Ch 25 Metamorphic Facies. V.M. Goldschmidt (1911, 1912a) studied contact metamorphosed pelitic (mudrocks), calcareous (limestone and dolostone), and psammitic

Ch 25 Metamorphic FaciesCh 25 Metamorphic Facies

•V.M. Goldschmidt (1911, 1912a) studied contact V.M. Goldschmidt (1911, 1912a) studied contact metamorphosed pelitic (mudrocks) , calcareous metamorphosed pelitic (mudrocks) , calcareous (limestone and dolostone) , and psammitic (limestone and dolostone) , and psammitic (sandstone) hornfels near Oslo s. Norway(sandstone) hornfels near Oslo s. Norway•Fewer than six major minerals in the aureoles Fewer than six major minerals in the aureoles around granitoid intrusivesaround granitoid intrusives•First to note that the equilibrium mineral First to note that the equilibrium mineral assemblage of a metamorphic rock could be assemblage of a metamorphic rock could be related to Xrelated to Xbulkbulk

•Aluminous pelites contained Al-rich minerals, Aluminous pelites contained Al-rich minerals, such as cordierite, plagioclase, garnet, and/or an such as cordierite, plagioclase, garnet, and/or an AlAl22SiOSiO55 polymorph polymorph

•Calcareous rocks contained Ca-rich and Al-poor Calcareous rocks contained Ca-rich and Al-poor minerals such as Diopside, Wollastonite, and/or minerals such as Diopside, Wollastonite, and/or amphiboleamphibole

Certain mineral pairs (e.g. anorthite + enstatite) Certain mineral pairs (e.g. anorthite + enstatite) were consistently present in rocks of appropriate were consistently present in rocks of appropriate composition, whereas the compositionally composition, whereas the compositionally equivalent pair (diopside + andalusite) was notequivalent pair (diopside + andalusite) was not

If two alternative assemblages are X-equivalent, If two alternative assemblages are X-equivalent, we must be able to relate them by a reactionwe must be able to relate them by a reaction

In this case the reaction is simple:In this case the reaction is simple:

MgSiOMgSiO33 + CaAl + CaAl22SiSi22OO88 = CaMgSi = CaMgSi22OO66 + Al + Al22SiOSiO55

EnEn An An Di Di And And

Victor Moritz Goldschmidt- Victor Moritz Goldschmidt- OsloOslo

Pentii Eskola (1914, 1915) Orijärvi, S. Pentii Eskola (1914, 1915) Orijärvi, S. FinlandFinland

Rocks with K-feldspar + cordierite at Rocks with K-feldspar + cordierite at Oslo contained the compositionally Oslo contained the compositionally equivalent pair biotite + muscovite at equivalent pair biotite + muscovite at OrijärviOrijärvi

Eskola: difference must reflect differing Eskola: difference must reflect differing physical conditionsphysical conditions

His Finnish rocks (more hydrous and His Finnish rocks (more hydrous and lower volume assemblage) equilibrated lower volume assemblage) equilibrated at lower temperatures and higher at lower temperatures and higher pressures than the Norwegian ones of pressures than the Norwegian ones of Goldschmidt.Goldschmidt.

Metamorphic FaciesMetamorphic Facies

Oslo: Oslo: Kfs + Mg-CrdKfs + Mg-Crd

Orijärvi: Orijärvi: Biot + Ms + QtzBiot + Ms + Qtz

Reaction: Reaction: 2 KMg2 KMg33AlSiAlSi33OO1010(OH)(OH)22 + 6 KAl + 6 KAl22AlSiAlSi33OO1010(OH)(OH)22 + 15 + 15

SiOSiO22

BiotBiot MsMs QtzQtz

= 3 Mg= 3 Mg22AlAl44SiSi55OO1818 + 8 KAlSi + 8 KAlSi33OO88 + 8 H + 8 H22OO

Mg-CrdMg-Crd K-spar K-spar waterwater

Metamorphic Metamorphic FaciesFacies

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

““In any rock or metamorphic formation which has arrived at In any rock or metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant a chemical equilibrium through metamorphism at constant temperature and pressure conditions, temperature and pressure conditions, the mineral the mineral composition is controlled only by the chemical compositioncomposition is controlled only by the chemical composition. . We are led to a general conception which the writer We are led to a general conception which the writer proposes to call metamorphic facies.”proposes to call metamorphic facies.”

