This document discusses metamorphic rocks and the processes of metamorphism. Metamorphism occurs when rocks are subjected to high temperatures, high pressures, or chemical reactions, causing them to change form. There are several types of metamorphism including contact/thermal metamorphism near igneous intrusions, hydrothermal metamorphism involving hot fluids, and burial metamorphism under thick sedimentary layers. Metamorphism can produce both non-foliated and foliated rocks ranging from low to high grade, depending on temperature, pressure, and intensity of metamorphism.

1. Metamorphic RocksMetamorphic Rocks
Chapter 8Chapter 8

2. What Can MetamorphicWhat Can Metamorphic
Minerals/Rocks Tell Us?Minerals/Rocks Tell Us?
1.1. Type of MetamorphismType of Metamorphism
2.2. Temperature and Pressure Regimes of MetamorphismTemperature and Pressure Regimes of Metamorphism
3.3. Depth and Intensity of MetamorphismDepth and Intensity of Metamorphism
4.4. Environments prior to MetamorphismEnvironments prior to Metamorphism
5.5. History of the Formation of Mountain RangesHistory of the Formation of Mountain Ranges
6.6. Timing of Mountain FormationTiming of Mountain Formation
7.7. Types and Directions of StressTypes and Directions of Stress
8.8. Crustal Deformation – Folding and Faulting RegimesCrustal Deformation – Folding and Faulting Regimes
9.9. Plate MovementsPlate Movements
10.10. Tectonic SettingTectonic Setting

3. Metamorphic RocksMetamorphic Rocks
• Metamorphic means "Metamorphic means "changed formchanged form".".
• Rocks are changed orRocks are changed or
metamorphosed by:metamorphosed by:
1.1. High TemperaturesHigh Temperatures
2.2. High PressuresHigh Pressures
3.3. Chemical ReactionsChemical Reactions
caused bycaused by
chemically-activechemically-active
solutions and hotsolutions and hot
gases.gases.

4. MetamorphismMetamorphism
• The transition of one rock into anotherThe transition of one rock into another
due to a change indue to a change in conditions unlike thoseconditions unlike those
in which it formedin which it formed..
• In response to these new conditions, theIn response to these new conditions, the
rock will gradually change until a state ofrock will gradually change until a state of
equilibriumequilibrium is attained in its newis attained in its new
environment.environment.
• Metamorphism changes the rock’sMetamorphism changes the rock’s
mineralogymineralogy,, texturetexture, and, and chemicalchemical
compositioncomposition..

5. MetamorphismMetamorphism
• Metamorphism progresses incrementallyMetamorphism progresses incrementally
fromfrom low-grade to high-gradelow-grade to high-grade..
• During metamorphism the rockDuring metamorphism the rock mustmust
remain essentially solidremain essentially solid..

6. MetamorphismMetamorphism
• Metamorphic rocks are produced from a
parent rock or protolith.
• Igneous Rocks
• Sedimentary Rocks
• Other Metamorphic Rocks
• Most metamorphic rocks have the same overall
chemical composition as the parent rock from
which they formed.

7. Agents of MetamorphismAgents of Metamorphism
1.1. HeatHeat
2.2. PressurePressure (Stress)(Stress)
3.3. Chemically Active FluidsChemically Active Fluids
• Changes are bothChanges are both texturaltextural andand mineralogicalmineralogical..

8. Agents of MetamorphismAgents of Metamorphism
• HeatHeat
– Most important agent.Most important agent.
– Provides energy to drive chemicalProvides energy to drive chemical
reactions.reactions.

9. Changes Caused by HeatChanges Caused by Heat
• RecrystallizationRecrystallization
of individualof individual
grains.grains.
– Finer particlesFiner particles
coalesce to formcoalesce to form
larger crystalslarger crystals of theof the
same mineralogy.same mineralogy.
– RecrystallizationRecrystallization
results inresults in new, stablenew, stable
mineralsminerals..

