fluids
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Presentation Notes
Granulites 1. Granulites, as their name implies, have a granular texture composed of pyroxene, plagioclase and garnet. 2. Granulites form at the highest grades of metamorphism when portions of the protolith melt and exit the rock leaving behind a “restite” that is devoid of H2O or other fluids.
Protoliths
• Protolith: original rock that becomes metamorphosed
• Common Protolith/metamorphic rock relationships– Protolith Low Med High
Protoliths: 1. Protolith: original rock that becomes metamorphosed 2. Common Protolith/metamorphic rock relationships ProtolithLowMedHigh Shaleslate, phylliteschistgneiss Basaltgreenstoneamph.amph. Sandstonequartzitequartzitequartzite Limestonemarblemarblemarble
Large Crystal Textures
• Large metamorphic crystals are termed porphyroblasts
Large Crystal Textures: 1. Large metamorphic crystals are termed porphyroblasts 2. Common porphyroblast forming minerals include: Garnet, Andalusite, Staurolite, Kyanite, Plagioclase, Amphibole
Large Crystal Textures cont.
• Large crystals that are inherited from protolith are porphyroclasts.
• Augen: eye-shaped feldspar porphyroclasts in gneiss
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Presentation Notes
Large Crystal Textures continued: 1. Large crystals that are inherited from protolith are porphyroclasts. 2. Augen: eye-shaped feldspar porphyroclasts in gneiss
Banded Foliation
• Gneiss, migmatites, and impure marbles often display banding foliation.
• Banding is marked by alternating light and dark layers of mineral segregations.
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Presentation Notes
Banded Foliation: 1. Gneiss, migmatites, and impure marbles often display banding foliation. 2. Banding is marked by alternating light and dark layers of mineral segregations. 3. Light bands are usually composed of quartz and feldspar. Dark bands are composed of biotite, amphibole, and pyroxene.
Metamorphic Isograds• Isograd: this first appearance of an index
Metamorphic Isograds: 1. Isograd: this first appearance of an index metamorphic mineral. 2. Minerals: Chlorite, Muscovite, Biotite, Garnet, Staurolite, Kyanite, Sillimanite.
Relationship of Texture and Grade
• Increasing metamorphic grade results in larger grain size.
Increase in grain size
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Presentation Notes
Relationship of Texture and Grade: Increasing metamorphic grade results in larger grain size. High-T metamorphic rocks take longer to reach the higher T allowing for larger crystal growth.
Metamorphic Facies Concept
• Metamorphic Facies: regions on a T vs. P graph
Facies boundary
Geotherm
High T geothermHigh P geotherm
Very High T geotherm
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Presentation Notes
Metamorphic Facies Concept: 1. Metamorphic Facies: regions on a T vs. P graph. 2. Note that P increase downward on the Y axis, and that P is related to depth. 3. High-P geotherm is associated with blueschist and eclogite facies metamorphic rocks proximal to a subduction zone. 4. High-T geotherm is associated with greenschist, amphibolite, and granulite facies metamorphic rocks proximal to the volcanic and magmatic arc. 5. Very high-T geotherm is associated with hornfels facies contact metamorphic rocks.
High Pressure Metamorphic Facies
• Occur adjacent to trench where geothermal gradient is low (10 deg. C/km).
• High grade: garnet-bearing peridotite named “Eclogite”.
• Eclogites are considered to be ultra-high pressure metamorphic rocks and are only exposed at collisional plate boundaries.
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Presentation Notes
High Pressure Metamorphic Facies 1. Occur adjacent to trench where geothermal gradient is low (10 deg. C/km). 2. Low grade: Glaucophane + Kyanite + Jadite schist – “Blueschist”. 3. High grade: garnet-bearing peridotite named “Eclogite”. 4. Eclogites are considered to be ultra-high pressure metamorphic rocks and are only exposed at collisional plate boundaries.
Geotherms and Plate Tectonics• Subduction zones have unusually low geotherms- High P geotherm
(Blue schist & Eclogite facies)• Volcanic/Magmatic Arcs have unusually high geotherms- High T
Geotherms and Plate Tectonics: 1. Subduction zones have unusually low geotherms- High P geotherm (Blue schist & Eclogite facies). 2. Volcanic/Magmatic Arcs have unusually high geotherms- High T geotherm (Slate>Phyllite>Schist>Gneiss>Granulite; greenschist – amphibolite- granulite facies).
Geothermobarometry
• Mineral assemblages can be used to calculate P-T of crystallization during metamorphism
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Presentation Notes
Geothermobarometry: 1. Mineral assemblages can be used to calculate P-T of crystallization during metamorphism. 2. Some porphyroblasts such as garnet yield P-T data over time as they grow from core to rim.
P-T-time paths
• Geothermobarometry can be used to track P-T-time paths
• This allows the tectonic environment to be determined for the metamorphic rock
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Presentation Notes
P-T-time paths: 1. Geothermobarometry can be used to track P-T-time paths. 2. This allows the tectonic environment to be determined for the metamorphic rock
Exam Summary• Know the causes of metamorphism.• Be able to list protoliths of common metamorphic rocks.• Be able to list metamorphic facies and draw geotherms
on a P-T diagram.• Know metamorphic textural terms.• Be able to correlate geotherms with tectonic
environments.• Be able to list and describe the various types of
metamorphism.• Be familiar with the concept of metamorphic isograds.
Presenter
Presentation Notes
Exam Summary: 1. Know the causes of metamorphism. 2. Be able to list protoliths of common metamorphic rocks. 3. Be able to list metamorphic facies and draw geotherms on a P-T diagram. 4. Know metamorphic textural terms (porphyroblast, porphyroclast, etc.) 5. Be able to correlate geotherms with tectonic environments. 6. Be able to list and describe the various types of metamorphism. 7. Be familiar with the concept of metamorphic isograds. Know the sequence of isograds for High-T regional metamorphic rocks.
