ch 21 intro to met

7/31/2019 Ch 21 Intro to Met

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Chapter 21: MetamorphismChapter 21: Metamorphism

Fresh basalt andweathered basalt


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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. Thesechanges 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


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The Limits of MetamorphismLow-temperature limit grades into diagenesis

Processes are indistinguishable

Metamorphism begins in the range of 100-150oC

for the more unstable types ofprotolith

Some zeolites are considered diagenetic and

others metamorphic pretty arbitrary


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The Limits of MetamorphismThe Limits of Metamorphism

High-temperatureHigh-temperature limit grades intolimit 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


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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.


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Increasing temperature has several effects

1) Promotes recrystallizationincreased grain

size

2) Drive reactions (endothermic)

3) Overcomes kinetic barriers


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Metamorphic Agents and ChangesPressure

Normal gradients perturbed in several ways,

most commonly:

High T/P geotherms in areas of plutonicactivity or rifting

Low T/P geotherms in subduction zones


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Figure 21.1.Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) forseveral metamorphic areas. After Turner (1981).Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-

Hill.


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Metamorphic grade: a general increase indegree of metamorphism without specifying

the exact relationship between temperature

and pressure


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

directions

Resolved into three mutually perpendicularstress() components:

1 is the maximum principal stress

2 is an intermediate principal stress

3 is the minimum principal stress

In hydrostatic situations all three are equal


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Metamorphic Agents and ChangesMetamorphic Agents and Changes

StressStress StrainStraindeformationdeformation

Deviatoric stress affects the textures andDeviatoric stress affects the textures and

structures,structures,but not the equilibrium mineralbut not the equilibrium mineralassemblageassemblage

Strain energy may overcome kinetic barriers toStrain energy may overcome kinetic barriers to

reactionsreactions


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Foliation is a common result, which allows us to

estimate the orientation of1

1

> 2

= 3

foliation and no lineation

1 = 2 > 3lineation and no foliation

1 > 2 > 3both foliation and lineation

Figure 21.3. Flattening of a ductile homogeneous sphere (a) containing randomly oriented flat disks or flakes. In (b), the matrixflows 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


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Shearmotion 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 slipalong parallel planes and rotation. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

1


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FluidsFluidsEvidence for the existence of a metamorphic fluid:

Fluid inclusions

Fluids are required for hydrous or carbonatephases

Volatile-involving reactions occur at

temperatures and pressures that require finitefluid pressures


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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


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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


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Contact MetamorphismContact Metamorphism

Adjacent to igneous intrusionsAdjacent to igneous intrusions Thermal (Thermal ( metasomatic) effects of hot magmametasomatic) effects of hot magma

intruding cooler shallow rocksintruding cooler shallow rocks

Occurs over a wide range of pressures, includingOccurs over a wide range of pressures, including

very lowvery low

ContactContact aureoleaureole


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Most easily recognized where a pluton is introduced into

shallow rocks in a static environment

Hornfelses (granofelses) commonly with relict

textures and structures


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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)


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The Types of Metamorphism

Pyrometamorphism

Very high temperatures at low pressures,

generated by a volcanic or sub-volcanic body

Also developed in xenoliths


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The Types of MetamorphismOrogenic Metamorphism


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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.


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Polymetamorphic patterns

Continental collision

Batholiths are usually present in the highest grade areas

If plentiful and closely spaced, may be called regional

contact metamorphism


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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?


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The Types of MetamorphismBurial metamorphism occurs in areas that have not

experienced significant deformation or orogeny

Bengal Fan sedimentary pile > 22 km

Extrap. 250-300o

C at the base (P ~ 0.6 GPa)

Passive margins often become active

Areas of burial metamorphism may thus become

areas of orogenic metamorphism


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The Types of Metamorphism

3 Impact metamorphism at meteorite (or other

bolide) impact craters

3 Both correlate with dynamic metamorphism,

based on process

Fault-Zone and Impact Metamorphism

3 High rates of deformation and strain with only

minor recrystallization


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The Progressive Nature of Metamorphism

A rock at a high metamorphic grade probablyprogressed through a sequence of mineral

assemblages rather than hopping directly from an

unmetamorphosed rock to the metamorphic rockthat we find today


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Types of Protolith

Lump the common types of sedimentary and igneousrocks into six chemically based-groups

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

2. Mafic - high Fe, Mg, and Ca3. Shales (pelitic) - high Al, K, Si

4. Carbonates - high Ca, Mg, CO2

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


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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


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Orogenic Regional Metamorphism of the

Scottish Highlands

George Barrow (1893, 1912)

SE Highlands of Scotland - Caledonian Orogeny

~ 500 Ma

Nappes

Granites


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Barrows

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.

O i i l hi f h


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Orogenic Regional Metamorphism of the

Scottish Highlands

Barrow studied thepelitic rocks

Could subdivide the area into a series of

metamorphic zones, each based on the appearance

of a new mineral as metamorphic grade increased


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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 oroligoclase

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)


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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)


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To summarize:

An isograd represents the first appearance of a particular

metamorphic index mineral in the field as one progressesup 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 appearanceof a mineral, and not its disappearance, an index mineral

may still be stable in higher grade zones


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A variation occurs in the area just to the north of

Barrows, in the Banff and Buchan district

Pelitic compositions are similar, but the sequenceof isograds is:

chlorite

biotite cordierite

andalusite

sillimanite

The stability field of andalusite occurs at pressures less than


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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 Al2SiO

5showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite.

Also shown is the hydration of Al2SiO

5to pyrophyllite, which limits the occurrence of an Al

2SiO

5polymorph at low grades in the presence of excess

silica and water. The dia ram was calculated usin the ro ram TW Berman 1988 1990 1991 .

R i l B i l M t hi


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Regional Burial Metamorphism

Otago, New Zealand

Jurassic graywackes, tuffs, and volcanics in a deeptrough metamorphosed in the Cretaceous

Fine grain size and immature material is highly

susceptible to alteration (even at low grades)



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Otago, New ZealandSection 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.

R i l B i l M t hi


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Otago, New Zealand

Isograds mapped at the lower grades:

1) Zeolite

2) Prehnite-Pumpellyite

3) Pumpellyite (-actinolite)4) Chlorite (-clinozoisite)

5) Biotite

6) Almandine (garnet)7) Oligoclase (albite at lower grades is replaced by a

more calcic plagioclase)

R i l B i l M t hi


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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


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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.


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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


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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



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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


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Figure 21.14. Geologic

Map and cross-section of

the area around the Skiddawgranite, 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.



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Middle zone: slates more thoroughly recrystallized, containbiotite + 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


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Inner zone:

Thoroughly recrystallized

Lose 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



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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


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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 themineral assemblages in each (listed in order of increasing

grade):

F t it Z


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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



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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.



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1. The mineral associations in successive zones (in allmetamorphic 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



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a) Calcite + brucite + clinohumite + spinel zone to the Calcite+ clinohumite + forsterite + spinel sub-zone involves the

reaction:

2 Clinohumite + SiO2

9 Forsterite + 2 H2

O

b) Formation of the vesuvianite zone involves the reaction:

Monticellite + 2 Spurrite + 3 Merwinite + 4 Melilite

+ 15 SiO2 + 12 H2O 6 Vesuvianite + 2 CO2



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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)

Figure 21.18. CaO-MgO-SiO2

diagram at a fixed

pressure and temperature showing the


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Zones are numbered

(from outside inward)

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


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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 shifttoward 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.



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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

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