10. Sources of HeatSources of Heat
• Two Sources of Heat:
1. Contact Metamorphism – heat from
magma (mantle plumes, upwelling at mid-
ocean ridges, partial melting).

11. Sources of HeatSources of Heat
• Two Sources of Heat:
2. Geothermal Gradient – An increase in
temperature with depth (20o
– 30o
C/km).
• Subduction
(convergent plate
boundaries –
crustal thickening)
• Burial (large
basins – Gulf of
Mexico)

12. Types of PressureTypes of Pressure
• Confining Pressure – increases with depth
due to load from rocks above (analogous
to water pressure).
• “Squeezes” rock
equally in all directions.
• Spaces between mineral
grains close, producing
a more compact, more
dense rock.
• May cause minerals to
to recrystallize into new
minerals with more
compact structure.

13. • Differential Stress (or
Directed Stress) – due to
tectonic forces during
mountain building.
PressurePressure
• Stress is greater in one
direction.
• Rocks become folded.
• Rocks are shortened in the
direction of greatest stress.
• Rocks are thickened in the
the direction perpendicular
to that of greatest stress.
• Brittle Deformation –
fractured, pulverized.
• Ductile Deformation – flow,
flatten, elongate.

14. Chemically Active FluidsChemically Active Fluids
• Chemically Active Fluids –Chemically Active Fluids – ions in solutionions in solution..
• CausesCauses Hydrothermal MetamorphismHydrothermal Metamorphism ––
hot, chemically active, mineral laden watershot, chemically active, mineral laden waters
interact with a surrounding preexistinginteract with a surrounding preexisting
rock (rock (country or host rockcountry or host rock).).
• Most hydrothermal metamorphism takesMost hydrothermal metamorphism takes
place atplace at low pressureslow pressures and relativelyand relatively lowlow
temperaturestemperatures..

15. Chemical MetamorphismChemical Metamorphism
• Dissolves material formDissolves material form
regions of high stressregions of high stress,,
along mineral grainalong mineral grain
boundaries, andboundaries, and
precipitating this materialprecipitating this material
in areas of low stressin areas of low stress..
• As a result, minerals tendAs a result, minerals tend
toto grow longer in thegrow longer in the
direction perpendicular todirection perpendicular to
compressional stresscompressional stress..
• EnhancesEnhances migration ofmigration of
ionsions..
• Aids inAids in recrystallizationrecrystallization
of existing minerals.of existing minerals.

16. Chemical MetamorphismChemical Metamorphism
• Sources ofSources of FluidsFluids::
1.1. GroundwaterGroundwater
• Pore spaces of sedimentary rocksPore spaces of sedimentary rocks
• Fractures in rocksFractures in rocks
1.1. Hydrated mineralsHydrated minerals such as clayssuch as clays
and micasand micas
2.2. Water (volatiles) from magmaWater (volatiles) from magma

17. How Metamorphism Alters RocksHow Metamorphism Alters Rocks
• Textural Changes:Textural Changes:
– TextureTexture – Size, shape, and arrangement– Size, shape, and arrangement
of mineral grains.of mineral grains.
• FoliationFoliation –– PreferredPreferred
orientationorientation ofof
minerals in a sub-minerals in a sub-
parallel to parallelparallel to parallel
alignment caused byalignment caused by
differential stressesdifferential stresses..

18. 1.1. RotationRotation of platy and/or elongatedof platy and/or elongated
minerals grains into a new orientation.minerals grains into a new orientation.
Foliation Forms By…Foliation Forms By…

19. 2.2. RecrystallizationRecrystallization ofof
minerals to form newminerals to form new
grains growing in thegrains growing in the
direction ofdirection of preferredpreferred
orientationorientation..
Foliation Forms By…Foliation Forms By…
• Changing the shapeChanging the shape ofof
equidimensional grains intoequidimensional grains into
elongated shapes that are alignedelongated shapes that are aligned
in a preferred orientation via...in a preferred orientation via...
• Ductile DeformationDuctile Deformation ((flatteningflattening))
1.1. Solid-state plastic flow that involvesSolid-state plastic flow that involves
intracrystalline glidingintracrystalline gliding of individualof individual
units within each grain.units within each grain.
2.2. Dissolving materialDissolving material from areas offrom areas of
high stress and depositing thathigh stress and depositing that
material in locations of low stress.material in locations of low stress.

