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Transcript
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
• Metamorphism progresses from low to
high grades
• Rocks remain solid during metamorphism
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
Origin of pressure in metamorphism
Confining or
hydrostatic pressure:
equal in all directions
Directed pressure:
largely in one direction
or along a particular
axis
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
Progressive metamorphism of a shale
Shale
Progressive metamorphism of a shale
Slate
Progressive metamorphism of a shale
Phyllite
Progressive metamorphism of a shale
Schist
Progressive metamorphism of a shale
Gneiss
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
Contact metamorphism
Produced mostly by local heat source
Hydrothermal metamorphism
Requires hot circulation of fluids
Regional metamorphism
in Mountain Belts
Requires crustal thickening and shortening
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
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)
Location of metamorphic zones
in a subduction zone
Metamorphic Environments
• Index minerals and metamorphic grade
• Certain minerals, called index minerals,
are good indicators of the metamorphic
conditions in which they form
Index Minerals in metamorphic rocks
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)
Metamorphic Environments in Subduction Zones
Greenschist Hand Sample
Greenschist Thin Section
Mica
Schist
Blueschist
Amphibolite
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
Sample of
quartzite
Thin section
of quartzite
Flattening of quartz grains in quartzite
Common metamorphic rocks
• Nonfoliated rocks
• Marble
– Coarse, crystalline
– Parent rock usually limestone
– Composed of calcite crystals
– Fabric can be random or oriented
Marble (Random fabric = annealing; nonfoliated)
Change in metamorphic grade with depth
Common metamorphic rocks
• Foliated rocks
• Slate
– Very fine-grained
– Excellent rock cleavage
– Made by low-grade metamorphism of shale
Example of slate
Slate roof
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
Phyllite (left) and Slate (right)
lack visible mineral grains
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)
Mica Schist - note well developed foliation
A mica garnet schist
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
Gneiss displays bands of light and
dark minerals
Diorite to Gneiss Morph
(orthogneiss - from igneous protolith)
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
Outcrop of foliated gneiss
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
Flattened Pebble Conglomerate = flattening
Development of foliation due
to directed pressure