chapter 21: metamorphism rocks as chemical systems (ch. 5)rocks as chemical systems (ch. 5) a...

Chapter 21: MetamorphismChapter 21: Metamorphism• Rocks as chemical systems (Ch. 5)Rocks as chemical systems (Ch. 5)

• a particular assemblage of coexisting phases a particular assemblage of coexisting phases (thermodynamic equilibrium and the phase rule)(thermodynamic equilibrium and the phase rule)

• A basaltic composition can be either:A basaltic composition can be either: MeltMelt CpxCpx + + plagplag ( ( olivine, ilmenite…) olivine, ilmenite…) Or any combination of melt + minerals along the Or any combination of melt + minerals along the

liquid line of descentliquid line of descent If uplifted and eroded If uplifted and eroded surface, will surface, will weatherweather a a

combinations of combinations of claysclays, , oxidesoxides……

• The IUGS-SCMR has proposed the following definition The IUGS-SCMR has proposed the following definition of metamorphism:of metamorphism:

““Metamorphism is a Metamorphism is a subsolidussubsolidus process leading to process leading to changes in mineralogy and/or texture (for example grain changes in mineralogy and/or texture (for example grain size) and often in chemical composition in a rock. These size) and often in chemical composition in a rock. These changes are due to physical and/or chemical conditions changes are due to physical and/or chemical conditions that differ from those normally occurring at the surface that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial below this surface. They may coexist with partial melting.”melting.”

Chapter 21: MetamorphismChapter 21: Metamorphism

The Limits of MetamorphismThe Limits of Metamorphism• Low-temperatureLow-temperature limit grades into limit grades into diagenesisdiagenesis

The boundary is somewhat arbitraryThe boundary is somewhat arbitrary Diagenetic/weathering processes are Diagenetic/weathering processes are

indistinguishable from metamorphic indistinguishable from metamorphic Metamorphism begins in the range of 100-150Metamorphism begins in the range of 100-150ooC for C for

the more unstable types of the more unstable types of protolithprotolith Some zeolites are considered diagenetic and others Some zeolites are considered diagenetic and others

metamorphic – pretty arbitrarymetamorphic – pretty arbitrary

The Limits of MetamorphismThe Limits of Metamorphism• High-temperatureHigh-temperature limit grades into limit grades into meltingmelting • Over the melting range solids and liquids coexistOver the melting range solids and liquids coexist• If we heat a metamorphic rock until it melts, at If we heat a metamorphic rock until it melts, at

what point in the melting process does it become what point in the melting process does it become “igneous”? “igneous”?

• Xenoliths, restites, and other enclaves are Xenoliths, restites, and other enclaves are considered part of the igneous realm because melt considered part of the igneous realm because melt is dominant, but the distinction is certainly vague is dominant, but the distinction is certainly vague and disputableand disputable

• MigmatitesMigmatites (“mixed rocks”) are gradational (“mixed rocks”) are gradational

Metamorphic Agents and ChangesMetamorphic Agents and Changes

• TemperatureTemperature: typically the most : typically the most important factor in important factor in metamorphismmetamorphism

Figure 1-9. Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311.

Metamorphic Agents and ChangesMetamorphic Agents and ChangesIncreasing temperature has several effectsIncreasing temperature has several effects

1) 1) Promotes recrystallizationPromotes recrystallization increased grain increased grain sizesize Larger surface/volume ratio of a mineral Larger surface/volume ratio of a mineral

lower stabilitylower stability Increasing temperature eventually overcomes Increasing temperature eventually overcomes

kinetic barriers to recrystallization, and fine kinetic barriers to recrystallization, and fine aggregates coalesce to larger grainsaggregates coalesce to larger grains

Metamorphic Agents and ChangesMetamorphic Agents and ChangesIncreasing temperature has several effectsIncreasing temperature has several effects

2) 2) DriveDrive reactions reactions that consume unstable that consume unstable mineral(s) and produces new minerals that are mineral(s) and produces new minerals that are stable under the new conditionsstable under the new conditions

3) 3) Overcomes kinetic barriers that might otherwise Overcomes kinetic barriers that might otherwise preclude the attainment of equilibriumpreclude the attainment of equilibrium

Metamorphic Agents and ChangesMetamorphic Agents and Changes• PressurePressure

““Normal” gradients may be perturbed in several Normal” gradients may be perturbed in several ways, typically:ways, typically:

High T/P geotherms in areas of plutonic High T/P geotherms in areas of plutonic activity or riftingactivity or rifting

Low T/P geotherms in subduction zonesLow T/P geotherms in subduction zones

Figure 21-1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.

