metamorphism
DESCRIPTION
Metamorphism. The transformation of rock by temperature and pressure Metamorphic rocks are produced by transformation of: Igneous, sedimentary and igneous rxs. Thanks to CU Boulder Geology Dept for use of some of these slides. Metamorphism. - PowerPoint PPT PresentationTRANSCRIPT
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Metamorphism
• The transformation of rock by temperature and pressure
• Metamorphic rocks are produced by transformation of:
• Igneous, sedimentary and igneous rxs
Thanks to CU Boulder Geology Dept for use of some of these slides
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Metamorphism
• Metamorphism progresses from low to high grades
• Rocks remain solid during metamorphism
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What causes metamorphism?
• Heat• Most important agent • Heat drives recrystallization - creates new,
stable minerals• Pressure (stress)
• Increases with depth• Pressure can be applied equally in all
directions or differentially
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Origin of pressure in metamorphism
Confining or hydrostatic pressure: equal in all directions
Directed pressure: largely in one direction or along a particular axis
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Main factor affecting metamorphism
• Parent rock• Metamorphic rocks typically have the same
chemical composition as the rock they were formed from
• Different minerals, but made of the same stuff.
• Exception: gases (carbon dioxide, CO2) and water (H2O) may be released
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Shale
Progressive metamorphism of a shale
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Slate
Progressive metamorphism of a shale
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Phyllite
Progressive metamorphism of a shale
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Schist
Progressive metamorphism of a shale
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Progressive metamorphism of a shale
Gneiss
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Metamorphism
• Three types of metamorphic settings:• Contact metamorphism – from a rise in
temperature within host rock• Hydrothermal metamorphism – chemical
alterations from hot, ion-rich water• Regional metamorphism -- Occurs in the
cores of mountain belts and makes great volumes of metamorphic rock
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Contact metamorphism
Produced mostly by local heat source
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Hydrothermal metamorphism
Requires hot circulation of fluids
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Regional metamorphismin Mountain Belts
Requires crustal thickening and shortening
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Metamorphism and plate tectonics
• Most regional metamorphism occurs along convergent plate boundaries
• Compressional stresses deform plate edge• Occurs in major mountain belts: Alps,
Himalayas, and Appalachians• Not necessarily in all mountains, depends
on orogenic forces, i.e., uplift versus compression
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Metamorphism and plate tectonics
• Metamorphism at subduction zones• Cores of subduction zones contain linear
belts of metamorphic rocks
– High-P, low-T zones near trench
– High-T, low-P zones in region of igneous activity (arc)
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Location of metamorphic zones in a subduction zone
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Metamorphic Environments
• Index minerals and metamorphic grade
• Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form
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Index Minerals in metamorphic rocks
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Metamorphic Environments
• Metamorphic grade• A group of minerals that form in a
particular P-T environment
Zeolite (really low T,P; <200C)Greenschist (low T, P; 200-450C, 10-15 km)Blueschist (low T, high P - subduction zones)Amphibolite (high T, P; 450-650C, 15-20 km)Granulite (super high T, P; >700C, >25km)
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Metamorphic Environments in Subduction Zones
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Greenschist Hand Sample
Greenschist Thin Section
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MicaSchist
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Blueschist Amphibolite
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Common metamorphic rocks
• Nonfoliated rocks• Quartzite
– Formed from a parent rock of quartz-rich sandstone
– Quartz grains are fused together
– Forms in intermediate T, P conditions
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Sample ofquartzite
Thin sectionof quartzite
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Flattening of quartz grains in quartzite
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Common metamorphic rocks
• Nonfoliated rocks• Marble
– Coarse, crystalline
– Parent rock usually limestone
– Composed of calcite crystals
– Fabric can be random or oriented
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Marble (Random fabric = annealing; nonfoliated)
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Change in metamorphic grade with depth
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Common metamorphic rocks
• Foliated rocks• Slate
– Very fine-grained
– Excellent rock cleavage
– Made by low-grade metamorphism of shale
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Example of slate
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Slate roof
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Common metamorphic rocks
• Foliated rocks• Phyllite
– Grade of metamorphism between slate and schist– Made of small platy minerals– Glossy sheen with rock cleavage– Composed mainly of muscovite and/or chlorite
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Phyllite (left) and Slate (right) lack visible mineral grains
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Common metamorphic rocks
• Foliated rocks• Schist
– Medium- to coarse-grained
– Comprised of platy minerals (micas)
– The term schist describes the texture
– To indicate composition, mineral names are used (such as mica schist)
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Mica Schist - note well developed foliation
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A mica garnet schist
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Common metamorphic rocks
• Foliated rocks• Gneiss
– Medium- to coarse-grained
– Banded appearance
– High-grade metamorphism
– Composed of light-colored feldspar layers with bands of dark mafic minerals
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Gneiss displays bands of light and dark minerals
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Diorite to Gneiss Morph(orthogneiss - from igneous protolith)
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What are metamorphic textures?
• Texture refers to the size, shape, and arrangement of mineral grains within a rock
• Foliation – planar arrangement of mineral grains within a rock
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Outcrop of foliated gneiss
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Metamorphic textures
• Foliation• Foliation can form in various ways:
– Rotation of platy or elongated minerals
– Recrystallization of minerals in a preferred orientation
– Changing the shape of equidimensional grains into elongated and aligned shapes
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Flattened Pebble Conglomerate = flattening
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Development of foliation due to directed pressure