Eskola ‘s dual basis facies : XEskola ‘s dual basis facies : Xbulkbulk & & mineralogymineralogy–A metamorphic facies is a set of A metamorphic facies is a set of repeatedly associated metamorphic repeatedly associated metamorphic mineral assemblagesmineral assemblages


– Eskola was aware of the P-T Eskola was aware of the P-T implications and correctly deduced implications and correctly deduced the relative temperatures and the relative temperatures and pressures of facies he proposedpressures of facies he proposed

– Modern lab results: can now assign Modern lab results: can now assign relatively accurate temperature and relatively accurate temperature and pressure limits to individual faciespressure limits to individual facies


Eskola (1920) proposed 5 original rock Eskola (1920) proposed 5 original rock facies:facies:– GreenschistGreenschist– AmphiboliteAmphibolite– Hornfels Hornfels (this term has too many conflicting (this term has too many conflicting

definitions)definitions)

– 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 rocksrocks


In his final account, Eskola (1939) In his final account, Eskola (1939) added:added:– GranuliteGranulite– Epidote-amphiboliteEpidote-amphibolite– Glaucophane-schist Glaucophane-schist (now called (now called

BlueschistBlueschist))... and changed the name of the hornfels ... and changed the name of the hornfels

facies to the facies to the pyroxene hornfelspyroxene hornfels facies facies


Mafic Metamorphic Mafic Metamorphic FaciesFacies

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. text.

•The limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phase •The 30The 30ooC/km C/km geothermal geothermal gradient is an gradient is an example of an example of an elevated orogenic elevated orogenic geothermal geothermal gradient. gradient.

Table 25-1. The definitive mineral Table 25-1. The definitive mineral assemblages that characterize each facies assemblages that characterize each facies (for mafic rocks).(for mafic rocks).


Miyashiro (1961,1973) extended the facies concept to encompass broader progressive sequences: facies series

A traverse up grade through a metamorphic terrane should follow one of several possible metamorphic field gradients (Fig. 21-1, next slide), and, if extensive enough, cross through a sequence of facies

Facies SeriesFacies Series

How to traverse up grade? Walk toward the migmatite.

Mineral changes and associations along T-P Mineral changes and associations along T-P gradients characteristic of the three facies gradients characteristic of the three facies seriesseries– HydrationHydration of original mafic minerals generally of original mafic minerals generally

requiredrequired– If water unavailable, mafic igneous rocks will remain If water unavailable, mafic igneous rocks will remain

largely unaffected, even as associated sediments are largely unaffected, even as associated sediments are completely re-equilibratedcompletely re-equilibrated

– Coarse-grained intrusives are the least permeable Coarse-grained intrusives are the least permeable and likely to resist metamorphic changesand likely to resist metamorphic changes

– Tuffs and graywackes are the most permeable and so Tuffs and graywackes are the most permeable and so subject to metamorphic changes.subject to metamorphic changes.

Metamorphism of Mafic Metamorphism of Mafic RocksRocks

The greenschist, amphibolite and granulite The greenschist, amphibolite and granulite facies constitute the most common facies facies constitute the most common facies series of regional metamorphism on Mafic series of regional metamorphism on Mafic rocksrocks

Greenschist, Amphibolite, Greenschist, Amphibolite, and Granulite Faciesand Granulite Facies

Metamorphism Of A Basaltic Metamorphism Of A Basaltic RockRockExample:Example: Progressive Thermal Metamorphism Progressive Thermal Metamorphism

of a Diabase (usually a shallow dike, of a Diabase (usually a shallow dike, basaltic in composition)basaltic in composition)

Each successive case is hotter: closer to the magma

Progressive thermal metamorphism of a Diabase (coarse basaltic)

Stage 1: lots of relict textures from Diabase Augite and Plagioclase

• Plag becomes Albite NaAlSi3O8, so it loses Ca and (Ca -> calcite)

• Augite (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6

hydrates -> Chlorite(Mg,Fe)3(Si,Al)4O10(OH)2

.

(Mg,Fe)3(OH)6

And Actinolite• Ca2(Mg,Fe)5Si8O22(OH)2.