Metamorphic Agents and ChangesMetamorphic Agents and Changes• Metamorphic gradeMetamorphic grade: a general increase in : a general increase in

degree of metamorphism without specifying degree of metamorphism without specifying the exact relationship between temperature the exact relationship between temperature and pressureand pressure


• Lithostatic pressureLithostatic pressure is uniform stress is uniform stress (hydrostatic) (hydrostatic)• Deviatoric stressDeviatoric stress = unequal pressure in different = unequal pressure in different

directionsdirections• Deviatoric stress can be resolved into three Deviatoric stress can be resolved into three

mutually perpendicular stress (mutually perpendicular stress () components:) components:11 is the is the maximummaximum principal stress principal stress

22 is an is an intermediateintermediate principal stress principal stress

33 is the is the minimumminimum principal stress principal stress

• In In hydrostatichydrostatic situations all three are equal situations all three are equal


• StressStress is an applied force acting on a rock (over a is an applied force acting on a rock (over a particular cross-sectional area)particular cross-sectional area)

• StrainStrain is the response of the rock to an applied is the response of the rock to an applied stress (= yielding or stress (= yielding or deformationdeformation))

• Deviatoric stress affects the textures and Deviatoric stress affects the textures and structures, structures, but not the equilibrium mineral but not the equilibrium mineral assemblageassemblage

• Strain energy may overcome kinetic barriers to Strain energy may overcome kinetic barriers to reactionsreactions


Deviatoric stresses come in three principal types: Deviatoric stresses come in three principal types: TensionTension CompressionCompression ShearShear

Tension:Tension: 33 is negative, and the resulting is negative, and the resulting strain is strain is

extensionextension, or pulling apart, or pulling apart original shapeoriginal shape strain strain ellipsoidellipsoid

Figure 21-2. The three main types of deviatoric stress with an example of possible resulting structures. a. Tension, in which one stress in negative. “Tension fractures” may open normal to the extension direction and become filled with mineral precipitates. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

1

3

Compression:Compression: 11 is dominant, is dominant, foldingfolding or more or more

homogenous homogenous flatteningflattening

Figure 21-2. The three main types of deviatoric stress with an example of possible resulting structures. b. Compression, causing flattening or folding. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

1

3

• FoliationFoliation is a common result, which allows us to is a common result, which allows us to estimate the estimate the orientationorientation of of 11

11 > > 22 = = 3 3 foliation and no lineation foliation and no lineation

11 = = 22 > > 3 3 lineation and no foliation lineation and no foliation

11 > > 22 > > 3 3 both foliation and lineation both foliation and lineationFigure 21-3. Flattening of a ductile homogeneous sphere (a) containing randomly oriented flat disks or flakes. In (b), the matrix flows with progressive flattening, and the flakes are rotated toward parallelism normal to the predominant stress. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

1


ShearShear motion occurs along planes at an angle to motion occurs along planes at an angle to 11

Figure 21-2. The three main types of deviatoric stress with an example of possible resulting structures. b. Shear, causing slip along parallel planes and rotation. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

1


FluidsFluidsEvidence for the existence of a metamorphic fluid:Evidence for the existence of a metamorphic fluid:

Fluid inclusionsFluid inclusions Fluids are required for hydrous or carbonate Fluids are required for hydrous or carbonate

phasesphases Volatile-involving reactions occur at Volatile-involving reactions occur at

temperatures and pressures that require finite temperatures and pressures that require finite fluid pressuresfluid pressures

Metamorphic Agents and ChangesMetamorphic Agents and Changes• PPfluidfluid indicates the total fluid pressure, which is the sum of indicates the total fluid pressure, which is the sum of

the the partial pressurespartial pressures of each component (P of each component (Pfluidfluid = = ppH2OH2O + +

ppCO2CO2 + …) + …)

• May also consider the May also consider the mole fractionsmole fractions of the components, of the components, which must sum to 1.0 (Xwhich must sum to 1.0 (XH2OH2O + X + XCO2CO2 + … = 1.0) + … = 1.0)

Metamorphic Agents and ChangesMetamorphic Agents and Changes• GradientsGradients in T, P, X in T, P, Xfluidfluid across an area across an area

• ZonationZonation in the mineral assemblages in the mineral assemblages

The Types of MetamorphismThe Types of MetamorphismDifferent approaches to Different approaches to classificationclassification

1. Based on principal process or agent1. Based on principal process or agent Dynamic MetamorphismDynamic Metamorphism Thermal MetamorphismThermal Metamorphism Dynamo-thermal MetamorphismDynamo-thermal Metamorphism

The Types of MetamorphismThe Types of MetamorphismDifferent approaches to classificationDifferent approaches to classification

2. Based on setting2. Based on setting Contact MetamorphismContact Metamorphism

PyrometamorphismPyrometamorphism Regional MetamorphismRegional Metamorphism

Orogenic MetamorphismOrogenic Metamorphism Burial MetamorphismBurial Metamorphism Ocean Floor MetamorphismOcean Floor Metamorphism

Hydrothermal MetamorphismHydrothermal Metamorphism Fault-Zone Metamorphism Fault-Zone Metamorphism ImpactImpact or or ShockShock MetamorphismMetamorphism

Contact MetamorphismContact Metamorphism

• Adjacent to igneous intrusionsAdjacent to igneous intrusions

• Result of thermal (and possibly metasomatic) Result of thermal (and possibly metasomatic) effects of hot magma intruding cooler shallow effects of hot magma intruding cooler shallow rocks rocks

