Download Metamorphic Rocks

Metamorphosis…
"When Gregor Samsa woke up one
morning from unsettling dreams,
he found himself changed into a
monstrous bug.“
Metamorphosis, by Franz Kafka
One of the oldest rocks in the world. A gneiss
produced by metamorphosis of an even older shale.
Metamorphism
• The transformation of rock by
temperature and pressure
• Metamorphic rocks are produced by
transformation of:
• Igneous, sedimentary and even other
metamorphic rocks
Origin of Metamorphic Rocks
• Metamorphism begins when
– Temperature exceeds 200OC
– Pressure exceeds 300 M Pa (approx. 10,000
ft deep)
• Metamorphism ends when melting
begins
Metamorphism
• Recrystallization of minerals in the solid
state
• Caused by changes in T, P or pore fluids
• New environment = new minerals
• Growing minerals create a new texture
• Metamorphism progresses from low to high
grades
Metamorphism
• Textural changes occur during
metamorphism
– New minerals grow during metamorphism
replacing old minerals
– Precursor rock textures are modified as old
minerals are replaced by new
– Environment of formation influences crystal
growth
– Diagnostic features of the original rock are
masked or destroyed
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, i.e. directed
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: at higher T and P gases may be
released, e.g., carbon dioxide (CO2) and
water (H2O)
Source of pressure
Confining or
hydrostatic pressure:
equal in all directions
Directed pressure:
largely in one direction
or along a particular
axis
Source of Heat
• Source of heat
– Proximity to igneous intrusions
• Contact metamorphism
• Zones if different metamorphic grade ring
the intrusion
– Depth of burial
• 15-30oC increase per km
• Regional scale burial, mountain building
events
Source of Fluids
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
Circulation of hot fluids through cracks and porous
rock
Important
source of ores
Regional Metamorphism:
Subduction zones …..
…and/or deep burial
Why it is called regional
Fig. 6.15.
Regional
Metamorphic
Gradients
The three types reviewed
Origin of Metamorphic ‘Grade’
• T & P combined determine degree of
metamorphism & mineral assemblage
– Low-grade metamorphism- 200 to 350 OC and
relatively low pressures
– Intermediate-grade metamorphism - 350 to 550
OC and moderate to high pressures
– High-grade metamorphism - very high
temperatures, usually above 550OC and/or very
high pressures
Mineral stability depends largely on
temperature and pressure. Example: the
Al2SiO5 system
Other minerals behave similarly
Metamorphic Index Minerals
Regional Metamorphic Zones
• Index minerals
– A mineral that forms within a specific,often
narrow range of conditions
– Identifies a specific grade of metamorphism
– Allows further subdivision of rock types
Index Minerals in metamorphic rocks
Different kinds of tectonic settings can
produce distinct types of metamorphism
Regional metamorphism
High pressure is dominant factor
– Results in rocks with foliated textures
– Prevalent in intensely deformed mountain
ranges
– May occur over wide temperature range
– Higher pressure and temperature will
produce increased metamorphic grade
– Metamorphism occurs in large belts
– Prograde metamorphism common
Regional Metamorphism and plate tectonics
• Most regional metamorphism occurs along
convergent plate boundaries
• Compressional stresses deform along the plate
boundaries
• Cores of subduction zones contain linear belts
of metamorphic rocks
• Occurs in major mountain belts: Alps,
Himalayas, and Appalachians
High-P, low-T zones near trench
High-T, low-P zones in region of igneous activity (arc)
Change in metamorphic grade with depth
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 facies
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
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
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
Development of foliation due to
directed pressure
Flattened Pebble Conglomerate = flattening
Granites
Granites
Foliated vs. Nonfoliated textures
under the microscope
Common metamorphic rocks
• Foliated rocks
• Slate
– Very fine-grained
– Excellent rock cleavage
– Made by low-grade metamorphism of shale
Example of slate
Fig. 6.10a. 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
Schist
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
Progressive metamorphism of shale
Metamorphic rocks exposed at Mt. Everest.
Deformation occurs at various scales
Outcrop of foliated gneiss
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
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)
Marble
Question:
Where do we see metamorphic rocks in outcrops?
North
American
Craton
Shield
Western
North
American
Mobile Belt
Platform
Eastern
North
American
Mobile Be
Answer:
In continental shields and uplifted basement rocks