• Color is greener

Sphene is CaTiSiO5

HOT

GREENSCHIST 1 200C

Progressive thermal metamorphism of a diabase (coarse basalt)

Stage 2- closer to igneous body, thoroughly recrystallized

Epidote forms as more Ca released from Calcite

Even greener

Epidote is Ca2Al2(Fe+3Al)(SiO4)(Si2O7)O(OH)

EVEN HOTTERGreenschist 2

Progressive thermal metamorphism of a diabase

Stage 3: Warmer yet, Actinolite, Chlorite, Epidote, Calcite become unstable as Hornblende and more calcic plagioclase become stable “Amphibolite”

Note coarser grain size as well

REALLY HOT 500C

Amphibolite

Progressive thermal metamorphism of a diabase (coarse basalt)

Stage 4: Right up against a magma (hot), dehydration at high grade -> Cpx + Plagioclase rock

Note that mineralogy is now ~ same as original diabase, but the granoblastic texture is new

VERY HOT> 700C

Granulite

Origin of granulite facies: general agreement on Origin of granulite facies: general agreement on two pointstwo points

1) Granulites represent unusually hot conditions1) Granulites represent unusually hot conditions– Temperatures > 700Temperatures > 700ooC (geothermometry has C (geothermometry has

yielded some very high temperatures, even yielded some very high temperatures, even in excess of 1000in excess of 1000ooC)C)

– Granoblastic: Phaneritic, equidimensional, Granoblastic: Phaneritic, equidimensional, not foliated, no porphyroblastsnot foliated, no porphyroblasts

Granulite FaciesGranulite Facies

2) Granulites are dry, no significant water2) Granulites are dry, no significant water– Rocks don’t melt despite high Temps due to lack of Rocks don’t melt despite high Temps due to lack of

available wateravailable water– Granulite facies represent deeply buried and Granulite facies represent deeply buried and

dehydrated roots of the continental crustdehydrated roots of the continental crust– Fluid inclusions in granulite facies rocks of S. Fluid inclusions in granulite facies rocks of S.

Norway are CONorway are CO22-rich, whereas those in the -rich, whereas those in the amphibolite facies rocks are Hamphibolite facies rocks are H22O-richO-rich

Granulite FaciesGranulite Facies

Fig. 25-2.Fig. 25-2. Temperature-Temperature-pressure diagram pressure diagram showing the showing the generally generally accepted limits of accepted limits of the various facies the various facies used in this text. used in this text. Winter (2001) An Winter (2001) An Introduction to Introduction to Igneous and Igneous and Metamorphic Metamorphic Petrology. Petrology. Prentice Hall.Prentice Hall.

Ophiolite suite = Ocean Ophiolite suite = Ocean LithosphereLithosphere

MaficMafic rocks (not pelites) develop definitive mineral rocks (not pelites) develop definitive mineral assemblages under high P/T conditionsassemblages under high P/T conditions

High P/T geothermal gradients characterize High P/T geothermal gradients characterize subduction zonessubduction zones

Mafic Mafic blueschistsblueschists are easily recognizable by their are easily recognizable by their color, and are useful indicators of ancient subduction color, and are useful indicators of ancient subduction zones. Formation requires abundant water present zones. Formation requires abundant water present near subducted ophiolites.near subducted ophiolites.

The great density of The great density of eclogiteseclogites: subducted basaltic : subducted basaltic oceanic crust becomes more dense than the oceanic crust becomes more dense than the surrounding mantlesurrounding mantle

Begins to sink , extended necks thin and break off Begins to sink , extended necks thin and break off piecespieces

Mafic Assemblages of the High Mafic Assemblages of the High P/T Series: Blueschist and P/T Series: Blueschist and

Eclogite FaciesEclogite Facies

Proposed metamorphic P-T-t paths may be Proposed metamorphic P-T-t paths may be tested by:tested by:

1) Checking for partial overprints of one mineral 1) Checking for partial overprints of one mineral assemblage upon anotherassemblage upon another– The relict minerals may indicate a portion of either The relict minerals may indicate a portion of either

the prograde or retrograde path (or both) the prograde or retrograde path (or both) depending upon when they were createddepending upon when they were created

Pressure-Temperature-Time (P-T-t) Pressure-Temperature-Time (P-T-t) PathsPaths

Muscovite + chlorite + quartz + staurolite = andalusite / sillimanite + biotite + H20

Staurolite (St) as relict within poikiloblastic andalusite (And).

Staurolite (St) as relict within poikiloblastic andalusite (And).

Proposed metamorphic P-T-t paths may be Proposed metamorphic P-T-t paths may be tested by:tested by:

2) Applying geothermometers and 2) Applying geothermometers and geobarometers to the core vs. rim geobarometers to the core vs. rim compositions of chemically zoned minerals to compositions of chemically zoned minerals to document the changing P-T conditions document the changing P-T conditions experienced by a rock during their growthexperienced by a rock during their growth


Classic view: regional metamorphism is a Classic view: regional metamorphism is a result of deep burial or intrusion of hot result of deep burial or intrusion of hot magmasmagmas

Plate tectonics: regional metamorphism is a Plate tectonics: regional metamorphism is a result of crustal thickening and heat input result of crustal thickening and heat input during orogeny at convergent plate during orogeny at convergent plate boundaries (not simple burial)boundaries (not simple burial)

Heat flow higher than the normal continental Heat flow higher than the normal continental geotherm is required for typical greenschist to geotherm is required for typical greenschist to amphibolite medium P/T reaction.amphibolite medium P/T reaction.