• Occur over a wide range of pressures, including Occur over a wide range of pressures, including very lowvery low

• Contact Contact aureoleaureole

The Types of MetamorphismThe Types of MetamorphismContact Metamorphism Contact Metamorphism The size and shape of an aureole is controlled by:The size and shape of an aureole is controlled by:

The nature of the plutonThe nature of the pluton

The nature of the country rocksThe nature of the country rocks

Size Shape Orientation

Temperature Composition

CompositionComposition Depth and metamorphic grade prior to intrusionDepth and metamorphic grade prior to intrusion Permeability Permeability

The Types of MetamorphismThe Types of MetamorphismContact Metamorphism Contact Metamorphism Most easily recognized where a pluton is introduced into Most easily recognized where a pluton is introduced into

shallow rocks in a static environmentshallow rocks in a static environment

The rocks near the pluton are often high-grade The rocks near the pluton are often high-grade rocks with an isotropic fabric: rocks with an isotropic fabric: hornfelseshornfelses (or (or granofelses) in which relict textures and granofelses) in which relict textures and structures are commonstructures are common

The Types of MetamorphismThe Types of MetamorphismContact Metamorphism Contact Metamorphism PolymetamorphicPolymetamorphic rocks are common, usually representing rocks are common, usually representing

an orogenic event followed by a contact onean orogenic event followed by a contact one

• Spotted phylliteSpotted phyllite (or slate) (or slate)• Overprint may be due to: Overprint may be due to:

Lag time between the creation of the magma at depth Lag time between the creation of the magma at depth during T maximum, and its migration to the lower during T maximum, and its migration to the lower grade rocks abovegrade rocks above

Plutonism may reflect a separate phase of post-Plutonism may reflect a separate phase of post-orogenic collapse magmatism (Chapter 18)orogenic collapse magmatism (Chapter 18)

The Types of MetamorphismThe Types of MetamorphismPyrometamorphism

Very high temperatures at very low pressures, Very high temperatures at very low pressures, generated by a volcanic or subvolcanic bodygenerated by a volcanic or subvolcanic body

Also developed in xenolithsAlso developed in xenoliths

The Types of MetamorphismThe Types of MetamorphismRegional Metamorphism Regional Metamorphism sensu latosensu lato: metamorphism : metamorphism

that affects a large body of rock, and thus covers a that affects a large body of rock, and thus covers a great lateral extentgreat lateral extent

Three principal types: Orogenic metamorphism Burial metamorphism Ocean-floor metamorphism

The Types of MetamorphismThe Types of MetamorphismOrogenic Metamorphism Orogenic Metamorphism is the type of metamorphism is the type of metamorphism

associated with associated with convergent plate marginsconvergent plate margins• Dynamo-thermal, involving one or more episodes of Dynamo-thermal, involving one or more episodes of

orogeny with combined elevated geothermal gradients orogeny with combined elevated geothermal gradients and deformation (deviatoric stress)and deformation (deviatoric stress)

• FoliatedFoliated rocks are a characteristic product rocks are a characteristic product

The Types of MetamorphismThe Types of MetamorphismOrogenic Orogenic

MetamorphismMetamorphism

Figure 21-6. Schematic model for the sequential (a c) development of a “Cordilleran-type” or active continental margin orogen. The dashed and black layers on the right represent the basaltic and gabbroic layers of the oceanic crust. From Dewey and Bird (1970) J. Geophys. Res., 75, 2625-2647; and Miyashiro et al. (1979) Orogeny. John Wiley & Sons.

The Types of MetamorphismThe Types of MetamorphismOrogenic MetamorphismOrogenic Metamorphism

• Uplift and erosionUplift and erosion• Metamorphism often continues after major Metamorphism often continues after major

deformation ceasesdeformation ceases Metamorphic pattern is simpler than the Metamorphic pattern is simpler than the

structural onestructural one• Pattern of increasing metamorphic grade from Pattern of increasing metamorphic grade from

both directions toward the core areaboth directions toward the core area


• Most orogenic belts have several episodes of Most orogenic belts have several episodes of deformation and metamorphism, creating a more deformation and metamorphism, creating a more complex complex polymetamorphicpolymetamorphic pattern pattern

• Continental collisionContinental collision


• Batholiths are usually present in the highest grade areasBatholiths are usually present in the highest grade areas

• If plentiful and closely spaced, may be called If plentiful and closely spaced, may be called regional regional contact metamorphismcontact metamorphism

The Types of MetamorphismThe Types of MetamorphismBurial metamorphismBurial metamorphism = for low-grade = for low-grade metamorphism in sedimentary basins due to burialmetamorphism in sedimentary basins due to burial

• Southland Syncline in New Zealand: a thick pile (> 10 km) of Southland Syncline in New Zealand: a thick pile (> 10 km) of Mesozoic volcaniclastics had accumulated Mesozoic volcaniclastics had accumulated

• Mild deformation and no igneous intrusions discoveredMild deformation and no igneous intrusions discovered

• Fine-grained, high-temperature phases, glassy ash: very Fine-grained, high-temperature phases, glassy ash: very susceptible to metamorphic alterationsusceptible to metamorphic alteration

• Metamorphic effects attributed to increased pressure and Metamorphic effects attributed to increased pressure and temperature due to burialtemperature due to burial

• Range from diagenesis to the formation of zeolites, prehnite, Range from diagenesis to the formation of zeolites, prehnite, pumpellyite, laumontite, etc.pumpellyite, laumontite, etc.

