Metamorphic RocksMetamorphic Rocks

Chapter 8Chapter 8

What Can Metamorphic What Can Metamorphic Minerals/Rocks Tell Us?Minerals/Rocks Tell Us?

1.1. Type of Metamorphism Type of Metamorphism 2.2. Temperature and Pressure Regimes of MetamorphismTemperature and Pressure Regimes of Metamorphism3.3. Depth and Intensity of MetamorphismDepth and Intensity of Metamorphism4.4. Environments prior to MetamorphismEnvironments prior to Metamorphism5.5. History of the Formation of Mountain RangesHistory of the Formation of Mountain Ranges6.6. Timing of Mountain FormationTiming of Mountain Formation7.7. Types and Directions of Stress Types and Directions of Stress 8.8. Crustal Deformation – Folding and Faulting RegimesCrustal Deformation – Folding and Faulting Regimes9.9. Plate MovementsPlate Movements10.10. Tectonic SettingTectonic Setting

Metamorphic RocksMetamorphic Rocks• Metamorphic means "Metamorphic means "changed formchanged form".".

• Rocks are changed or Rocks are changed or metamorphosed by: metamorphosed by:

1.1. High Temperatures High Temperatures 2.2. High Pressures High Pressures 3.3. Chemical ReactionsChemical Reactions

caused by caused by chemically-active chemically-active solutions and hot solutions and hot gases. gases.

MetamorphismMetamorphism

• The transition of one rock into another The transition of one rock into another due to a change in due to a change in conditions unlike those conditions unlike those in which it formedin which it formed..

• In response to these new conditions, the In response to these new conditions, the rock will gradually change until a state of rock will gradually change until a state of equilibriumequilibrium is attained in its new is attained in its new environment.environment.

• Metamorphism changes the rock’s Metamorphism changes the rock’s mineralogymineralogy, , texturetexture, and , and chemical chemical compositioncomposition..

MetamorphismMetamorphism

• Metamorphism progresses incrementally Metamorphism progresses incrementally from from low-grade to high-gradelow-grade to high-grade. .

• During metamorphism the rock During metamorphism the rock must must remain essentially solidremain essentially solid..

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.

Agents of MetamorphismAgents of Metamorphism

1.1. HeatHeat

2.2. PressurePressure (Stress) (Stress)

3.3. Chemically Active FluidsChemically Active Fluids

• Changes are both Changes are both texturaltextural and and mineralogicalmineralogical..

Agents of MetamorphismAgents of Metamorphism

• HeatHeat– Most important agent. Most important agent.

– Provides energy to drive chemical Provides energy to drive chemical reactions.reactions.

Changes Caused by HeatChanges Caused by Heat

• RecrystallizationRecrystallization of individual of individual grains.grains.– Finer particles Finer particles

coalesce to form coalesce to form larger crystalslarger crystals of the of the same mineralogy.same mineralogy.

– Recrystallization Recrystallization results in results in new, stable new, stable mineralsminerals..

Sources of HeatSources of Heat

• Two Sources of Heat:1. Contact Metamorphism – heat from

magma (mantle plumes, upwelling at mid-ocean ridges, partial melting).

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)

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.

• 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.

Chemically Active FluidsChemically Active Fluids

• Chemically Active Fluids – Chemically Active Fluids – ions in solutionions in solution..

• Causes Causes Hydrothermal Metamorphism Hydrothermal Metamorphism – – hot, chemically active, mineral laden waters hot, chemically active, mineral laden waters interact with a surrounding preexisting interact with a surrounding preexisting rock (rock (country or host rockcountry or host rock). ).

• Most hydrothermal metamorphism takes Most hydrothermal metamorphism takes place at place at low pressureslow pressures and relatively and relatively low low temperaturestemperatures..