20. Foliation

21. Types of Foliation and FoliatedTypes of Foliation and Foliated
Metamorphic RocksMetamorphic Rocks
• In order ofIn order of increasing foliationincreasing foliation 
grain-sizegrain-size  grade of metamorphismgrade of metamorphism ::
• Slaty CleavageSlaty Cleavage
• Schistosity
• Schistosity
• Gneissic Banding
• Partial Melting
• SlateSlate
• PhyllitePhyllite
• SchistSchist
• GneissGneiss
• MigmatiteMigmatite

22. Foliated Metamorphic RocksFoliated Metamorphic Rocks
SlateSlate
PhyllitePhyllite
SchistSchist GneissGneiss

24. ClassificationClassification
ofof
MetamorphicMetamorphic
RocksRocks

25. Slaty CleavageSlaty Cleavage
• The parallel
mineral alignment
allows the rock to
split easily into the
flat plates.

26. Slaty CleavageSlaty Cleavage
• ProducesProduces fine-fine-
grained rocksgrained rocks
((SlateSlate).).
• Composed of minuteComposed of minute
micas formed frommicas formed from
clay minerals thatclay minerals that
alter to micasalter to micas
(muscovite, biotite,(muscovite, biotite,
chlorite).chlorite).
• Low-GradeLow-Grade
MetamorphismMetamorphism..
• Low T and P.Low T and P.

27. SchistositySchistosity
• Under more extreme
temperature-pressure
regimes, the minute
mica and chlorite
grains in slate begin to
grow many times
larger.
• Produces rocks with a
phyllitic sheen
(Phyllite).
• Low-GradeLow-Grade
MetamorphismMetamorphism..
• Low T and P.Low T and P.

28. SchistositySchistosity
• SchistositySchistosity is when theseis when these
platy mineralsplaty minerals grow largegrow large
enough to be discernableenough to be discernable
with the unaided eye andwith the unaided eye and
exhibit planar or layeredexhibit planar or layered
structurestructure..
• SchistsSchists contain:contain:
• Platy minerals (micaPlaty minerals (mica
and chlorite)and chlorite)
• Flattened or lens-Flattened or lens-
shaped quartz andshaped quartz and
feldspar grainsfeldspar grains
• Intermediate-GradeIntermediate-Grade
Metamorphism.Metamorphism.
• Intermediate T and P.Intermediate T and P.

29. • Gneissic BandingGneissic Banding – Ion– Ion
migration results in themigration results in the
segregation of mineralssegregation of minerals
intointo compositionalcompositional
bandsbands..
• Dark bandsDark bands == darkdark
silicate minerals (biotitesilicate minerals (biotite
crystals).crystals).
• Light bandsLight bands = light= light
silicate mineralssilicate minerals
(feldspars and quartz).(feldspars and quartz).
• High-GradeHigh-Grade
Metamorphism.Metamorphism.
• High T and P.High T and P.
Gneissic BandingGneissic Banding

30. • Very High-GradeVery High-Grade
MetamorphismMetamorphism
• Very High T and P.Very High T and P.
• Results inResults in partial meltingpartial melting – light– light
minerals = quartz, K-feldspar,minerals = quartz, K-feldspar,
muscovite, and Na-plagioclasemuscovite, and Na-plagioclase
begin to melt.begin to melt.
• FormsForms compositional bands.compositional bands.
• Light bandsLight bands consist ofconsist of partiallypartially
melted light mineralsmelted light minerals..
• TheThe unmelted portionunmelted portion comprisescomprises
thethe dark bandsdark bands composed ofcomposed of
biotite and amphibole.biotite and amphibole.
• The light-colored bands exhibitThe light-colored bands exhibit
tortuous foldstortuous folds..
• Forms rocks calledForms rocks called MigmatitesMigmatites..
• Transitional betweenTransitional between
metamorphic and igneous rocks.metamorphic and igneous rocks.
Ptigmatic FoldingPtigmatic Folding