Miyashiro’s (1961,1973) facies series don’t look correct for a subduction zone.

We think that the sequence of facies from basaltic is Greenschist, then Amphibolite, then Eclogite, as if PT conditions follow something like the red line.

Chapter 26: Metamorphic Chapter 26: Metamorphic ReactionsReactions

Yellow Isograds Yellow Isograds are reaction are reaction lineslines

1. Phase 1. Phase TransformationsTransformations

Isochemical phase transformations (the Isochemical phase transformations (the polymorphs of SiOpolymorphs of SiO22 or Al or Al22SiOSiO55 or graphite- or graphite-diamond or calcite-aragonite SINGLE diamond or calcite-aragonite SINGLE COMPONENTCOMPONENT

The transformations The transformations depend on temperature depend on temperature and pressure onlyand pressure onlyAragonite is the stable CaCO3 polymorph commonly found in blueschist facies terranes


Independent of other minerals, fluids, etc., present.Example:Andalusite -> Sill as T and P increase regardless of other phases Stau, Mus, Qtz


Small Small S for most polymorphic S for most polymorphic transformationstransformations

small small G between two alternative G between two alternative polymorphs, even several tens of degrees polymorphs, even several tens of degrees from the equilibrium boundaryfrom the equilibrium boundary

little driving force for the reaction to little driving force for the reaction to proceed proceed common common metastablemetastable relics in the relics in the stability field of otherstability field of other

Coexisting polymorphs may therefore Coexisting polymorphs may therefore represent represent non-equilibrium statesnon-equilibrium states (overstepped equilibrium curves)(overstepped equilibrium curves)

Staurolite poikiloblast

2. Exsolution2. Exsolution

Figure 6-16. T-X phase diagram of the system albite-orthoclase at 0.2 GPa H2O pressure. After Bowen and Tuttle

(1950). J. Geology, 58, 489-511. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Some solid solutions are unstable at lower T, and exsolve as T falls.

Classic example K-spar KAlSi3O8 and Albite NaAlSi3O8 form a solid solution at 1000C but separate into Microcline + Albite -> Perthitic Texture

3a. Solid-Solid Net-Transfer 3a. Solid-Solid Net-Transfer ReactionsReactions

Differ from polymorphic Differ from polymorphic transformations: involve transformations: involve solids of differing solids of differing compositioncomposition

Material must diffuse Material must diffuse from one site to another from one site to another for the reaction to for the reaction to proceedproceed

NaAlSiNaAlSi22OO66 + SiO + SiO22 = NaAlSi = NaAlSi33OO88 JdJd QtzQtz Ab Ab

MgSiOMgSiO33 + CaAl + CaAl22SiSi22OO88 = CaMgSi = CaMgSi22OO66 + Al + Al22SiOSiO55 EnEn AnAn Di Di And And

4 (Mg,Fe)SiO4 (Mg,Fe)SiO33 + CaAl + CaAl22SiSi22OO88 = = OpxOpx An Plag An Plag

(Mg,Fe)(Mg,Fe)33AlAl22SiSi33OO1212 + Ca(Mg,Fe)Si + Ca(Mg,Fe)Si22OO66 + SiO + SiO22 GntGnt Cpx Qtz Cpx Qtz

Reaction curves typically pretty straightS and V change little

3b. Solid-Solid Net-Transfer 3b. Solid-Solid Net-Transfer Reactions with conserved Reactions with conserved

waterwater If minerals contain volatiles, but the If minerals contain volatiles, but the

volatiles are conservedvolatiles are conserved in the reaction in the reaction so that no fluid phase is generated or so that no fluid phase is generated or consumedconsumed

For example, the reaction:For example, the reaction:MgMg33SiSi44OO1010(OH)(OH)22 + 4 MgSiO + 4 MgSiO33 = =

MgMg77SiSi88OO2222(OH)(OH)22 Talc EnstatiteTalc Enstatite

AnthophylliteAnthophyllite

involves hydrous phases, but involves hydrous phases, but conserves Hconserves H22OO

It may therefore be treated as a solid-It may therefore be treated as a solid-solid net-transfer reactionsolid net-transfer reaction

4. Devolatilization 4. Devolatilization ReactionsReactions

For example the location of the reaction line For example the location of the reaction line on aon a

