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Metamorphism and Metamorphic RocksChapter 7
MetamorphismMetamorphism
… … is the transformation of rock by is the transformation of rock by temperature and pressuretemperature and pressure
Metamorphic rocks are produced by Metamorphic rocks are produced by transformation of:transformation of:
Sedimentary and Igneous rocks, and by Sedimentary and Igneous rocks, and by the further alteration of other the further alteration of other metamorphic rocksmetamorphic rocks
0 kmSedimentaryrock
Metamorphicrock
Igneousrock
50 km
10 km
~200ºC
~800ºC
Incr
easi
ng d
epth
and
tem
pera
ture
Melting
Metamorphism
Sedimentaryrock
Sediment
Metamor-phism occurs between about 10 and 50 km of depth
The rocks don’t melt
NorthCascades
CanadianShield
RockyMountains
Grand Canyon
AppalachianMountains
BlackHills
LlanoUplift
Originally buried deep,metamorphic rocks are seen when erosionremoves covering rocks, and in the coresof mountains
Glaciers exposed the
Best US exposures in New England and the South
MetamorphismMetamorphism
Metamorphism progresses from low to Metamorphism progresses from low to high gradeshigh grades
Rocks remain solid during metamorphismRocks remain solid during metamorphism
Metamorphism occurs above 50km melting Metamorphism occurs above 50km melting depth for felsic mineralsdepth for felsic minerals
What causes metamorphism?What causes metamorphism?
1. Heat1. Heat Most important agent Most important agent Heat drives recrystallization - creates new, Heat drives recrystallization - creates new,
stable mineralsstable minerals Increasing Heat with DepthIncreasing Heat with Depth
Temperature Increase with Depth
“Geothermal Gradient” due to:
Radioactive Isotopes
Intruding Magma
Friction Between Moving Bodies of Rock
What causes metamorphism?
2. Pressure (stress)Increases with depthPressure can be applied equally in all directions or differentiallyAll Directions = “Confining Pressure”also called “lithostatic pressure”
Differential = “Directed Pressure”
Origin of pressure in metamorphism
(due to burial)
(Convergent Margin)
Confining pressure aka “lithostatic”
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 sheet-like Micas, their parallel orientation gives a layered look; layering unrelated to the original bedding in the parent rock.
Main factors affecting Main factors affecting metamorphismmetamorphism
3. Parent rock3. Parent rock Metamorphic rocks typically have the Metamorphic rocks typically have the
same chemical composition as the rock same chemical composition as the rock they were formed from.they were formed from.
Different minerals, but made of the same Different minerals, but made of the same atoms.atoms.
Metamorphic SettingsMetamorphic Settings Three types of metamorphic settings:Three types of metamorphic settings:
ContactContact metamorphism – due heat from adjacent rocks metamorphism – due heat from adjacent rocks HydrothermalHydrothermal metamorphism – chemical alterations from metamorphism – chemical alterations from
hot, ion-rich waterhot, ion-rich water RegionalRegional metamorphism -- Occurs in the cores of metamorphism -- Occurs in the cores of
mountain belts and subduction zones (Converging mountain belts and subduction zones (Converging Margins) . Makes great volumes of metamorphic rock. Margins) . Makes great volumes of metamorphic rock. Includes:Includes:
– BurialBurial Metamorphism – e.g. Burial of sediments Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliateddeeper than 10 km – non-foliated
– DynamothermalDynamothermal Metamorphism – Directed pressure Metamorphism – Directed pressure in Plate Tectonic Processes - foliatedin Plate Tectonic Processes - foliated
1. Contact Metamorphism
Baking due to nearby Magma
Effect strongest in rocks in immediate contact
Contact metamorphismProduced mostly by local heat source
Contact MetamorphismMetamorphicAureole
2. Hydrothermal Metamorphism
Due circulation of water near Magma
Important at mid-ocean ridge
Hydrothermal Metamorphism
3. Regional Metamorhism3. Regional Metamorhism
Most Most DynamothermalDynamothermal metamorphism metamorphism occurs along convergent plate boundariesoccurs along convergent plate boundaries
Example 1: Continent-Continent CollisionsExample 1: Continent-Continent CollisionsCompressional stresses deforms plate edgeCompressional stresses deforms plate edgeContinents CollideContinents CollideMajor Folded Mountain Belts: Alps, Major Folded Mountain Belts: Alps,
Himalayas, and Appalachian Mts.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 densityThey float, cannot be subducted, so they get squashed.