31. Other Textural ChangesOther Textural Changes
• Porphyroblastic Texture
• Large crystals, called
porphyroblasts, surrounded by a
fine-grained matrix of other
minerals.
• Minerals in the parent rock
recrystallize to form new
minerals.
• Minerals like garnet, staurolite,
and andalusite form a small
number of very large grains.
• Minerals like muscovite, biotite,
and quartz form a large number
of very small grains.

32. Mineralogical ChangesMineralogical Changes
• Clays alter to micas (muscovite, biotite,
chlorite).
• New minerals may grow during
metamorphism.
– CaCO3 + SiO2 = CaSiO3 + CO2
– Wollastonite found in Skarns
• Hydrothermal solutions may add new
elements.
– Ore deposits

33. Metamorphic EnvironmentsMetamorphic Environments
• Contact / ThermalContact / Thermal
• HydrothermalHydrothermal
• BurialBurial
• Fault ZoneFault Zone
• ImpactImpact
These metamorphic environments may overlap.These metamorphic environments may overlap.
Non-Foliated RocksNon-Foliated Rocks
Low PressureLow Pressure
Low to High TemperaturesLow to High Temperatures
Low- to High-Grade MetamorphismLow- to High-Grade Metamorphism
Foliated RocksFoliated Rocks
Low to Very High PressureLow to Very High Pressure
Low to Very High TemperaturesLow to Very High Temperatures
Low- to High-Grade MetamorphismLow- to High-Grade Metamorphism• RegionalRegional

34. Contact/ThermalContact/Thermal
MetamorphismMetamorphism
• Alteration of rock by heat adjacent to hotAlteration of rock by heat adjacent to hot
molten lava or magma.molten lava or magma.
• Low Pressure.Low Pressure.
• Low to HighLow to High
Temperatures.Temperatures.
• Low- to High-GradeLow- to High-Grade
Metamorphism.Metamorphism.
• Produces Non-Produces Non-
FoliatedFoliated
Metamorphic Rocks.Metamorphic Rocks.

35. Metamorphic EnvironmentsMetamorphic Environments
(Plate Tectonics Model)(Plate Tectonics Model)

36. • Forms aForms a zone of alterationzone of alteration, called a, called a contactcontact
aureoleaureole, in the, in the host rockhost rock..
Contact/Thermal MetamorphismContact/Thermal Metamorphism
• Large aureoles consist of
distinct zones of
metamorphism that
consist of high-
temperature minerals
near the magma body
and progressively lower-
temperature minerals
farther away.

37. Contact Metamorphism
along a narrow (approx. 1
meter wide) diabase dike in
the Deep River Basin of
North Carolina.
Diabase weathers tan.
Contact metamorphic
aureole rocks (hornfels) are
gray.
Host rocks are red
siltstones.

38. Hydrothermal MetamorphismHydrothermal Metamorphism
• Hydrothermal Metamorphism –
chemical alteration due to the circulation
of hot, ion-rich fluids through fissures
and cracks in the host rock.
• Closely associated withClosely associated with igneousigneous
activityactivity andand contact metamorphismcontact metamorphism..
• Low Pressure.Low Pressure.
• Low to High Temperatures.Low to High Temperatures.
• Low- to High-GradeLow- to High-Grade Metamorphism.Metamorphism.
• Produces Non-Foliated MetamorphicProduces Non-Foliated Metamorphic
Rocks.Rocks.
• Often precipitate economicallyOften precipitate economically
importantimportant mineral and ore depositsmineral and ore deposits..

