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Metamorphic Zones, Index Minerals, Metamorphic Zones, Index Minerals, Isograds, Facies and Facies SeriesIsograds, Facies and Facies Series

the onslaught of terminology to understand how we categorize the onslaught of terminology to understand how we categorize metamorphic rocks and their conditions of formation!metamorphic rocks and their conditions of formation!

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Review: Types of ProtolithReview: Types of Protolith

1.Pelitic/mudrocks -1.Pelitic/mudrocks - high Al, K, Si high Al, K, Si

2. Quartzo-feldspathic -2. Quartzo-feldspathic - high Si, Na, K, Al high Si, Na, K, Al

3. Calcareous-3. Calcareous- high Ca, Mg, CO high Ca, Mg, CO22

4. Mafic- 4. Mafic- high Ca, Mg, Fehigh Ca, Mg, Fe

5. Ultramafic-5. Ultramafic- very high Mg, Fe, low Si, Al very high Mg, Fe, low Si, Al

WHY IS CHEMICAL COMPOSITION OF WHY IS CHEMICAL COMPOSITION OF PROTOLITH IMPORTANT?PROTOLITH IMPORTANT?

Metamorphic GradeMetamorphic Grade

Refers to maximum T or P of Refers to maximum T or P of metamorphismmetamorphism

What grades have we talked about?What grades have we talked about?

The idea of grade is general. We can better The idea of grade is general. We can better express the maximum P, T constraints express the maximum P, T constraints using the concepts of metamorphic using the concepts of metamorphic zonezone and and faciesfacies. More on this! . More on this!

Orogenic Regional Metamorphism of Orogenic Regional Metamorphism of the Scottish Highlands: Development of the Scottish Highlands: Development of

the Index Mineral Conceptthe Index Mineral Concept

• George Barrow (1893, George Barrow (1893, 1912)1912)

• SE Highlands of SE Highlands of Scotland - Caledonian Scotland - Caledonian OrogenyOrogeny ~ 500 Ma ~ 500 Ma

• Lots of foldingLots of folding• 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 Barrow studied 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 , each based on the appearance of a new mineral as metamorphic appearance of a new mineral as metamorphic grade increasedgrade 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)

Each of these minerals is an Each of these minerals is an INDEX mineral.INDEX mineral.

Chlorite zoneChlorite zone Biotite zoneBiotite zone Garnet zoneGarnet zone Staurolite zoneStaurolite zone Kyanite zoneKyanite zone Sillimanite zone

WHAT IS AN INDEX MINERALWHAT IS AN INDEX MINERAL

• Sequence = Sequence = “Barrovian zones”“Barrovian zones”

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

• ANOTHER DEFINTION:ANOTHER DEFINTION:• 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). Also reflects of constant metamorphic grade). Also reflects the FIRST APPEARANCE of the index the FIRST APPEARANCE of the index mineral. mineral.

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 represents the first appearance of a particular represents the first appearance of a particular metamorphic metamorphic index mineralindex mineral in the field as one progresses in 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

• Because classic isograds are based on the first appearance Because classic isograds are based on the first appearance of a mineral, and not its disappearance, of 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

• Pelitic compositions are similar, but the sequence Pelitic compositions are similar, but the sequence of isograds is: of isograds is: chloritechlorite biotitebiotite garnetgarnet 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).

1 GPa = 10kbars

• Eskola (1915) developed the concept of Eskola (1915) developed the concept of metamorphic facies:metamorphic facies:

• What is a metamorphic facies?What is a metamorphic facies?

Metamorphic FaciesMetamorphic Facies


Fig. 25-2.Fig. 25-2. Temperature-Temperature-pressure diagram pressure diagram showing the generally showing the generally accepted limits of the accepted limits of the various facies used in this various facies used in this text. Boundaries are text. Boundaries are approximate and approximate and gradational. The gradational. The “typical” or average “typical” or average continental geotherm is continental geotherm is from Brown and Mussett from Brown and Mussett (1993). (1993). Winter (2001) An Winter (2001) An Introduction to Igneous Introduction to Igneous and Metamorphic and Metamorphic Petrology. Prentice Hall.Petrology. Prentice Hall.

• The range of temperature and pressure conditions The range of temperature and pressure conditions represented by each faciesrepresented by each facies Eskola aware of the P-T implications and correctly Eskola aware of the P-T implications and correctly

deduced the deduced the relativerelative temperatures and pressures of temperatures and pressures of facies he proposedfacies he proposed

Can now assign relatively accurate temperature and Can now assign relatively accurate temperature and pressure limits to individual faciespressure limits to individual facies


Eskola (1920) proposed 5 original facies:Eskola (1920) proposed 5 original facies: GreenschistGreenschist AmphiboliteAmphibolite HornfelsHornfels SanidiniteSanidinite EclogiteEclogite

• Easily defined on the basis of mineral Easily defined on the basis of mineral assemblages that develop in assemblages that develop in maficmafic rocks rocks

• More facies have been added since original More facies have been added since original designationsdesignations


Fig. 25-1Fig. 25-1 The metamorphic facies proposed by Eskola and their relative temperature-pressure relationships. After The metamorphic facies proposed by Eskola and their relative temperature-pressure relationships. After Eskola (1939) Eskola (1939) Die Entstehung der GesteineDie Entstehung der Gesteine. Julius Springer. Berlin. . Julius Springer. Berlin.


