Chapter 21: Metamorphism

Chapter 21: MetamorphismChapter 21: Metamorphism

Fresh basalt and weathered basalt

The IUGS-SCMR proposed this definition:

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

Chapter 21: Metamorphism

The Limits of Metamorphism Low-temperature limit grades into diagenesis• Processes are indistinguishable • Metamorphism begins in the range of 100-150oC

for the more unstable types of protolith• Some zeolites are considered diagenetic and

others metamorphic – 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• Xenoliths, restites, and other enclaves?Xenoliths, restites, and other enclaves?• MigmatitesMigmatites (“mixed rocks”) are gradational (“mixed rocks”) are gradational

Metamorphic Agents and Changes

• Temperature: typically the most important factor in metamorphism

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 ChangesIncreasing temperature has several effects

1) Promotes recrystallization increased grain size

2) Drive reactions (endothermic)3) Overcomes kinetic barriers

Metamorphic Agents and ChangesPressure• “Normal” gradients perturbed in several ways,

most commonly: High T/P geotherms in areas of plutonic

activity or rifting Low 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 Changes• Metamorphic grade: a general increase in

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

Metamorphic Agents and Changes

• Lithostatic pressure - uniform stress (hydrostatic)• Deviatoric stress = pressure unequal in different

directions• Resolved into three mutually perpendicular stress

() components:1 is the maximum principal stress2 is an intermediate principal stress3 is the minimum principal stress

• In hydrostatic situations all three are equal

Metamorphic Agents and ChangesMetamorphic Agents and Changes

• StressStress• StrainStrain 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

• Foliation is a common result, which allows us to estimate the orientation of 1

1 > 2 = 3 foliation and no lineation 1 = 2 > 3 lineation and no foliation 1 > 2 > 3 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 Strain ellipsoid

Metamorphic Agents and ChangesShear motion occurs along planes at an angle to 1

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

Metamorphic Agents and ChangesFluidsFluidsEvidence for the existence of a metamorphic fluid:• Fluid inclusions• Fluids are required for hydrous or carbonate

phases• Volatile-involving reactions occur at

temperatures and pressures that require finite fluid pressures

Metamorphic Agents and Changes• Pfluid = partial pressures of each component

(Pfluid = pH2O + pCO2 + …)

• Mole fractions of components must sum to 1.0 (XH2O + XCO2

+ … = 1.0)

• pH2O = XH2O x Pfluid

• Gradients in T, P, Xfluid • Zonation in mineral assemblages

The Types of MetamorphismDifferent approaches to classification

1. Based on principal process or agent• Dynamic Metamorphism• Thermal Metamorphism• Dynamo-thermal Metamorphism

The Types of MetamorphismDifferent approaches to classification

2. Based on setting• Contact Metamorphism

Pyrometamorphism• Regional Metamorphism

Orogenic Metamorphism Burial Metamorphism Ocean Floor Metamorphism

• Hydrothermal Metamorphism• Fault-Zone Metamorphism • Impact or Shock Metamorphism

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

• The nature of the pluton

• The nature of the country rocks

Size Shape Orientation

Temperature Composition

Composition Depth and metamorphic grade prior to intrusion Permeability

Contact MetamorphismContact Metamorphism• Adjacent to igneous intrusionsAdjacent to igneous intrusions

• Thermal (Thermal (±± metasomatic) effects of hot magma metasomatic) effects of hot magma intruding cooler shallow rocks intruding cooler shallow rocks

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

• Contact Contact aureoleaureole

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

shallow rocks in a static environment

Hornfelses (granofelses) commonly with relict textures and structures

The Types of MetamorphismContact Metamorphism Polymetamorphic rocks are common, usually

representing an orogenic event followed by a contact one

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

Lag time for magma migration A separate phase of post-orogenic collapse

magmatism (Chapter 18)

The Types of MetamorphismPyrometamorphism Very high temperatures at low pressures,

generated by a volcanic or sub-volcanic body

Also developed in xenoliths

The Types of MetamorphismRegional Metamorphism sensu lato: metamorphism

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

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

The Types of MetamorphismOrogenic Metamorphism is the type of

metamorphism associated with convergent plate margins• Dynamo-thermal: one or more episodes of

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

• Foliated rocks are a characteristic product

The Types of MetamorphismOrogenic

Metamorphism

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 MetamorphismOrogenic Metamorphism

The Types of MetamorphismOrogenic Metamorphism• Uplift and erosion• Metamorphism often continues after major

deformation ceases Metamorphic pattern is simpler than the

structural one• Pattern of increasing metamorphic grade from

both directions toward the core area

From Understanding Earth, Press and Siever. Freeman.

The Types of MetamorphismOrogenic Metamorphism

• Polymetamorphic patterns• Continental collision• Batholiths are usually present in the highest grade areas

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

The Types of MetamorphismBurial metamorphism• Southland Syncline in New Zealand: thick pile (> 10 km)

of Mesozoic volcaniclastics

• Mild deformation, no igneous intrusions discovered

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

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

• Diagenesis grades into the formation of zeolites, prehnite, pumpellyite, laumontite, etc.