Chemical MetamorphismChemical Metamorphism

• Dissolves material form Dissolves material form regions of high stressregions of high stress, , along mineral grain along mineral grain boundaries, and boundaries, and precipitating this material precipitating this material in areas of low stressin areas of low stress..

• As a result, minerals tend As a result, minerals tend to to grow longer in the grow longer in the direction perpendicular to direction perpendicular to compressional stresscompressional stress..

• Enhances Enhances migration of migration of ionsions..

• Aids in Aids in recrystallizationrecrystallization of existing minerals.of existing minerals.

Chemical MetamorphismChemical Metamorphism

• Sources of Sources of FluidsFluids::1.1. GroundwaterGroundwater• Pore spaces of sedimentary rocksPore spaces of sedimentary rocks• Fractures in rocksFractures in rocks

2.2. Hydrated mineralsHydrated minerals such as clays such as clays and micasand micas

3.3. Water (volatiles) from magma Water (volatiles) from magma

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 – – Preferred Preferred orientationorientation of of minerals in a sub-minerals in a sub-parallel to parallel parallel to parallel alignment caused by alignment caused by differential stressesdifferential stresses..

1.1. Rotation Rotation of platy and/or elongated of platy and/or elongated minerals grains into a new orientation.minerals grains into a new orientation.

Foliation Forms By…Foliation Forms By…

2.2. Recrystallization Recrystallization of of minerals to form new minerals to form new grains growing in the grains growing in the direction of direction of preferred preferred orientationorientation..

Foliation Forms By…Foliation Forms By…

• Changing the shapeChanging the shape of of equidimensional grains into equidimensional grains into elongated shapes that are aligned elongated 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 involves Solid-state plastic flow that involves

intracrystalline glidingintracrystalline gliding of individual of individual units within each grain.units within each grain.

2.2. Dissolving materialDissolving material from areas of from areas of high stress and depositing that high stress and depositing that material in locations of low stress.material in locations of low stress.

Foliation

Types of Foliation and Foliated Types of Foliation and Foliated Metamorphic RocksMetamorphic Rocks

• In order of In order of increasing foliation increasing foliation grain-size grain-size grade of metamorphismgrade of metamorphism : :• Slaty CleavageSlaty Cleavage

• Schistosity

• Schistosity

•Gneissic Banding

• Partial Melting

• SlateSlate

• Phyllite Phyllite

• Schist Schist

• Gneiss Gneiss

•MigmatiteMigmatite

Foliated Metamorphic RocksFoliated Metamorphic Rocks

SlateSlatePhyllitePhyllite

SchistSchist GneissGneiss

Classification Classification of of

Metamorphic Metamorphic RocksRocks

Slaty CleavageSlaty Cleavage

• The parallel mineral alignment allows the rock to split easily into the flat plates.

Slaty CleavageSlaty Cleavage• Produces Produces fine-fine-

grained rocksgrained rocks ((SlateSlate).).

• Composed of minute Composed of minute micas formed from micas formed from clay minerals that clay minerals that alter to micasalter to micas (muscovite, biotite, (muscovite, biotite, chlorite).chlorite).

• Low-Grade Low-Grade MetamorphismMetamorphism..

• Low T and P.Low T and P.

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-Grade Low-Grade MetamorphismMetamorphism..

• Low T and P.Low T and P.

SchistositySchistosity• Schistosity Schistosity is when these is when these

platy minerals platy minerals grow largegrow large enough to be discernable enough to be discernable with the unaided eye and with the unaided eye and exhibit planar or layered exhibit planar or layered structurestructure..

• Schists Schists contain: contain: • Platy minerals (mica Platy minerals (mica

and chlorite)and chlorite)• Flattened or lens-Flattened or lens-

shaped quartz and shaped quartz and feldspar grainsfeldspar grains

• Intermediate-Grade Intermediate-Grade Metamorphism.Metamorphism.

• Intermediate T and P.Intermediate T and P.