39. Metamorphic EnvironmentsMetamorphic Environments
(Plate Tectonics Model)(Plate Tectonics Model)

40. Burial MetamorphismBurial Metamorphism
• Burial MetamorphismBurial Metamorphism is associated withis associated with
very thick accumulations of sedimentaryvery thick accumulations of sedimentary
strata in a subsiding basin.strata in a subsiding basin.
• Caused byCaused by confining pressureconfining pressure and theand the
geothermal gradientgeothermal gradient..
• Low Pressure.Low Pressure.
• Low Temperatures.Low Temperatures.
• Low-GradeLow-Grade
Metamorphism.Metamorphism.
• Produces Non-Produces Non-
FoliatedFoliated
MetamorphicMetamorphic
Rocks.Rocks.

41. Metamorphic EnvironmentsMetamorphic Environments
(Plate Tectonics Model)(Plate Tectonics Model)

42. Fault Zone MetamorphismFault Zone Metamorphism
• Movement along fault zone fractures pulverizes rock.Movement along fault zone fractures pulverizes rock.
• Near the surface, rocks to deform in aNear the surface, rocks to deform in a brittlebrittle manner.manner.
• Deep in the fault zones where temperature andDeep in the fault zones where temperature and
pressure cause the rocks to deform in apressure cause the rocks to deform in a ductileductile
manner.manner.
• Low Pressure.Low Pressure.
• Low Temperatures.Low Temperatures.
• Low-GradeLow-Grade
Metamorphism.Metamorphism.
• Produces Non- to Weakly-Produces Non- to Weakly-
Foliated MetamorphicFoliated Metamorphic
Rocks.Rocks.

43. • Fault BrecciasFault Breccias are rocks composed ofare rocks composed of broken and crushed rockbroken and crushed rock
fragmentsfragments that develop shallow in the faults where the rock isthat develop shallow in the faults where the rock is
“ground up”.“ground up”.
• Fault Gouge is a soft, uncemented claylike material formed in
shallow fault zones.
Fault Zone MetamorphismFault Zone Metamorphism

44. • MylonitesMylonites occur deep in the fault zones whereoccur deep in the fault zones where
temperature and pressure cause the rocks to deform intemperature and pressure cause the rocks to deform in
a ductile manner forminga ductile manner forming elongated grainselongated grains that give thethat give the
rock a lineated appearance.rock a lineated appearance.
Fault Zone MetamorphismFault Zone Metamorphism

45. Impact MetamorphismImpact Metamorphism
• Impact or Shock Metamorphism occurs when high
speed projectiles called meteorites (fragments of
comets or asteroids) strike the Earth’s surface.

46. • Often the highOften the high
pressure forms ofpressure forms of
quartz (quartz (coesitecoesite))
and carbonand carbon
((diamonddiamond) are) are
found.found.
Impact MetamorphismImpact Metamorphism
Tektites – Impact ejecta composed of beads of silica-rich glassTektites – Impact ejecta composed of beads of silica-rich glass
aerodynamically shaped during flight. Ejected great distancesaerodynamically shaped during flight. Ejected great distances
from the impact crater.from the impact crater.
• Upon impact, the energy is transferred to into the surroundingUpon impact, the energy is transferred to into the surrounding
rocks asrocks as heat energy and shock wavesheat energy and shock waves thatthat pulverizepulverize,, shattershatter, and, and
sometimessometimes meltmelt the surrounding rock.the surrounding rock.
• ProducesProduces impactilesimpactiles – mixtures of– mixtures of fused rock fragmentsfused rock fragments plusplus
glass-rich ejectaglass-rich ejecta that resemble volcanic bombs.that resemble volcanic bombs.