The Types of Metamorphism

Hydrothermal metamorphism• Hot H2O-rich fluids• Usually involves metasomatism• Difficult type to constrain: hydrothermal effects

often play some role in most of the other types of metamorphism

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

• Restricted to large, relatively undisturbed sedimentary piles away from active plate margins

The Gulf of Mexico? Bengal Fan?

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

• Bengal Fan sedimentary pile > 22 km• Extrap. 250-300oC at the base (P ~ 0.6 GPa)

• Passive margins often become active

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

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

• Considerable metasomatic alteration, notably loss of Ca and Si and gain of Mg and Na

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

• Exchange between basalt and hot seawater

• Another example of hydrothermal metamorphism

The Types of Metamorphism

Impact metamorphism at meteorite (or other bolide) impact craters

Both correlate with dynamic metamorphism, based on process

Fault-Zone and Impact Metamorphism High rates of deformation and strain with only

minor recrystallization

(a) Shallow fault zone with fault breccia

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

Figure 21.7. Schematic cross section across fault zones. After Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.

Prograde Metamorphism• Prograde: increase in metamorphic grade with time

as a rock is subjected to gradually more severe conditions

Prograde metamorphism: changes in a rock that accompany increasing metamorphic grade

• Retrograde: decreasing grade as rock cools and recovers from a metamorphic or igneous event

Retrograde metamorphism: any accompanying changes

The Progressive Nature of Metamorphism

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

The Progressive Nature of MetamorphismRetrograde metamorphism typically of minor

significance• Prograde reactions are endothermic and easily

driven by increasing T• Devolatilization reactions are easier than

reintroducing the volatiles• Geothermometry indicates that the mineral

compositions commonly preserve the maximum temperature

Types of ProtolithLump the common types of sedimentary and igneous

rocks into six chemically based-groups1. Ultramafic - very high Mg, Fe, Ni, Cr2. Mafic - high Fe, Mg, and Ca3. Shales (pelitic) - high Al, K, Si4. Carbonates - high Ca, Mg, CO2

5. Quartz - nearly pure SiO2.6. Quartzo-feldspathic - high Si, Na, K, Al

Why Study Metamorphism?• Interpretation of the conditions and evolution of

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

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

Orogenic Regional Metamorphism of the Scottish Highlands

• George Barrow (1893, 1912)• SE Highlands of Scotland - Caledonian Orogeny

~ 500 Ma• Nappes• Granites

Barrow’s Area

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 the Scottish Highlands

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

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

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

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

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

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

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

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

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

• Sequence = “Barrovian zones”• The P-T conditions referred to as “Barrovian-type”

metamorphism (fairly typical of many belts)• Now extended to a much larger area of the Highlands• Isograd = line that separates the zones (a line in the field

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:• An isograd represents the first appearance of a particular

metamorphic index mineral in the field as one progresses up metamorphic grade

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

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

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

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

• Pelitic compositions are similar, but the sequence of isograds is: chlorite biotite cordierite andalusite sillimanite

The stability field of andalusite occurs at pressures less than 0.37 GPa (~ 10 km), while kyanite sillimanite at the sillimanite 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 MetamorphismOtago, New Zealand

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

• Fine grain size and immature material is highly susceptible to alteration (even at low 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:1) Zeolite2) Prehnite-Pumpellyite3) Pumpellyite (-actinolite)4) Chlorite (-clinozoisite)5) Biotite6) Almandine (garnet)7) Oligoclase (albite at lower grades is replaced by a

more calcic plagioclase)

Regional 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.

Paired 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.

Paired Metamorphic Belts of Japan

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 in the Skiddaw Aureole, UK

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

• Intrusions are shallow• Contact effects overprinted on an earlier low-grade

regional orogenic metamorphism


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

• Unaltered slates• Outer zone of spotted slates• Middle zone of andalusite slates• Inner zone of hornfels• Skiddaw granite

Increasing Metamorphic Grade

Contact

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 biotite + 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:Thoroughly recrystallizedLose 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 textural basis

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

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

grade is difficult to determine accurately

Contact Metamorphism and Skarn Formation at Crestmore, CA, USA

• Crestmore quarry in the Los Angeles basin• Quartz monzonite porphry intrudes Mg-bearing

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

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

• Forsterite Zone: calcite + brucite + clinohumite + spinel calcite + clinohumite + forsterite + spinel calcite + forsterite + spinel + clintonite

• Monticellite Zone: calcite + 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 cross-section 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. The mineral associations in successive zones (in all metamorphic terranes) vary by the formation of new minerals as grade increases

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


a) Calcite + brucite + clinohumite + spinel zone to the Calcite + clinohumite + forsterite + spinel sub-zone involves the reaction: 2 Clinohumite + SiO2 9 Forsterite + 2 H2O

b) Formation of the vesuvianite zone involves the reaction: Monticellite + 2 Spurrite + 3 Merwinite + 4 Melilite + 15 SiO2 + 12 H2O 6 Vesuvianite + 2 CO2


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

If we think of the aureole as a chemical system, we note that most of the minerals consist of the components CaO-MgO-SiO2-CO2-H2O (with minor Al2O3)

Zones are numbered (from outside inward)

Figure 21.18. 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.

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Chapter 21: Metamorphism