Transcript Metamorphic Rocks

Metamorphism and Metamorphic Rocks
Vocabulary
Foliated: wavy bands or wrinkles
 Porphyroblast: large changed crystals
surrounded by small grains
 Rock cycle: continuous changing cycle of
rocks ( igneous->sedimentary>metamorphic)

Metamorphism

… is the transformation of rock by
temperature and pressure

Metamorphic rocks are produced by
transformation of:
 Sedimentary and Igneous rocks, and by
the further alteration of other
metamorphic rocks
Sedimentary
rock
0 km
Metamorphic
rock
Igneous
rock
Sediment
Increasing depth
and temperature
10 km
~200ºC
50 km
~800ºC
Sedimentary
rock
Metamorphism
Metamor
-phism
occurs
between
about 10
and 50
km of
depth
The rocks don’t melt
Melting
Glaciers exposed the
Canadian
Shield
Rocky
Mountains
North
Cascades
Black
Hills
Appalachian
Mountains
Grand
Canyon
Llano
Uplift
Originally buried deep,
metamorphic rocks are seen when erosion
removes covering rocks, and in the cores
of mountains
Best US exposures
in New England
and the South
Metamorphism

Metamorphism progresses from low to
high grades

Rocks remain solid during metamorphism

Metamorphism occurs above 50km melting
depth for felsic minerals
What causes metamorphism?
1. Heat
 Most important agent
 Heat drives recrystallization - creates new,
stable minerals
 Increasing Heat with Depth
What causes metamorphism?
2. Pressure (stress)
Increases with depth
Pressure can be applied equally in all
directions or differentially
All Directions = “Confining Pressure”
also called “lithostatic pressure”
Differential = “Directed Pressure”
Origin of pressure in
metamorphism
Confining pressure aka “lithostatic”
(due to burial)
(Convergent Margin)
Confining Pressure
Directed Pressure
Directed Pressure causes rocks to become folded, and
minerals to reorient perpendicular to the stress: “foliation”
Source: Kenneth Murray/Photo Researchers Inc.
Foliation
Minerals Recrystallize Perpendicular to
the Directed Pressure
If the minerals are flat, such as sheetlike Micas, their parallel orientation
gives a layered look; layering unrelated
to the original bedding in the parent
rock.
Main factors affecting
metamorphism
3. Parent rock
 Metamorphic rocks typically have the
same chemical composition as the rock
they were formed from.
 Different minerals, but made of the same
atoms.
Metamorphic Settings

Three types of metamorphic settings:
 Contact metamorphism – due heat from adjacent rocks
 Hydrothermal metamorphism – chemical alterations from
hot, ion-rich water
 Regional metamorphism -- Occurs in the cores of
mountain belts and subduction zones (Converging
Margins) . Makes great volumes of metamorphic rock.
Includes:
– Burial Metamorphism – e.g. Burial of sediments
deeper than 10 km – non-foliated
– Dynamothermal Metamorphism – Directed pressure
in Plate Tectonic Processes - foliated
1. Contact Metamorphism
Baking due to nearby Magma
Effect strongest in rocks in
immediate contact
Contact metamorphism
Produced mostly by local heat source
Contact Metamorphism
Metamorphic
Aureole
2. Hydrothermal
Metamorphism
Due to the circulation of water
near Magma
Important at mid-ocean ridge
Hydrothermal Metamorphism
3. Regional Metamorhism
 Most
Dynamothermal metamorphism
occurs along convergent plate boundaries
Example 1: Continent-Continent Collisions
 Compressional stresses deforms plate edge
 Continents Collide
 Major Folded Mountain Belts: Alps,
Himalayas, and Appalachian Mts.
Dynamothermal Metamorphism,
Before collision
Sediments are “unconsolidated”. They will fold if pushed.
Dynamothermal Metamorphism,
After continental collision
Felsic continental materials and sediments are buoyant, they have low density
They float, cannot be subducted, so they get squashed.
2. Regional Metamorphism
(continued)

Most Dynamothermal
metamorphism occurs along convergent
plate boundaries
 Example 2: In Subduction Zones
Metamorphism in a Subduction Zone
Metamorphism and plate tectonics
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Metamorphism at subduction zones
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Cores of subduction zones contain linear belts of
metamorphic rocks
High-P, low-T zones near trench
 High-T, low-P zones in regions near igneous
activity within shallow Lithosphere (Crust)
 High P, high-T zones in regions near igneous
activity deeper in Lithosphere (Uppermost
Mantle)

High-temperature/low-pressure
metamorphism
Oceanic sediments
CONTINENTAL
CRUST
Basalt
Low-temperature/
high-pressure
metamorphism
Hightemperature/
highpressure
metamorphism
Metamorphic Grade and
Index Minerals

Certain minerals, called index
minerals, are good indicators of
the metamorphic conditions in which
they form
Note
Temperature
gradient
Index Minerals in metamorphic rocks
580oC
220oC
460oC
Note Quartz and Feldspar are not index minerals: Why?
690oC
Any Useful Thermometers and
Pressure Gauges?
Sillimanite
Kyanite
Polymorphs of Al2SiO5
Andalusite
CANADA
New England
Dynamothermal
Metamorphism
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MAINE
Augusta
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Montpelier
NEW
VERMONT HAMPSHIRE
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Concord

Boston

Albany
NEW YORK
ATLANTIC
OCEAN
MASSACHUSETTS
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Binghamton
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R.I.
Hartford
Providence
CONNECTICUT
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PENNSYLVANIA
Scranton
r
NEW
JERSEY
i
ft
va l
l
ey
Low
grade
Long Medium
Island grade
Newark
High grade
Unmetamorphosed
Chlorite/muscovite zone
Biotite zone
Garnet zone
Staurolite zone
Sillimanite zone
Increasing pressure and temperature
DIAGENESIS
LOW GRADE
HIGH GRADE
INTERMEDIATE GRADE
Chlorite and muscovite
Biotite
Garnet
Staurolite
MELTING
Sillimanite
Metamorphic Environments
Metamorphic grade or Facies
 A group of minerals that form in a
particular P-T environment
 Can be used to deduce T-P conditions of
formation
Metamorphic Environments in Subduction Zones
We can look at minerals in Metamorphic Rocks
and determine where they formed.
Water facilitates metamorphic reactions by allowing movement of atoms and ions
Greenschist Hand Sample
Greenschist Thin Section
Chl-Ep
Mica
Schist
Blueschist glaucph
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
Field Geologists are
grateful for
quartzites. They don’t
foliate, so you can
see the folds.
Mudrocks foliate;
much harder to map.
Flattening of quartz grains in quartzite
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Fracture
Sandstone
Fracture
Quartzite
Common metamorphic rocks

Nonfoliated rocks (cont.)
 Marble
– Coarse, crystalline
– Parent rock usually limestone
– Composed of calcite crystals
– Fabric can be random or oriented
Marble (nonfoliated)
Common metamorphic rocks
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Foliated rocks
– Type formed depends on metamorphic grade
– Grade depends on depth
Mudstones are sediments, can be squashed by burial and/or in continent-continent collisions
Change in metamorphic grade with depth
Increasing Directed Pressure and increasing Temps
Common metamorphic rocks
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Foliated rocks
Slate
–Very fine-grained
–Excellent rock cleavage, often perp.
to original
–Made by low-grade metamorphism
of shale
Example of slate
Common metamorphic rocks
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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 (l) and Slate (r)
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)
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
Outcrop of foliated gneiss
Metamorphic textures
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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