The Types of MetamorphismThe Types of Metamorphism

• Coombs (1961) also proposed Coombs (1961) also proposed hydrothermal hydrothermal metamorphismmetamorphism, caused by hot H, caused by hot H22O-rich fluids and O-rich fluids and

usually involving metasomatismusually involving metasomatism• Difficult type of metamorphism to constrain, since Difficult type of metamorphism to constrain, since

hydrothermal effects often play some role in most hydrothermal effects often play some role in most of the other types of metamorphismof the other types of metamorphism

The Types of MetamorphismThe Types of MetamorphismBurial metamorphismBurial metamorphism occurs in areas that have not occurs in areas that have not experienced significant deformation or orogenyexperienced significant deformation or orogeny

• Restricted to large, relatively undisturbed Restricted to large, relatively undisturbed sedimentary piles away from active plate marginssedimentary piles away from active plate margins The Gulf of Mexico?The Gulf of Mexico? Bengal Fan? Bengal Fan?

The Types of MetamorphismThe Types of MetamorphismBurial MetamorphismBurial Metamorphism

• Bengal Fan Bengal Fan sedimentary pile > 22 km sedimentary pile > 22 km• Extrapolating Extrapolating 250-300 250-300ooC at the base (P ~ 0.6 GPa)C at the base (P ~ 0.6 GPa)• Well into the metamorphic range, and the weight of Well into the metamorphic range, and the weight of

the overlying sediments sufficient to the overlying sediments sufficient to impart a impart a foliation at depthfoliation at depth

• Passive margins often become activePassive margins often become active

• Areas of burial metamorphism may thus become Areas of burial metamorphism may thus become areas of orogenic metamorphismareas of orogenic metamorphism

The Types of MetamorphismThe Types of MetamorphismOcean-Floor Metamorphism Ocean-Floor Metamorphism affects the oceanic affects the oceanic crust at ocean ridge spreading centerscrust at ocean ridge spreading centers

• Wide range of temperatures at relatively low Wide range of temperatures at relatively low pressurepressure

• Metamorphic rocks exhibit considerable Metamorphic rocks exhibit considerable metasomatic alteration, notably metasomatic alteration, notably loss of Ca and Siloss of Ca and Si and and gain of Mg and Nagain of Mg and Na

• These changes can be correlated with exchange These changes can be correlated with exchange between basalt and hot seawaterbetween basalt and hot seawater


• May be considered another example of May be considered another example of hydrothermalhydrothermal metamorphismmetamorphism

• Highly altered chlorite-quartz rocks- distinctive high-Mg, Highly altered chlorite-quartz rocks- distinctive high-Mg, low-Ca compositionlow-Ca composition

Ocean-Floor MetamorphismOcean-Floor Metamorphism


• Impact metamorphism (Impact metamorphism (“shock metamorphism”“shock metamorphism”) occurs at ) occurs at meteorite (or other bolide) impact cratersmeteorite (or other bolide) impact craters

• Both fault-zone and impact metamorphism correlate with Both fault-zone and impact metamorphism correlate with dynamic metamorphismdynamic metamorphism, based on process, based on process

Fault-Zone and Impact MetamorphismFault-Zone and Impact Metamorphism occur in occur in areas experiencing relatively high rates of deform-areas experiencing relatively high rates of deform-ation and strain with only minor recrystallizationation and strain with only minor recrystallization

(a) (a) Shallow fault Shallow fault zone with zone with fault fault brecciabreccia

(b)(b) Slightly deeper Slightly deeper fault zone (exposed fault zone (exposed by erosion) with by erosion) with some ductile flow some ductile flow and fault and fault mylonitemylonite

Figure 21-7. Schematic cross section across fault zones. After Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.

The Progressive Nature of MetamorphismThe Progressive Nature of Metamorphism

• Prograde:Prograde: increase in metamorphic grade with time increase in metamorphic grade with time as a rock is subjected to gradually more severe as a rock is subjected to gradually more severe conditionsconditions ProgradePrograde metamorphism: metamorphism: changes in a rock that changes in a rock that

accompany increasing metamorphic gradeaccompany increasing metamorphic grade• Retrograde: Retrograde: decreasing grade as rock cools and decreasing grade as rock cools and

recovers from a metamorphic or igneous eventrecovers from a metamorphic or igneous event Retrograde metamorphism: Retrograde metamorphism: any accompanying any accompanying

changeschanges


• A rock at a high metamorphic grade probably A rock at a high metamorphic grade probably progressedprogressed through a sequence of mineral through a sequence of mineral assemblages rather than hopping directly from an assemblages rather than hopping directly from an unmetamorphosed rock to the metamorphic rock unmetamorphosed rock to the metamorphic rock that we find todaythat we find today


All rocks that we now find must also have cooled to All rocks that we now find must also have cooled to surface conditionssurface conditions

At what point on its cyclic At what point on its cyclic P-T-t pathP-T-t path did its present did its present mineral assemblage last equilibrate?mineral assemblage last equilibrate?