• Gneissic BandingGneissic Banding – Ion – Ion migration results in the migration results in the segregation of minerals segregation of minerals into into compositional compositional bandsbands..

• Dark bandsDark bands = = dark dark silicate minerals (biotite silicate minerals (biotite crystals). crystals).

• Light bandsLight bands = light = light silicate minerals silicate minerals (feldspars and quartz). (feldspars and quartz).

• High-Grade High-Grade Metamorphism.Metamorphism.

• High T and P.High T and P.

Gneissic BandingGneissic Banding

• Very High-Grade Very High-Grade MetamorphismMetamorphism

• Very High T and P.Very High T and P.• Results in Results in partial meltingpartial melting – light – light

minerals = quartz, K-feldspar, minerals = quartz, K-feldspar, muscovite, and Na-plagioclase muscovite, and Na-plagioclase begin to melt.begin to melt.

• Forms Forms compositional bands. compositional bands. • Light bands Light bands consist ofconsist of partially partially

melted light mineralsmelted light minerals. . • The The unmelted portionunmelted portion comprises comprises

the the dark bandsdark bands composed of composed of biotite and amphibole.biotite and amphibole.

• The light-colored bands exhibit The light-colored bands exhibit tortuous foldstortuous folds..

• Forms rocks called Forms rocks called MigmatitesMigmatites..• Transitional between Transitional between

metamorphic and igneous rocks.metamorphic and igneous rocks.

Ptigmatic FoldingPtigmatic Folding

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.

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

Metamorphic EnvironmentsMetamorphic Environments

• Contact / ThermalContact / Thermal• HydrothermalHydrothermal• BurialBurial• Fault ZoneFault Zone• Impact Impact

These metamorphic environments may overlap.These metamorphic environments may overlap.

Non-Foliated RocksNon-Foliated RocksLow PressureLow PressureLow to High TemperaturesLow to High TemperaturesLow- to High-Grade MetamorphismLow- to High-Grade Metamorphism

Foliated RocksFoliated RocksLow to Very High PressureLow to Very High PressureLow to Very High TemperaturesLow to Very High TemperaturesLow- to High-Grade MetamorphismLow- to High-Grade Metamorphism• RegionalRegional

Contact/Thermal Contact/Thermal MetamorphismMetamorphism

• Alteration of rock by heat adjacent to hot Alteration of rock by heat adjacent to hot molten lava or magma. molten lava or magma.

• Low Pressure.Low Pressure.• Low to High Low to High

Temperatures.Temperatures.• Low- to High-Grade Low- to High-Grade

Metamorphism.Metamorphism.• Produces Non-Produces Non-

Foliated Foliated Metamorphic Rocks.Metamorphic Rocks.

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

• Forms a Forms a zone of alterationzone of alteration, called a, called a contact contact 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.

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.

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 with Closely associated with igneous igneous activityactivity and and 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 Metamorphic Produces Non-Foliated Metamorphic

Rocks.Rocks.• Often precipitate economically Often precipitate economically

important important mineral and ore depositsmineral and ore deposits..

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

Burial MetamorphismBurial Metamorphism• Burial Metamorphism Burial Metamorphism is associated with is associated with

very thick accumulations of sedimentary very thick accumulations of sedimentary strata in a subsiding basin.strata in a subsiding basin.

• Caused byCaused by confining pressure confining pressure and theand the geothermal gradientgeothermal gradient..

• Low Pressure.Low Pressure.• Low Temperatures.Low Temperatures.• Low-Grade Low-Grade

Metamorphism.Metamorphism.• Produces Non-Produces Non-

Foliated Foliated Metamorphic Metamorphic Rocks.Rocks.

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

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 a Near the surface, rocks to deform in a brittle brittle manner. manner. • Deep in the fault zones where temperature and Deep in the fault zones where temperature and

pressure cause the rocks to deform in a pressure cause the rocks to deform in a ductile ductile manner.manner.