P-T phase diagram of the P-T phase diagram of the dehydrationdehydration reaction: reaction: KAlKAl22SiSi33AlOAlO1010(OH)(OH)22 + SiO + SiO22 = KAlSi = KAlSi33OO88 + Al + Al22SiOSiO55 + +

HH22OO MuscMusc Qtz Qtz K-spar K-spar Sill Sill

WaterWater

depends upon the partial pressure of Hdepends upon the partial pressure of H22O O ((ppH2OH2O))

4. Devolatilization 4. Devolatilization ReactionsReactions

Here the equilibrium Here the equilibrium curve represents curve represents equilibrium between equilibrium between the reactants and the reactants and products under products under water-saturated water-saturated conditionsconditions ( (ppH2OH2O = = PPLithostaticLithostatic))

P-T phase diagram for the reaction Ms + Qtz = Kfs + Al2SiO5 + H2O showing the shift in

equilibrium conditions as pH2O varies

(assuming ideal H2O-CO2 mixing). Calculated

using the program TWQ by Berman (1988, 1990, 1991). After Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

KAlKAl22SiSi33AlOAlO1010(OH)(OH)22 + SiO + SiO22 = KAlSi = KAlSi33OO88 + Al + Al22SiOSiO55 + H + H22OO MsMs Qtz Qtz K-spar K-spar Sill Sill water water

Suppose HSuppose H22O is withdrawn from the system at some point on O is withdrawn from the system at some point on the water-saturated equilibrium curve: the water-saturated equilibrium curve: ppH2OH2O < P < Plithostaticlithostatic

According to Le Châtelier’s Principle, removing water at According to Le Châtelier’s Principle, removing water at equilibrium will be compensated by the reaction running to the equilibrium will be compensated by the reaction running to the right, thereby producing more waterright, thereby producing more water

This has the effect of stabilizing the right side of the reaction at This has the effect of stabilizing the right side of the reaction at the expense of the left sidethe expense of the left side

So as water is withdrawn the Kfs + Sill + HSo as water is withdrawn the Kfs + Sill + H22O field expands O field expands slightly at the expense of the Ms + Qtz field, and slightly at the expense of the Ms + Qtz field, and the reaction the reaction curve shifts toward lower temperaturecurve shifts toward lower temperature

Pfluid < PLith by drying out the rock and reducing the fluid content

Pfluid = PLith, but the water in the fluid can become diluted by adding another fluid component, such as CO2 or some other volatile phase

4. Decarbonization 4. Decarbonization ReactionsReactions

Figure 26-1. A portion of the equilibrium boundary for the calcite-aragonite phase transformation in the CaCO3 system. After

Johannes and Puhan (1971), Contrib. Mineral. Petrol., 31, 28-38. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 26-5. T-XCO2 phase diagram for the reaction Cal + Qtz

= Wo + CO2 at 0.5 GPa assuming ideal H2O-CO2 mixing,

calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

CaCO3 + SiO2 = CaSiO3 + CO2 (26-6)

Cal Qtz Wollastonite + CO2

Excess CO2 drives reaction to Cal + Qtz

CaCO3 + SiO2 <= CaSiO3 + CO2

5. 5. Continuous ReactionsContinuous Reactions Occur when F Occur when F 1 1, and the reactants and , and the reactants and

products coexist over a temperature (or products coexist over a temperature (or grade) intervalgrade) interval

Fig. 26-9. Schematic isobaric T-XMg

diagram representing the simplified metamorphic reaction Chl + Qtz Grt + H2O. From Winter (2001) An

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

6. Ion Exchange Reactions6. Ion Exchange Reactions Reciprocal exchange of components between Reciprocal exchange of components between

2 or more minerals2 or more minerals

– MgSiOMgSiO33 + CaFeSi + CaFeSi22OO66 = FeSiO = FeSiO33 + CaMgSi + CaMgSi22OO66

– Enstatite + Hedenbergite = Ferrosilite + DiopsideEnstatite + Hedenbergite = Ferrosilite + Diopside Expressed as pure end-members, but really Expressed as pure end-members, but really

involves Mg-Fe (or other) exchange between involves Mg-Fe (or other) exchange between intermediate solutionsintermediate solutions

Basis for many geothermobarometersBasis for many geothermobarometershttp://www.springerlink.com/content/ghr3426g33686522/http://www.springerlink.com/content/ghr3426g33686522/

Documents

Ch 25 Metamorphic Facies. V.M. Goldschmidt (1911, 1912a) studied contact metamorphosed pelitic (mudrocks), calcareous (limestone and dolostone), and psammitic