47. Regional MetamorphismRegional Metamorphism
• Regional MetamorphismRegional Metamorphism is ais a large-scalelarge-scale,,
intenseintense metamorphism that occurs duringmetamorphism that occurs during
mountain-building (mountain-building (orogenicorogenic) events.) events.
• Associated with compressional forces inAssociated with compressional forces in
convergent plate boundariesconvergent plate boundaries..
• Rocks are subjected toRocks are subjected to
high temperatureshigh temperatures andand
differential pressuresdifferential pressures..
• CausesCauses faultingfaulting andand
foldingfolding of the crustof the crust
causing.causing.
• ShorteningShortening andand
thickeningthickening of the crust.of the crust.

48. Regional MetamorphismRegional Metamorphism
• Produces the majority of metamorphic
rocks.
Low GradeLow Grade High Grade
• Low to Very HighLow to Very High
Pressures.Pressures.
• Low to Very HighLow to Very High
Temperatures.Temperatures.
• Low- to Very High-Low- to Very High-
GradeGrade
Metamorphism.Metamorphism.
• Produces FoliatedProduces Foliated
Metamorphic Rocks.Metamorphic Rocks.
Increasing metamorphism

49. RegionalRegional
MetamorphismMetamorphism
– Cottonwood– Cottonwood
Mountains,Mountains,
Death ValleyDeath Valley
CaliforniaCalifornia

50. Metamorphic Index MineralsMetamorphic Index Minerals
• Metamorphic Index MineralsMetamorphic Index Minerals formform
during metamorphism, under specificduring metamorphism, under specific
pressure and temperature conditions.pressure and temperature conditions.
• These minerals can be used as toThese minerals can be used as to
determine the metamorphic pressuredetermine the metamorphic pressure
and temperature conditions underand temperature conditions under
which they formedwhich they formed..

51. Metamorphic Index MineralsMetamorphic Index Minerals
• ChloriteChlorite andand muscovitemuscovite form at relativelyform at relatively lowlow
temperatures.temperatures.
• BiotiteBiotite andand garnetgarnet form at somewhatform at somewhat higher temperatureshigher temperatures
and pressuresand pressures..
• StauroliteStaurolite andand kyanitekyanite form atform at intermediate to highintermediate to high
temperatures and pressurestemperatures and pressures..
• SillimaniteSillimanite forms at theforms at the highesthighest temperatures andtemperatures and
pressures.pressures.

54. Metamorphic EnvironmentsMetamorphic Environments
(Plate Tectonics Model)(Plate Tectonics Model)

55. Glossary ofGlossary of
Metamorphic RocksMetamorphic Rocks

56. Classification of Metamorphic RocksClassification of Metamorphic Rocks
• Metamorphic rocks are separated intoMetamorphic rocks are separated into
two groups on the basis oftwo groups on the basis of texturetexture..
1.1. Non-FoliatedNon-Foliated (or granular) – Metamorphic(or granular) – Metamorphic
rocks are those which are composed ofrocks are those which are composed of
equidimensional grainsequidimensional grains such as quartz orsuch as quartz or
calcite.calcite.
• There isThere is no preferred orientationno preferred orientation..
• The grains form a mosaic.The grains form a mosaic.

57. Non-Foliated Metamorphic RocksNon-Foliated Metamorphic Rocks
MarbleMarble – Composed of finely- to– Composed of finely- to
coarsely-crystalline calcite or dolomite.coarsely-crystalline calcite or dolomite.
Derived from the metamorphism ofDerived from the metamorphism of
limestone or dolostone. Commonlylimestone or dolostone. Commonly
white or gray. May be pink.white or gray. May be pink.
QuartziteQuartzite – Composed of finely- to– Composed of finely- to
coarsely-crystalline quartz. Derivedcoarsely-crystalline quartz. Derived
from the metamorphism of quartzfrom the metamorphism of quartz
sandstone. Variable in color.sandstone. Variable in color.

