GY303 Igneous & GY303 Igneous & Metamorphic PetrologyMetamorphic Petrology

Lecture 10: Metamorphic Rock Lecture 10: Metamorphic Rock AssociationsAssociations

Outline of this PresentationOutline of this Presentation

•• Graphical Representation of Metamorphic Graphical Representation of Metamorphic Reactions and Mineral AssemblagesReactions and Mineral Assemblages

•• Types of Metamorphic ReactionsTypes of Metamorphic Reactions•• Metamorphism of Mafic and Ultramafic Igneous Metamorphism of Mafic and Ultramafic Igneous

RocksRocks•• Metamorphism of Aluminous Clastic Sedimentary Metamorphism of Aluminous Clastic Sedimentary

RocksRocks•• Metamorphism of Calcareous (carbonate) RocksMetamorphism of Calcareous (carbonate) Rocks

MultiMulti--component Systemscomponent Systems

•• Ternary phase diagrams are Ternary phase diagrams are the most usefulthe most useful

•• Quaternary systems are used Quaternary systems are used but only as but only as ““projectedprojected”” from from an apex to a ternary an apex to a ternary ““faceface””

•• Relatively pure minerals with Relatively pure minerals with no significant solid solution no significant solid solution will plot as pointswill plot as points

•• Solid solution mineral phases Solid solution mineral phases will generally plot as lines or will generally plot as lines or areas on a ternary phase areas on a ternary phase diagramdiagram

Example of Ternary Metamorphic Example of Ternary Metamorphic Phase DiagramPhase Diagram•• Anhydrous CaOAnhydrous CaO--Al2O3Al2O3--SiO2 (CO2 saturated) systemSiO2 (CO2 saturated) system•• Most likely bulk compositions would fall into a divariant field Most likely bulk compositions would fall into a divariant field

consisting of 3 phasesconsisting of 3 phases

Figure 19-6: CaO-Al2O3-SiO2 Ternary

Bulk composition =An + Q + Aluminosilicate

EskolaEskola’’ss ACF TernaryACF Ternary

•• EskolaEskola (Father of Metamorphic Petrology)(Father of Metamorphic Petrology)•• ACF: (all components in molecular proportions) ACF: (all components in molecular proportions)

–– A = Al2O3 + Fe2O3 A = Al2O3 + Fe2O3 –– (Na2O + K2O) (Na2O + K2O) –– C = CaO C = CaO –– 3.3 P2O53.3 P2O5–– F = FeO + MgO + MnOF = FeO + MgO + MnO

•• Diagram is a good phase model for calcareous Diagram is a good phase model for calcareous mud rocksmud rocks

•• Assumes that quartz is present in rockAssumes that quartz is present in rock

ACF Calculation from Weight % ACF Calculation from Weight % OxidesOxides•• Oxides are converted to molecular Oxides are converted to molecular

proportionsproportions•• A = Al2O3 + Fe2O3 A = Al2O3 + Fe2O3 –– (Na2O + (Na2O +

K2O) = 0.1539 + 0.0019 K2O) = 0.1539 + 0.0019 –– (0.029 + (0.029 + 0.0002) = 0.12660.0002) = 0.1266

•• C = CaO C = CaO –– 3.33 (P2O5) = 0.2138 3.33 (P2O5) = 0.2138 ––3.33(0.0015) = 0.20893.33(0.0015) = 0.2089

•• F = FeO + MgO + MnO = 0.0237 + F = FeO + MgO + MnO = 0.0237 + 0.0670 + 0.0042 = 0.09490.0670 + 0.0042 = 0.0949

•• A, C, F are then converted to A, C, F are then converted to ternary proportions and plotted on ternary proportions and plotted on the ACF ternary graphthe ACF ternary graph

•• A% = A/(A+C+F)*100 = 0.1266 / A% = A/(A+C+F)*100 = 0.1266 / (0.1266 + 0.2089 + 0.0949) *100 = (0.1266 + 0.2089 + 0.0949) *100 = 29.5%29.5%