• Low Pressure.Low Pressure.• Low Temperatures.Low Temperatures.• Low-Grade Low-Grade

Metamorphism.Metamorphism.• Produces Non- to Weakly-Produces Non- to Weakly-

Foliated Metamorphic Foliated Metamorphic Rocks.Rocks.

• Fault BrecciasFault Breccias are rocks composed of are rocks composed of broken and crushed rock broken and crushed rock fragmentsfragments that develop shallow in the faults where the rock is that 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

• MylonitesMylonites occur deep in the fault zones where occur deep in the fault zones where temperature and pressure cause the rocks to deform in temperature and pressure cause the rocks to deform in a ductile manner forming a ductile manner forming elongated grainselongated grains that give the that give the rock a lineated appearance.rock a lineated appearance.

Fault Zone MetamorphismFault Zone Metamorphism

Impact MetamorphismImpact Metamorphism• Impact or Shock Metamorphism occurs when high

speed projectiles called meteorites (fragments of comets or asteroids) strike the Earth’s surface.

• Often the high Often the high pressure forms of pressure forms of quartz (quartz (coesitecoesite) ) and carbon and carbon ((diamonddiamond) are ) are found.found.

Impact MetamorphismImpact Metamorphism

Tektites – Impact ejecta composed of beads of silica-rich glass Tektites – Impact ejecta composed of beads of silica-rich glass aerodynamically shaped during flight. Ejected great distances aerodynamically shaped during flight. Ejected great distances

from the impact crater.from the impact crater.

• Upon impact, the energy is transferred to into the surrounding Upon impact, the energy is transferred to into the surrounding rocks as rocks as heat energy and shock wavesheat energy and shock waves that that pulverizepulverize, , shattershatter, and , and sometimes sometimes meltmelt the surrounding rock. the surrounding rock.

• Produces Produces impactilesimpactiles – mixtures of – mixtures of fused rock fragmentsfused rock fragments plus plus glass-rich ejectaglass-rich ejecta that resemble volcanic bombs. that resemble volcanic bombs.

Regional MetamorphismRegional Metamorphism• Regional Metamorphism Regional Metamorphism is a is a large-scalelarge-scale, ,

intenseintense metamorphism that occurs during metamorphism 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 to Rocks are subjected to high temperatureshigh temperatures and and differential pressuresdifferential pressures. .

• Causes Causes faulting faulting andand foldingfolding of the crust of the crust causing.causing.

• ShorteningShortening and and thickeningthickening of the crust. of the crust.

Regional MetamorphismRegional Metamorphism• Produces the majority of metamorphic

rocks.

Low GradeLow Grade High Grade

• Low to Very High Low to Very High Pressures. Pressures.

• Low to Very High Low to Very High Temperatures. Temperatures.

• Low- to Very High-Low- to Very High-Grade Grade Metamorphism.Metamorphism.

• Produces Foliated Produces Foliated Metamorphic Rocks.Metamorphic Rocks.

Increasing metamorphism

Regional Regional Metamorphism Metamorphism – Cottonwood – Cottonwood

Mountains, Mountains, Death Valley Death Valley

CaliforniaCalifornia

Metamorphic Index Minerals Metamorphic Index Minerals

• Metamorphic Index MineralsMetamorphic Index Minerals form form during metamorphism, under specific during metamorphism, under specific pressure and temperature conditions.pressure and temperature conditions.

• These minerals can be used as to These minerals can be used as to determine the metamorphic pressure determine the metamorphic pressure and temperature conditions under and temperature conditions under which they formedwhich they formed..

Metamorphic Index Minerals Metamorphic Index Minerals • ChloriteChlorite and and muscovitemuscovite form at relatively form at relatively lowlow temperatures. temperatures. • BiotiteBiotite and and garnetgarnet form at somewhat form at somewhat higher temperatures higher temperatures

and pressuresand pressures. . • StauroliteStaurolite and and kyanitekyanite form at form at intermediate to high intermediate to high

temperatures and pressurestemperatures and pressures. . • SillimaniteSillimanite forms at the forms at the highesthighest temperatures and pressures. temperatures and pressures.