58. • HornfelsHornfels
– Fine-grained.Fine-grained.
– Low-grade metamorphosed shales andLow-grade metamorphosed shales and
siltstones.siltstones.
– Metamorphosed by contact metamorphism.Metamorphosed by contact metamorphism.
Non-Foliated Metamorphic RocksNon-Foliated Metamorphic Rocks

59. • Metamorphic rocks areMetamorphic rocks are
separated into two groupsseparated into two groups
on the basis ofon the basis of texturetexture..
2.2. FoliatedFoliated – Laminated– Laminated
structure in astructure in a
metamorphic rockmetamorphic rock
resulting from theresulting from the
parallel alignment ofparallel alignment of
sheet-like mineralssheet-like minerals
(usually micas).(usually micas).
Muscovite
Biotite
Classification of Metamorphic RocksClassification of Metamorphic Rocks

60. Foliated Metamorphic RocksFoliated Metamorphic Rocks
• Fine-grained.Fine-grained.
• Composed ofComposed of minute mica flakesminute mica flakes too smalltoo small
to be visible with the unaided eye.to be visible with the unaided eye.
• Dull appearance. Variable in color.Dull appearance. Variable in color.
• Excellent slaty cleavage.Excellent slaty cleavage.
• Low-grade metamorphismLow-grade metamorphism ofof shaleshale,,
mudstonemudstone,, siltstonesiltstone, or, or volcanic ashvolcanic ash..
SlateSlate PhyllitePhyllite
• Fine-grained.Fine-grained.
• Composed ofComposed of minute mica flakesminute mica flakes
larger than in slatelarger than in slate but still too smallbut still too small
to be visible with the unaided eye.to be visible with the unaided eye.
• Glossy sheen.Glossy sheen.
• Wavy appearance.Wavy appearance.
• Low-grade metamorphismLow-grade metamorphism ofof shaleshale,,
mudstonemudstone,, siltstonesiltstone, or, or volcanic ashvolcanic ash..

61. Foliated Metamorphic RocksFoliated Metamorphic Rocks
SchistSchist
• Medium- to coarse-grained.
• Composed of platy minerals – mica flakes
visible with the unaided eye that form visible
planar alignment – schistosity.
• Often contain accessory minerals (garnet,
staurolite, andalusite, kyanite)
• Medium- to high-grade metamorphism of
shale, mudstone, siltstone, or volcanic ash
during major mountain building episodes
GneissGneiss
• Medium- to coarse-grained.
• Compositionally banded, in which granular and
elongated minerals predominate.
• Light bands = quartz, muscovite, K-feldspar, and
plagioclase feldspar.
• Dark bands = biotite and amphibole.
• May contain large accessory minerals
• High-grade metamorphism of igneous or sedimentary
rocks during major mountain building episodes.

62. • MigmatitesMigmatites
• Medium- to coarse-grained.Medium- to coarse-grained.
• Compositionally bandedCompositionally banded, in which granular, in which granular
and elongated (as opposed to platy) mineralsand elongated (as opposed to platy) minerals
predominate.predominate.
• Product ofProduct of Partial MeltingPartial Melting – Temperatures– Temperatures
are reached where some minerals begin toare reached where some minerals begin to
melt (Quartz, K-Feldspar).melt (Quartz, K-Feldspar).
• When partially melted material cools, theWhen partially melted material cools, the
igneous material forms the light bandsigneous material forms the light bands
composed of quartz, muscovite, potassiumcomposed of quartz, muscovite, potassium
feldspar, and plagioclase feldspar.feldspar, and plagioclase feldspar.
• The unmelted portion comprises the darkThe unmelted portion comprises the dark
bands composed of biotite and amphibole.bands composed of biotite and amphibole.
• The light-colored bands exhibitThe light-colored bands exhibit tortuous foldstortuous folds..
• Very high-grade metamorphism of igneous orVery high-grade metamorphism of igneous or
sedimentary rocks.sedimentary rocks.
• Transitional between metamorphic andTransitional between metamorphic and
igneous rocks.igneous rocks.
Migmatite –Migmatite –
Near Great Falls, MDNear Great Falls, MD
Foliated Metamorphic RocksFoliated Metamorphic Rocks