•• C% = C/(A+C+F)*100 = 48.5%C% = C/(A+C+F)*100 = 48.5%•• F% = F/(A+C+F)*100 = 22.0%F% = F/(A+C+F)*100 = 22.0%

0.00150.0015141.94141.940.210.21P2O5P2O5

0.00020.000294.2094.200.020.02K2OK2O

0.02900.029061.9861.981.801.80Na2ONa2O

0.21380.213856.0856.0811.9911.99CaOCaO

0.06700.067040.3040.302.702.70MgOMgO

0.00420.004270.9470.940.300.30MnOMnO

0.02370.023771.8571.851.701.70FeOFeO

0.00190.0019159.64159.640.300.30Fe2O3Fe2O3

0.15390.1539101.96101.9615.6915.69Al2O3Al2O3

1.07711.077160.0860.0864.7264.72SiO2SiO2

Mol. Mol. Prop. Prop. OxideOxide

M.W. M.W. OxideOxide

SampleSampleWt %Wt %

OxideOxide

ACF MineralsACF Minerals

•• Minerals: Anorthite, Epidote, Minerals: Anorthite, Epidote, Grossular, Calcite, Grossular, Calcite, Wollastonite, Dolomite, Wollastonite, Dolomite, Diopside, Tremolite, Diopside, Tremolite, Hornblende, Talc, Hornblende, Talc, Orthopyroxene, Chlorite, Orthopyroxene, Chlorite, Garnet, Cordierite, Staurolite, Garnet, Cordierite, Staurolite, AluminosilicateAluminosilicate

•• Note that Hornblende, Chlorite Note that Hornblende, Chlorite and Garnet have significant and Garnet have significant solid solutionsolid solution

•• Chlorite and Garnet overlap in Chlorite and Garnet overlap in compositioncomposition-- which is present which is present depends on metamorphic depends on metamorphic gradegrade

5050505000Ca(Mg,Fe)Si2O6 Ca(Mg,Fe)Si2O6 (1CaO + (1CaO + 1(MgO,FeO) + 1(MgO,FeO) + 0Al2O30Al2O3

DiopsideDiopside

0000100100Al2SiO5Al2SiO5AluminoAlumino--silicatesilicate

75752525Ca3Al2Si3O12 Ca3Al2Si3O12 (3CaO + 1Al2O3 (3CaO + 1Al2O3 + 0FeO)+ 0FeO)

GrossularGrossular

0050505050CaAl2Si2O8CaAl2Si2O8(1CaO + 1Al2O3 (1CaO + 1Al2O3 + 0FeO)+ 0FeO)

AnorthiteAnorthite

F%F%C%C%A%A%Molecular Molecular FormulaFormula

MineralMineral

AA’’KF TernaryKF Ternary

•• Useful as a companion to ACF diagrams for KUseful as a companion to ACF diagrams for K--bearing metamorphic minerals in aluminous bearing metamorphic minerals in aluminous metameta--sedimentary (metapelite) rockssedimentary (metapelite) rocks

•• AA’’KF:KF:–– AA’’ = Al2O3 + Fe2O3 = Al2O3 + Fe2O3 –– (Na2O + K2O + CaO)(Na2O + K2O + CaO)–– K = K2OK = K2O–– F = FeO + MgO + MnOF = FeO + MgO + MnO

•• Minerals: Muscovite, Biotite, KMinerals: Muscovite, Biotite, K--Feld, Feld, Aluminosilicate, Staurolite, Cordierite, Garnet, Aluminosilicate, Staurolite, Cordierite, Garnet, ChloriteChlorite

ACF/AACF/A’’KF Diagram KF Diagram

•• Combined diagrams are a good way to view mineral Combined diagrams are a good way to view mineral assemblages in most metasedimentary rocksassemblages in most metasedimentary rocks

Figure 19-7 : ACF & A’KF Ternary graphs

AKFM TetrahedronAKFM Tetrahedron

•• Projects bulk composition from Ms to the AFM ternary Projects bulk composition from Ms to the AFM ternary ““faceface””

•• Assumes Assumes Q+MsQ+Ms are always present in the metamorphic are always present in the metamorphic rockrock

•• Good for visualizing the variability of Chlorite, Biotite, Good for visualizing the variability of Chlorite, Biotite, Garnet and Staurolite solid solutionGarnet and Staurolite solid solution

AKFM Projection (From Ms)AKFM Projection (From Ms)

AFM TernaryAFM Ternary

•• Good for FeGood for Fe--Mg solid solution phasesMg solid solution phases

Ternary Ternary ““TieTie--LinesLines””

•• Define mineral phases in Define mineral phases in equilibrium with bulk equilibrium with bulk compositioncomposition

