Metamorphism
Metamorphism
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Metamorphism
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process whereby preexisting rock undergoes pronounced changes in texture
and minerology at T > 200-300ºC while in the solid state
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Agents of metamorphism are heat, pressure, chemically active fluids
Agents of Metamorphism
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Heat - temperature increases with depth
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Geothermal Gradient - variable; 30ºC/km on continent
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Effect of Heat
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increased ductility
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change in mineral assemblage
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Pressure (Stress) - Increases with depth (0.3 kbars/km)
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Confining pressure - felt equally in all directions
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Directed pressure - added pressure from tectonic activity (tension, compression,
shear)
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Effect of pressure
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favors dense, compact structures
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minerals grow perpendicular to maximum pressure - produces a parallel arrangement
of platy minerals called foliation. Gives rock a layered or banded appearence.
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Chemically Active Fluids
Metamorphic Textures
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Foliation
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rock cleavage - slaty cleavage
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schistosity - parallel alignment of visible platy minerals
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gneissic - banded; layers of light and dark minerals
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Lineation - parallel alignment of needle-like grains
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Porphyroblastic - larger crystals (porphyroblasts) in a finer matrix
Common Metamorphic Rocks
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Foliated Metamorphic Rocks
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Slate - very fine grained, clay rich
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slaty cleavage
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bedding often present
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metamorphic grade: low
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Phyllite
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cleavage surface has sheen
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mica and/or chlorite grains
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Schist
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> 50% visible platy minerals; may have layers or qtz and/or feldspar
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bedding rare
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grade: medium to high
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rock name based on predominant mineral
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Ex: muscovite schist (clay rich sed. rock commonly parent)
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Gneiss
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light bands of qtz and feldspars, dark bands of biotite and hornblende
(amphibole)
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grade: high
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parent: igneous rock, shale
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Non-Foliated Metamorphic Rocks
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Low Fe-Mg
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Marble
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fine to coarsely crystalline calcite
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Parent: limestone or dolomite
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Quartzite
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metamorphosed qtz sandstone; almost all qtz
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Metaconglomerate
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High Fe-Mg (probably basalt parent)
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Hornfels
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uniform, fine-grained
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varied composition
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associated with contact metamorphism
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Greenstone - dark green, generally aphanitic
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Amphibolite - amphibole, plag.
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Granulite - pyx, gt, sillimanite
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Eclogite - pyx, gt
Types of Metamorphism
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Types of Metamorphism - Contact, Burial, Regional, Hydrothermal
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Burial
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low grade metamorphism of deeply buried sediments; bedding preserved
Contact Metamorphism
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Contact
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magma comes in contact with cooler rock
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aureole - zone of metamophosed rock around pluton; non-foliated
Regional Metamorphism
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Associated with subduction (compression)
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Majority of metamorphic rocks
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Can map pattern of regional metamorphism using index minerals which give
clues to pressure and temperature of metamorphism
Regional Metamorphism
Index Minerals
Metamorphic Zones
Hydrothermal Metamorphism
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Hydrothermal
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frequently associated with MOR
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hot seawater alters basalts
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on continent, alteration of country rocks by fluids from pluton at depth
Plate Tectonics and Metamorphism