Formation of Zeolites

Temperature

Pressure

GreenschistFacies

Epidote-Amphibolite

Facies

AmphiboliteFacies

Pyroxene-HornfelsFacies

Glaucophane-Schist Facies Eclogite

Facies

GranuliteFacies

SanadiniteFacies


Fig. 25-2.Fig. 25-2. Temperature-Temperature-pressure diagram pressure diagram showing the generally showing the generally accepted limits of the accepted limits of the various facies used in this various facies used in this text. Boundaries are text. Boundaries are approximate and approximate and gradational. The gradational. The “typical” or average “typical” or average continental geotherm is continental geotherm is from Brown and Mussett from Brown and Mussett (1993). (1993). Winter (2001) An Winter (2001) An Introduction to Igneous Introduction to Igneous and Metamorphic and Metamorphic Petrology. Prentice Hall.Petrology. Prentice Hall.

• The definitive mineral assemblages that characterize each facies The definitive mineral assemblages that characterize each facies (for mafic rocks).(for mafic rocks).

Metamorphic Facies defined for Metamorphic Facies defined for mafic protolithmafic protolith

Facies Definitive Mineral Assemblage in Mafic Rocks

Zeolite zeolites: especially laumontite, wairakite, analcime

Prehnite-Pumpellyite prehnite + pumpellyite (+ chlorite + albite)

Greenschist chlorite + albite + epidote (or zoisite) + quartz ± actinolite

Amphibolite hornblende + plagioclase (oligoclase-andesine) ± garnet

Granulite orthopyroxene (+ clinopyrixene + plagioclase ± garnet ±

hornblende)

Blueschist glaucophane + lawsonite or epidote (+albite ± chlorite)

Eclogite pyrope garnet + omphacitic pyroxene (± kyanite)

Contact Facies

After Spear (1993)

Table 25-1. Definitive Mineral Assemblages of Metamorphic Facies

Mineral assemblages in mafic rocks of the facies of contact meta-morphism do not differ substantially from that of the corresponding regional facies at higher pressure.

It is convenient to consider metamorphic facies in 4 groups:It is convenient to consider metamorphic facies in 4 groups:

1) Facies of high pressure1) Facies of high pressure The The blueschistblueschist and and eeccllooggiittee facies: low molar volume facies: low molar volume

phases under conditions of high pressurephases under conditions of high pressure BlueschistBlueschist facies occurs in areas of low T/P gradients, facies occurs in areas of low T/P gradients,

characteristically developed in characteristically developed in subduction zonessubduction zones EEccllooggiitteess are stable under normal geothermal are stable under normal geothermal

conditionsconditionsMay develop wherever mafic May develop wherever mafic magmas solidify in the deep crust magmas solidify in the deep crust or mantle: crustal chambers or or mantle: crustal chambers or dikes, sub-crustal magmatic dikes, sub-crustal magmatic underplates, subducted crust that underplates, subducted crust that is redistributed into the mantleis redistributed into the mantle

2) Facies of medium pressure2) Facies of medium pressure Most metamorphic rocks now exposed belong to the Most metamorphic rocks now exposed belong to the

greenschistgreenschist, , amphiboliteamphibolite, or , or granulitegranulite facies facies The The greenschistgreenschist and and amphiboliteamphibolite facies conform to the facies conform to the

“typical” geothermal“typical” geothermal gradientgradient


Fig. 25-9.Fig. 25-9. Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison. for comparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

3) Facies of low pressure3) Facies of low pressure Albite-epidote hornfelsAlbite-epidote hornfels, , hornblende hornfelshornblende hornfels, and , and

pyroxene hornfelspyroxene hornfels facies: contact metamorphic facies: contact metamorphic terranes and regional terranes with very high terranes and regional terranes with very high geothermal gradient.geothermal gradient.


SanidiniteSanidinite facies is facies is rare- limited to rare- limited to xenoliths in basic xenoliths in basic magmas and the magmas and the innermost portions of innermost portions of some contact aureoles some contact aureoles adjacent to hot basic adjacent to hot basic intrusivesintrusives


ZeoliteZeolite and and prehnite-prehnite-pumpellyitepumpellyite facies are facies are thus not always thus not always represented, and the represented, and the greenschistgreenschist facies is facies is the lowest grade the lowest grade developed in many developed in many regional terranesregional terranes

4) Facies of low grades4) Facies of low grades Rocks often fail to recrystallize thoroughly at very low Rocks often fail to recrystallize thoroughly at very low

grades, and equilibrium is not always attainedgrades, and equilibrium is not always attained

ReviewReview

• Metamorphic zone (e.g., chlorite zone)Metamorphic zone (e.g., chlorite zone)

• Index MineralIndex Mineral

• IsogradIsograd

• Metamorphic FaciesMetamorphic Facies


A traverse up grade through a metamorphic terrane should A traverse up grade through a metamorphic terrane should follow one of several possible metamorphic field follow one of several possible metamorphic field gradients, and, if extensive enough, cross through a gradients, and, if extensive enough, cross through a sequence of faciessequence of facies

Facies Series/Field GradientFacies Series/Field Gradient

Fig. 25-3.Fig. 25-3. Temperature-Temperature-pressure diagram pressure diagram showing the three showing the three major types of major types of metamorphic metamorphic facies series facies series proposed by proposed by Miyashiro (1973, Miyashiro (1973, 1994). 1994). Winter Winter (2001) An (2001) An Introduction to Introduction to Igneous and Igneous and Metamorphic Metamorphic Petrology. Petrology. Prentice Hall.Prentice Hall.

Field gradient

Pressure-Temperature Time Paths

•Facies concept leads to idea that metamorphic petrologists try to reconstruct CONDITIONS of metamorphism.

•Also important is TIME. Time tells us about the RATES of processes.

Regional Metamorphism

3 stages:

Burial/crustal thickening--why does trajectory have steep slope?

Heating stage

Uplift stage

Regional Metamorphism

What are prograde vs. retrograde metamorphic paths or reactions?

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.

Example of Contact Metamorphism

What does this diagram show? ------------------>

Explain how the metamorphic grade and assemblages MIGHT change with distance from this dike.

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