•• Example (x) is in equilibrium Example (x) is in equilibrium with with St+Pl+GaSt+Pl+Ga on the ACF on the ACF ternaryternary

•• Example (x) is in equilibrium Example (x) is in equilibrium with with Ga+St+BiGa+St+Bi in the AFMin the AFM

•• Example (x) is in equilibrium Example (x) is in equilibrium with with Ga+St+Ms+BiGa+St+Ms+Bi in the in the AA’’KF indicating KF indicating disdis--equlibriumequlibrium or that the or that the composition is on a composition is on a univariant curveunivariant curve

Metamorphic Reactions on Ternary Metamorphic Reactions on Ternary Phase DiagramsPhase Diagrams

•• In general the breakdown of a mineral In general the breakdown of a mineral because it is unstable at current PT because it is unstable at current PT conditions will cause a reconditions will cause a re--alignment of tie alignment of tie lines and new 3lines and new 3--phase stability fieldsphase stability fields

•• An example would be the loss of staurolite An example would be the loss of staurolite in a schist due to the reaction:in a schist due to the reaction:St + Ms + Q = Ga + Bi + St + Ms + Q = Ga + Bi + SilSil + H2O+ H2O

Metamorphic Reactions cont.Metamorphic Reactions cont.

•• Figure 20Figure 20--5 (A) & (B): Compositions x, y, z contain different stable 5 (A) & (B): Compositions x, y, z contain different stable assemblages at lower assemblages at lower staurolitestaurolite gradegrade

•• At higher grade (At higher grade (staurolitestaurolite unstable) all 3 compositions contain the same unstable) all 3 compositions contain the same mineral assemblage mineral assemblage Ga+Sill+BiGa+Sill+Bi (but with different proportions).(but with different proportions).

Types of Metamorphic ReactionsTypes of Metamorphic Reactions

•• Most are dehydration reactions:Most are dehydration reactions:–– Garnet + chlorite + muscovite = Garnet + chlorite + muscovite = staurolitestaurolite + +

biotite + quartz + H2Obiotite + quartz + H2O

•• DecarbonizationDecarbonization reactions are common in reactions are common in calcareous calcareous protolithsprotoliths::–– CaCO3 + SiO2 = CaSiO3 (CaCO3 + SiO2 = CaSiO3 (wollastonitewollastonite) + CO2) + CO2

Metamorphism of Mafic & Metamorphism of Mafic & Ultramafic RocksUltramafic Rocks

•• Seafloor Metamorphism: hydrothermal Seafloor Metamorphism: hydrothermal seawater fluid circulates through new seawater fluid circulates through new ocean lithosphere ocean lithosphere

•• Mafic Low Grade: Chl + Mafic Low Grade: Chl + AbAb--Pl + Act + Pl + Act + EpEp•• Mafic Med. Grade: Pl + Mafic Med. Grade: Pl + HBlHBl + Ga+ Ga•• Mafic High Grade: Mafic High Grade: CpxCpx + Opx + Garnet + + Opx + Garnet +

HblHbl

Ultramafic Ultramafic ProtolithsProtoliths

•• Low grade: Chl + Ol + Talc + Low grade: Chl + Ol + Talc + TremTrem•• Med grade: Chl + Ol + Med grade: Chl + Ol + AnthAnth + + TremTrem•• High grade: Ol + Opx + High grade: Ol + Opx + CpxCpx + + SpinelSpinel

ACF Ternary for Mafic CompositionsACF Ternary for Mafic Compositions

•• Lower Greenschist Facies (Barrovian)Lower Greenschist Facies (Barrovian)•• Chl+Ep+CtChl+Ep+Ct commoncommon

ACF Ternary for Mafic ACF Ternary for Mafic Compositions/Lower Amphibolite Compositions/Lower Amphibolite FaciesFacies•• Plagioclase is dominantly Plagioclase is dominantly AbAb•• Greenstones fall in Pl + Greenstones fall in Pl + EpEp + Mg+ Mg--Chl ternaryChl ternary

ACF for Mafic compositions/ Middle ACF for Mafic compositions/ Middle Grade Barrovian TypeGrade Barrovian Type

•• Middle Amphibolite FaciesMiddle Amphibolite Facies•• Mafic basalt/gabbro = Pl + Mafic basalt/gabbro = Pl + HblHbl + Ga+ Ga