2. Regional Metamorphism 2. Regional Metamorphism (continued)(continued)
Most Most Dynamothermal Dynamothermal metamorphism occurs along convergent metamorphism occurs along convergent plate boundariesplate boundaries
Example 2: In Subduction ZonesExample 2: In Subduction Zones
Metamorphism in a Subduction Zone
Metamorphism and plate tectonicsMetamorphism and plate tectonics
Metamorphism at subduction zonesMetamorphism at subduction zones Cores of subduction zones contain linear belts ofCores of subduction zones contain linear belts of
metamorphic rocksmetamorphic rocks High-P, low-T zones near trenchHigh-P, low-T zones near trench High-T, low-P zones in regions near igneous High-T, low-P zones in regions near igneous
activity within shallow Lithosphere (Crust)activity within shallow Lithosphere (Crust) High P, high-T zones in regions near igneous High P, high-T zones in regions near igneous
activity deeper in Lithosphere (Uppermost activity deeper in Lithosphere (Uppermost Mantle)Mantle)
Oceanic sediments
Basalt
High-temperature/low-pressuremetamorphism
Low-temperature/high-pressuremetamorphism
CONTINENTALCRUST
High-temperature/high-pressuremetamorphism
Metamorphic Grade and Metamorphic Grade and Index MineralsIndex Minerals
Certain minerals, called Certain minerals, called index index mineralsminerals, are good indicators of , are good indicators of the metamorphic conditions in which the metamorphic conditions in which they formthey form
Index Minerals in metamorphic rocks
Note Quartz and Feldspar are not index minerals: Why?
Note Temperature gradient
220oC
580oC
460oC 690oC
Here is an internally heated pressure vessel at the AMNH
With these you can study, for example:
1. the temperature and pressure conditions at which polymorphs change from one form to another. (next slide)
2. the reactions of minerals with fluids (for example salty or alkaline water) at high temperatures and pressures.
3. the conditions necessary to change one assemblage of minerals to another
http://research.amnh.org/earthplan/research/Equipment/Petrology
Any Useful Thermometers and Pressure Gauges?
Polymorphs of Al2SiO5
Kyanite
Sillimanite
Andalusite
7_21CANADA
NEW YORK
CANADA
U.S.A.
Albany
Boston
Scranton LongIsland
Binghamton
ATLANTICOCEAN
Unmetamorphosed
Chlorite/muscovite zone
Biotite zone
Garnet zone
Staurolite zoneSillimanite zoneHigh grade
Mediumgrade
Lowgrade
Augusta
PENNSYLVANIA
NEWHAMPSHIRE
MAINE
MASSACHUSETTS
Concord
Montpelier
VERMONT
CONNECTICUT
NEWJERSEY
R.I.
ProvidenceHartford
Newark
Increasing pressure and temperature
DIAGENESIS LOW GRADEChlorite and muscoviteBiotite
GarnetStaurolite
Sillimanite
INTERMEDIATE GRADE HIGH GRADE MELTING
New EnglandDynamothermalMetamorphism
r i f t
va lley
Metamorphic EnvironmentsMetamorphic 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 SectionChl-Ep
MicaSchist
Blueschist glaucph Amphibolite
Common metamorphic rocksCommon metamorphic rocks
Nonfoliated rocksNonfoliated rocks QuartziteQuartzite
– Formed from a parent rock of quartz-rich Formed from a parent rock of quartz-rich sandstonesandstone
– Quartz grains are fused togetherQuartz grains are fused together
– Forms in intermediate T, P conditionsForms in intermediate T, P conditions
Sample ofquartzite
Thin sectionof 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
7_18
Fracture
Fracture
Sandstone
Quartzite
Common metamorphic rocksCommon metamorphic rocks Nonfoliated rocks (cont.)Nonfoliated rocks (cont.)