• The preserved zonal distribution of metamorphic The preserved zonal distribution of metamorphic rocks suggests that each rock preserves the rocks suggests that each rock preserves the conditions of the conditions of the maximum metamorphic grade maximum metamorphic grade (temperature)(temperature)


Retrograde metamorphism is of only minor Retrograde metamorphism is of only minor significancesignificance

• Prograde reactions are endothermic and easily Prograde reactions are endothermic and easily driven by increasing Tdriven by increasing T

• Devolatilization reactions are easier than Devolatilization reactions are easier than reintroducing the volatilesreintroducing the volatiles

• Geothermometry indicates that the mineral Geothermometry indicates that the mineral compositions commonly preserve the maximum compositions commonly preserve the maximum temperature temperature

Types of ProtolithTypes of Protolith

Lump the common types of sedimentary and igneous Lump the common types of sedimentary and igneous rocks into six chemically based-groupsrocks into six chemically based-groups

1. Ultramafic -1. Ultramafic - very high Mg, Fe, Ni, Cr very high Mg, Fe, Ni, Cr

2. Mafic -2. Mafic - high Fe, Mg, and Ca high Fe, Mg, and Ca

3. Shales (pelitic) -3. Shales (pelitic) - high Al, K, Si high Al, K, Si

4. Carbonates-4. Carbonates- high Ca, Mg, CO high Ca, Mg, CO22

5. Quartz5. Quartz -- nearly pure SiO nearly pure SiO22..

6. Quartzo-feldspathic -6. Quartzo-feldspathic - high Si, Na, K, Al high Si, Na, K, Al

Some Examples of MetamorphismSome Examples of Metamorphism• Interpretation of the conditions and evolution of Interpretation of the conditions and evolution of

metamorphic bodies, mountain belts, and ultimately the metamorphic bodies, mountain belts, and ultimately the evolution of the Earth's crustevolution of the Earth's crust

• Metamorphic rocks may retain enough inherited Metamorphic rocks may retain enough inherited information from their protolith to allow us to interpret information from their protolith to allow us to interpret much of the pre-metamorphic history as wellmuch of the pre-metamorphic history as well

Orogenic Regional Metamorphism of Orogenic Regional Metamorphism of the Scottish Highlandsthe Scottish Highlands

• George Barrow (1893, 1912)George Barrow (1893, 1912)• SE Highlands of Scotland - Caledonian orogeny ~ 500 MaSE Highlands of Scotland - Caledonian orogeny ~ 500 Ma• NappesNappes• GranitesGranites

Barrow’s Barrow’s AreaArea

Figure 21-8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.

Orogenic Regional Metamorphism of Orogenic Regional Metamorphism of the Scottish Highlandsthe Scottish Highlands

• Barrow studied the Barrow studied the peliticpelitic rocks rocks• Could subdivide the area into a series of Could subdivide the area into a series of

metamorphic zonesmetamorphic zones, each based on the appearance , each based on the appearance of a new mineral as metamorphic grade increasedof a new mineral as metamorphic grade increased

The sequence of zones now recognized, and the typical The sequence of zones now recognized, and the typical metamorphic mineral assemblage in each, are:metamorphic mineral assemblage in each, are:

Chlorite zoneChlorite zone. Pelitic rocks are slates or phyllites and typically . Pelitic rocks are slates or phyllites and typically contain chlorite, muscovite, quartz and albitecontain chlorite, muscovite, quartz and albite

Biotite zoneBiotite zone. Slates give way to phyllites and schists, with biotite, . Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albitechlorite, muscovite, quartz, and albite

Garnet zoneGarnet zone. Schists with conspicuous red almandine garnet, . Schists with conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or usually with biotite, chlorite, muscovite, quartz, and albite or oligoclaseoligoclase

Staurolite zoneStaurolite zone. Schists with staurolite, biotite, muscovite, quartz, . Schists with staurolite, biotite, muscovite, quartz, garnet, and plagioclase. Some chlorite may persistgarnet, and plagioclase. Some chlorite may persist

Kyanite zoneKyanite zone. Schists with kyanite, biotite, muscovite, quartz, . Schists with kyanite, biotite, muscovite, quartz, plagioclase, and usually garnet and stauroliteplagioclase, and usually garnet and staurolite

Sillimanite zone. Schists and gneisses with sillimanite, biotite, muscovite, quartz, plagioclase, garnet, and perhaps staurolite. Some kyanite may also be present (although kyanite and sillimanite are both polymorphs of Al2SiO5)

• Sequence = Sequence = Barrovian zonesBarrovian zones

• The P-T conditions referred to as The P-T conditions referred to as Barrovian-typeBarrovian-type metamorphismmetamorphism ( (fairly typical of many belts)fairly typical of many belts)

• Now extended to a much larger area of the HighlandsNow extended to a much larger area of the Highlands• IsogradIsograd = line that separates the zones (a line in the field = line that separates the zones (a line in the field

of constant metamorphic grade)of constant metamorphic grade)

Figure 21-8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.