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

Glossary of Glossary of Metamorphic RocksMetamorphic Rocks

Classification of Metamorphic RocksClassification of Metamorphic Rocks

• Metamorphic rocks are separated into Metamorphic rocks are separated into two groups on the basis of two groups on the basis of texturetexture. .

1.1. Non-FoliatedNon-Foliated (or granular) – Metamorphic (or granular) – Metamorphic rocks are those which are composed of rocks are those which are composed of equidimensional grainsequidimensional grains such as quartz or such as quartz or calcite. calcite. • There is There is no preferred orientationno preferred orientation. .

• The grains form a mosaic. The grains form a mosaic.

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 of Derived from the metamorphism of limestone or dolostone. Commonly limestone 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. Derived coarsely-crystalline quartz. Derived from the metamorphism of quartz from the metamorphism of quartz sandstone. Variable in color.sandstone. Variable in color.

• HornfelsHornfels – Fine-grained. Fine-grained. – Low-grade metamorphosed shales and Low-grade metamorphosed shales and

siltstones.siltstones. – Metamorphosed by contact metamorphism.Metamorphosed by contact metamorphism.

Non-Foliated Metamorphic RocksNon-Foliated Metamorphic Rocks

• Metamorphic rocks are Metamorphic rocks are separated into two groups separated into two groups on the basis of on the basis of texturetexture. .

2.2. Foliated Foliated – Laminated – Laminated structure in a structure in a metamorphic rock metamorphic rock resulting from the resulting from the parallel alignment of parallel alignment of sheet-like minerals sheet-like minerals (usually micas). (usually micas).

Muscovite

Biotite

Classification of Metamorphic RocksClassification of Metamorphic Rocks

Foliated Metamorphic RocksFoliated Metamorphic Rocks

• Fine-grained.Fine-grained.

• Composed of Composed of minute mica flakesminute mica flakes too small too 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 of of shaleshale, , mudstonemudstone, , siltstonesiltstone, or , or volcanic ashvolcanic ash..

SlateSlate PhyllitePhyllite

• Fine-grained.Fine-grained.

• Composed of Composed of minute mica flakes minute mica flakes larger than in slatelarger than in slate but still too small but 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 of of shaleshale, , mudstonemudstone, , siltstonesiltstone, or , or volcanic ashvolcanic ash..

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.

• MigmatitesMigmatites• Medium- to coarse-grained.Medium- to coarse-grained.

• Compositionally bandedCompositionally banded, in which granular , in which granular and elongated (as opposed to platy) minerals and elongated (as opposed to platy) minerals predominate.predominate.

• Product of Product of Partial MeltingPartial Melting – Temperatures – Temperatures are reached where some minerals begin to are reached where some minerals begin to melt (Quartz, K-Feldspar).melt (Quartz, K-Feldspar).

• When partially melted material cools, the When partially melted material cools, the igneous material forms the light bands igneous material forms the light bands composed of quartz, muscovite, potassium composed of quartz, muscovite, potassium feldspar, and plagioclase feldspar. feldspar, and plagioclase feldspar.

• The unmelted portion comprises the dark The unmelted portion comprises the dark bands composed of biotite and amphibole.bands composed of biotite and amphibole.

• The light-colored bands exhibit The light-colored bands exhibit tortuous foldstortuous folds..

• Very high-grade metamorphism of igneous or Very high-grade metamorphism of igneous or sedimentary rocks.sedimentary rocks.

• Transitional between metamorphic and Transitional between metamorphic and igneous rocks.igneous rocks.

Migmatite – Migmatite – Near Great Falls, MDNear Great Falls, MD

Foliated Metamorphic RocksFoliated Metamorphic Rocks