MarbleMarble– Coarse, crystallineCoarse, crystalline
– Parent rock usually limestoneParent rock usually limestone
– Composed of calcite crystalsComposed of calcite crystals
– Fabric can be random or orientedFabric can be random or oriented
Marble (nonfoliated)
Common metamorphic rocksCommon metamorphic rocks Foliated rocksFoliated rocks
– Type formed depends on metamorphic gradeType formed depends on metamorphic grade
– Grade depends on depthGrade depends on depth
Change in metamorphic grade with depth
Increasing Directed Pressure and increasing Temps
Mudstones are sediments, can be squashed by burial and/or in continent-continent collisions
Common metamorphic rocksCommon metamorphic rocks Foliated rocksFoliated rocks
SlateSlate–Very fine-grainedVery fine-grained
–Excellent rock cleavage, often perp. Excellent rock cleavage, often perp. to originalto original
–Made by low-grade metamorphism Made by low-grade metamorphism of shaleof shale
Example of slate
Common metamorphic rocksCommon metamorphic rocks
Foliated rocksFoliated rocksPhyllitePhyllite
– Grade of metamorphism between slate and Grade of metamorphism between slate and schistschist
– Made of small platy mineralsMade of small platy minerals– Glossy sheen with rock cleavageGlossy sheen with rock cleavage– Composed mainly of muscovite and/or Composed mainly of muscovite and/or
chloritechlorite
Phyllite (l) and Slate (r)
lack visible mineral grains
Common metamorphic rocksCommon metamorphic rocks
Foliated rocksFoliated rocks
SchistSchist–Medium- to coarse-grained Medium- to coarse-grained
–Comprised of platy minerals (micas)Comprised of platy minerals (micas)
–The term The term schist schist describes the texturedescribes the texture
–To indicate composition, mineral names To indicate composition, mineral names are used (such as mica schist)are used (such as mica schist)
A mica garnet schist
Common metamorphic rocksCommon metamorphic rocks Foliated rocksFoliated rocks
GneissGneiss–Medium- to coarse-grainedMedium- to coarse-grained
–Banded appearanceBanded appearance
–High-grade metamorphismHigh-grade metamorphism
–Composed of light-colored feldspar Composed of light-colored feldspar layers with bands of dark mafic layers with bands of dark mafic mineralsminerals
Gneiss displays bands of light and dark minerals
What are metamorphic What are metamorphic textures?textures?
TextureTexture refers to the size, shape, and refers to the size, shape, and arrangement of mineral grains within a arrangement of mineral grains within a rockrock
FoliationFoliation – planar arrangement of – planar arrangement of mineral grains within a rockmineral grains within a rock
Outcrop of foliated gneiss
Metamorphic texturesMetamorphic textures
FoliationFoliation
Foliation can form in various ways:Foliation can form in various ways:
–Rotation of platy or elongated mineralsRotation of platy or elongated minerals
–Recrystallization of minerals in a Recrystallization of minerals in a preferred orientationpreferred orientation
–Changing the shape of equidimensional Changing the shape of equidimensional grains into elongated and aligned shapesgrains into elongated and aligned shapes
Flattened Pebble Conglomerate = flattening
Development of foliation due to directed pressure
Migmatites- When Partial Melting StartsHeat the rock, when the minerals with the lowest melting points (Qtz, Feldspar) at that pressure melt then recrystallize, we get separate bands of Metamorphic and Igneous rock
End of Chapter 7