To summarize:To summarize:

• An An isogradisograd (in this classical sense) represents the first (in this classical sense) represents the first appearance of a particular metamorphic appearance of a particular metamorphic index mineralindex mineral in in the field as one progresses the field as one progresses upup metamorphic grade metamorphic grade

• When one crosses an isograd, such as the biotite isograd, When one crosses an isograd, such as the biotite isograd, one enters the biotite one enters the biotite zonezone

• Zones thus have the same name as the isograd that forms Zones thus have the same name as the isograd that forms the the low-gradelow-grade boundary of that zone boundary of that zone

• Since classic isograds are based on the first appearance of Since classic isograds are based on the first appearance of a mineral, and not its disappearance, a mineral, and not its disappearance, an index mineral an index mineral may still be stable in higher grade zonesmay still be stable in higher grade zones

A variation occurs in the area just to the north of A variation occurs in the area just to the north of Barrow’s, in the Banff and Barrow’s, in the Banff and BuchanBuchan district district

• Here the pelitic compositions are similar, but the Here the pelitic compositions are similar, but the sequence of isograds is: sequence of isograds is: chloritechlorite biotitebiotite cordieritecordierite andalusiteandalusite sillimanitesillimanite

The stability field of andalusite occurs at pressures less than The stability field of andalusite occurs at pressures less than 0.37 GPa (~ 10 km), while kyanite 0.37 GPa (~ 10 km), while kyanite sillimanite at the sillimanite at the sillimanite isograd only above this pressuresillimanite isograd only above this pressure

Figure 21-9. The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and

sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in

the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).

Regional Burial MetamorphismRegional Burial MetamorphismOtago, New ZealandOtago, New Zealand

• Jurassic graywackes, tuffs, and volcanics in a deep Jurassic graywackes, tuffs, and volcanics in a deep trough metamorphosed in the Cretaceoustrough metamorphosed in the Cretaceous

• The fine grain size and immature nature of the The fine grain size and immature nature of the material is highly susceptible to alteration, even at material is highly susceptible to alteration, even at low gradeslow grades


Section X-Y shows more detail

Figure 21-10. Geologic sketch map of the South Island of New Zealand showing the Mesozoic metamorphic rocks east of the older Tasman Belt and the Alpine Fault. The Torlese Group is metamorphosed predominantly in the prehnite-pumpellyite zone, and the Otago Schist in higher grade zones. X-Y is the Haast River Section of Figure 21-11. From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.


Isograds mapped at the lower grades:Isograds mapped at the lower grades:

1) Zeolite1) Zeolite

2) Prehnite-Pumpellyite2) Prehnite-Pumpellyite

3) Pumpellyite3) Pumpellyite (-actinolite) (-actinolite)

4) Chlorite4) Chlorite (-clinozoisite) (-clinozoisite)

5) Biotite5) Biotite

6) Almandine6) Almandine (garnet) (garnet)

7) Oligoclase7) Oligoclase (albite at lower grades is replaced by a (albite at lower grades is replaced by a more calcic plagioclase)more calcic plagioclase)

Regional Burial MetamorphismRegional Burial MetamorphismFigure 21-11. Metamorphic zones of the Haast Group (along section X-Y in Figure 21-10). After Cooper and Lovering (1970) Contrib. Mineral. Petrol., 27, 11-24.


• Orogenic belts typically proceed directly from Orogenic belts typically proceed directly from diagenesis to chlorite or biotite zonesdiagenesis to chlorite or biotite zones

• The development of low-grade zones in New The development of low-grade zones in New Zealand may reflect the highly unstable nature of Zealand may reflect the highly unstable nature of the tuffs and graywackes, and the availability of hot the tuffs and graywackes, and the availability of hot water, whereas pelitic sediments may not react until water, whereas pelitic sediments may not react until higher gradeshigher grades

Paired Metamorphic Belts of JapanPaired Metamorphic Belts of Japan

Figure 21-12. The Sanbagawa and Ryoke metamorphic belts of Japan. From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill and Miyashiro (1994) Metamorphic Petrology. Oxford University Press.


• The NW belt (“inner” belt, inward, or away from The NW belt (“inner” belt, inward, or away from the trench) is the the trench) is the RyokeRyoke (or (or AbukumaAbukuma) ) BeltBelt Low P/TLow P/T Buchan-type of regional orogenic Buchan-type of regional orogenic

metamorphismmetamorphism Dominant meta-pelitic sediments, and isograds up to the Dominant meta-pelitic sediments, and isograds up to the

sillimanite zone have been mappedsillimanite zone have been mapped A A high-temperature-low-pressurehigh-temperature-low-pressure belt, and granitic belt, and granitic

plutons are commonplutons are common


• Outer belt, called the Outer belt, called the Sanbagawa BeltSanbagawa Belt• It is of a It is of a high-pressure-low-temperaturehigh-pressure-low-temperature nature nature

Only reaches the garnet zone in the pelitic rocksOnly reaches the garnet zone in the pelitic rocks BasicBasic rocks are more common than in the Ryoke belt, rocks are more common than in the Ryoke belt,

however, and in these however, and in these glaucophaneglaucophane is developed (giving is developed (giving way to hornblende at higher grades) way to hornblende at higher grades)

Rocks are commonly called Rocks are commonly called blueschistsblueschists


• Two belts are in contact along their whole length Two belts are in contact along their whole length across a major fault zone (the Median Line)across a major fault zone (the Median Line)

• Ryoke-AbukumaRyoke-Abukuma lithologies are similar to seds lithologies are similar to seds derived from a relatively derived from a relatively mature volcanic arcmature volcanic arc

• SanbagawaSanbagawa lithologies more akin to the oceanward lithologies more akin to the oceanward accretionary wedgeaccretionary wedge where distal arc-derived where distal arc-derived sediments and volcanics mix with oceanic crust and sediments and volcanics mix with oceanic crust and marine sedimentmarine sediment

Paired Metamorphic Belts of JapanPaired Metamorphic Belts of Japan• Fig. 16-15 suggests that the 600Fig. 16-15 suggests that the 600ooC isotherm, for example, C isotherm, for example,

could be as deep as 100 km in the trench-subduction zone could be as deep as 100 km in the trench-subduction zone area, and as shallow as 20 km beneath the volcanic arcarea, and as shallow as 20 km beneath the volcanic arc

Miyashiro (1961, 1973) suggested that the occurrence of coeval Miyashiro (1961, 1973) suggested that the occurrence of coeval metamorphic belts, an outer, high-P/T belt, and an inner, lower-P/T metamorphic belts, an outer, high-P/T belt, and an inner, lower-P/T belt ought to be a common occurrence in a number of subduction belt ought to be a common occurrence in a number of subduction zones, either modern or ancientzones, either modern or ancient

Figure 21-13. Some of the paired metamorphic belts in the circum-Pacific region. From Miyashiro (1994) Metamorphic Petrology. Oxford University Press.

Contact Metamorphism of Pelitic Rocks Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UKin the Skiddaw Aureole, UK

• Ordovician Skiddaw Slates (English Lake District) Ordovician Skiddaw Slates (English Lake District) intruded by several granitic bodiesintruded by several granitic bodies

• Intrusions are shallow, and contact effects Intrusions are shallow, and contact effects overprinted on an earlier low-grade regional overprinted on an earlier low-grade regional orogenic metamorphismorogenic metamorphism


• The aureole around the Skiddaw granite was sub-The aureole around the Skiddaw granite was sub-divided into three zones, principally on the basis of divided into three zones, principally on the basis of textures:textures:

Unaltered slatesUnaltered slates Outer zone of spotted slatesOuter zone of spotted slates Middle zone of andalusite slatesMiddle zone of andalusite slates Inner zone of hornfelsInner zone of hornfels Skiddaw graniteSkiddaw granite

Increasing Metamorphic Grade

Figure 21-14. Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. After Eastwood et al (1968). Geology of the Country around Cockermouth and Caldbeck. Explanation accompanying the 1-inch Geological Sheet 23, New Series. Institute of Geological Sciences. London.


• Middle zone: slates more thoroughly recrystallized, contain Middle zone: slates more thoroughly recrystallized, contain biotite + muscovite + cordierite + andalusite + quartzbiotite + muscovite + cordierite + andalusite + quartz

Figure 21-15. Cordierite-andalusite slate from the middle zone of the Skiddaw aureole. From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London. 1 mm


Inner zone:Inner zone:

Thoroughly recrystallizedThoroughly recrystallized

Lose foliationLose foliation

Figure 21-16. Andalusite-cordierite schist from the inner zone of the Skiddaw aureole. Note the chiastolite cross in andalusite (see also Figure 22-49). From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.

1 mm


• The zones determined on a The zones determined on a texturaltextural basis basis

• Better to use the sequential appearance of Better to use the sequential appearance of minerals and isograds to define the zonesminerals and isograds to define the zones

• But low-P isograds converge in P-TBut low-P isograds converge in P-T• Skiddaw sequence of mineral development with Skiddaw sequence of mineral development with

grade is difficult to determine accuratelygrade is difficult to determine accurately

Contact Metamorphism of Pelitic RocksContact Metamorphism of Pelitic Rocks

• Inner aureole at Comrie (a diorite intruded into the Inner aureole at Comrie (a diorite intruded into the Dalradian schists back up north in Scotland), the intrusion Dalradian schists back up north in Scotland), the intrusion was hotter and the rocks were metamorphosed to higher was hotter and the rocks were metamorphosed to higher grades than at Skiddawgrades than at Skiddaw

• Tilley describes coarse-grained non-foliated granofelses Tilley describes coarse-grained non-foliated granofelses containing very high-temperature minerals such as containing very high-temperature minerals such as orthopyroxene and K-feldspar that have formed due to orthopyroxene and K-feldspar that have formed due to the dehydration of biotite and muscovite in the country the dehydration of biotite and muscovite in the country rocksrocks

Contact Metamorphism and Skarn Contact Metamorphism and Skarn Formation at Crestmore, CA, USAFormation at Crestmore, CA, USA

• Crestmore quarry in the Los Angeles basinCrestmore quarry in the Los Angeles basin• Quartz monzonite porphry of unknown age intrudes Mg-Quartz monzonite porphry of unknown age intrudes Mg-

bearing carbonates (either late Paleozoic or Triassic)bearing carbonates (either late Paleozoic or Triassic)• Brunham (1959) mapped the following zones and the Brunham (1959) mapped the following zones and the

mineral assemblages in each (listed in order of mineral assemblages in each (listed in order of increasing grade):increasing grade):

Forsterite Zone: calcite + brucite + clinohumite + spinel calcite + clinohumite + forsterite + spinelcalcite + clinohumite + forsterite + spinel calcite + forsterite + spinel + clintonitecalcite + forsterite + spinel + clintonite

Monticellite Zone:Monticellite Zone: calcite + forsterite + monticellite + clintonitecalcite + forsterite + monticellite + clintonite calcite + monticellite + melilite + clintonite calcite + monticellite + spurrite (or tilleyite) + clintonite monticellite + spurrite + merwinite + melilite

Vesuvianite Zone: vesuvianite + monticellite + spurrite + merwinite +

melilite vesuvianite + monticellite + diopside + wollastonite

Garnet Zone: grossular + diopside + wollastonite


An idealized An idealized cross-sectioncross-section through the aureole through the aureole

Figure 21-17. Idealized N-S cross section (not to scale) through the quartz monzonite and the aureole at Crestmore, CA. From Burnham (1959) Geol. Soc. Amer. Bull., 70, 879-920.


1.1. The mineral associations in successive zones (in all The mineral associations in successive zones (in all metamorphic terranes) vary by the formation of new metamorphic terranes) vary by the formation of new minerals as grade increasesminerals as grade increases

This can only occur by a This can only occur by a chemical reactionchemical reaction in which some in which some minerals are consumed and others producedminerals are consumed and others produced


a) Ca) Calcite + brucite + clinohumite + spinel zone to the zone to the Calcite + clinohumite + forsterite + spinel sub-zone Calcite + clinohumite + forsterite + spinel sub-zone involves the reaction:involves the reaction: 2 Clinohumite + SiO2 Clinohumite + SiO22 9 Forsterite + 2 H 9 Forsterite + 2 H22OO

b) Formation of the vesuvianite zone involves the reaction:b) Formation of the vesuvianite zone involves the reaction: Monticellite + 2 Spurrite + 3 Merwinite + 4 Melilite Monticellite + 2 Spurrite + 3 Merwinite + 4 Melilite

+ 15 SiO+ 15 SiO22 + 12 H + 12 H22O O 6 Vesuvianite + 2 CO 6 Vesuvianite + 2 CO22


2) Find a way to 2) Find a way to displaydisplay data in simple, yet useful ways data in simple, yet useful ways

• If we think of the aureole as a chemical system, we note If we think of the aureole as a chemical system, we note that most of the minerals consist of the components that most of the minerals consist of the components CaO-MgO-SiOCaO-MgO-SiO22-CO-CO22-H-H22O (with minor AlO (with minor Al22OO33))

Zones are numbered Zones are numbered (from outside inward)(from outside inward)

Figure 21-17. CaO-MgO-SiO2 diagram at a fixed

pressure and temperature showing the compositional relationships among the minerals and zones at Crestmore. Numbers correspond to zones listed in the text. After Burnham (1959) Geol. Soc. Amer. Bull., 70, 879-920; and Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman.

Figures not usedFigures not used

Figure 21-4. A situation in which lithostatic pressure (Plith) exerted by the

mineral grains is greater than the intergranular fluid pressure (Pfluid). At a

depth around 10 km (or T around 300oC) minerals begin to yield or dissolve at the contact points and shift toward or precipitate in the fluid-filled areas, allowing the rock to compress. The decreased volume of the pore spaces will raise Pfluid until it equals Plith. Winter

(2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figures not usedFigures not used

Figure 21-5. Temperature distribution within a 1-km thick vertical dike and in the country rocks (initially at 0oC) as a function of time. Curves are labeled in years. The model assumes an initial intrusion temperature of 1200oC and cooling by conduction only. After Jaeger, (1968) Cooling and solidification of igneous rocks. In H. H. Hess and A. Poldervaart (eds.), Basalts, vol. 2. John Wiley & Sons. New York, pp. 503-536.

chapter 21: metamorphism rocks as chemical systems (ch. 5)rocks as chemical systems (ch. 5) a...

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