ACF at Upper Amphibolite Facies/ ACF at Upper Amphibolite Facies/ Mafic compositionsMafic compositions

•• Upper amphibolite Upper amphibolite faciesfacies/ Granulite / Granulite faciesfacies•• Mafic basalt/gabbro = Mafic basalt/gabbro = HblHbl + Pl + + Pl + CpxCpx

ACF Granulite Facies for Mafic ACF Granulite Facies for Mafic RocksRocks

•• Hornblende and other hydrated mineral phases Hornblende and other hydrated mineral phases become unstablebecome unstable

•• Opx may form in Opx may form in MgO+FeOMgO+FeO rich compositionsrich compositions

Metamorphism of Aluminous Clastic Metamorphism of Aluminous Clastic Rocks (Rocks (MetapelitesMetapelites))

•• Most sedimentary rocks are clastic, and Most sedimentary rocks are clastic, and most clastic sediments are aluminous most clastic sediments are aluminous shale, shale, claystoneclaystone or mudstonesor mudstones

•• Chemical weathering favors concentration Chemical weathering favors concentration of Al2O3 (most insoluble oxide)of Al2O3 (most insoluble oxide)

Metamorphism of Metamorphism of PelitesPelites: Barrovian : Barrovian FaciesFacies

•• PelitesPelites are the most likely crustal rock to are the most likely crustal rock to become metamorphosed become metamorphosed

•• PelitesPelites are chemically reactiveare chemically reactive•• AA’’KF and AFM ternary diagrams are most KF and AFM ternary diagrams are most

effective in displaying mineral effective in displaying mineral assemblagesassemblages

Barrovian IsogradsBarrovian Isograds•• Each isograd indicates increasing grade (T)Each isograd indicates increasing grade (T)

Example: Chlorite to Example: Chlorite to BiotiteBiotite

•• Isograd represents changing Isograd represents changing stability area on phase diagramstability area on phase diagram

Staurolite Staurolite ““OutOut”” ReactionReaction

•• In some cases the loss of a mineral phase In some cases the loss of a mineral phase is significantis significant

Staurolite Staurolite ““OutOut”” PT phase diagramPT phase diagram

•• St+Ms+QSt+Ms+Q = = Als+Bt+GaAls+Bt+Ga

Barrovian Pelitic Chlorite ZoneBarrovian Pelitic Chlorite Zone

•• Pelitic rocks (Aluminous shale protolith)Pelitic rocks (Aluminous shale protolith)•• FineFine--grained slates, grained slates, phyllitesphyllites•• Qtz+Ms(phengite)+Chl+AbQtz+Ms(phengite)+Chl+Ab

Barrovian Pelitic Biotite ZoneBarrovian Pelitic Biotite Zone

•• First appearance of small biotite crystals in First appearance of small biotite crystals in a finea fine--grained schist (grained schist (““spotted schistspotted schist””))

•• Qtz+Chl+Bi+AbQtz+Chl+Bi+Ab•• Note that the biotite producing reaction Note that the biotite producing reaction

may consume chlorite, but not all chlorite may consume chlorite, but not all chlorite will be consumed and some remains in will be consumed and some remains in equilibrium with biotite.equilibrium with biotite.

Barrovian Pelitic Garnet ZoneBarrovian Pelitic Garnet Zone

•• 11stst appearance of small almandine (Fe) garnets appearance of small almandine (Fe) garnets in schistin schist

•• Note that with the right bulk composition that Note that with the right bulk composition that garnet may form before biotitegarnet may form before biotite

•• Qtz+Ms+Bi+Ga+AbQtz+Ms+Bi+Ga+Ab•• Appearance of garnet is typically around 450CAppearance of garnet is typically around 450C•• As more garnet is produced chlorite is consumed As more garnet is produced chlorite is consumed

entirely as increasing grade (T).entirely as increasing grade (T).

““Chlorite out, Garnet Chlorite out, Garnet inin”” ReactionReaction•• Note that the Note that the Ga+Chl+BiGa+Chl+Bi 33--phase phase

triangle on AFM ternary keeps triangle on AFM ternary keeps shifting to the right at higher grade.shifting to the right at higher grade.

•• The Bulk composition remains in The Bulk composition remains in the same position therefore Chl is the same position therefore Chl is lost at the expense of Ga.lost at the expense of Ga.

Barrovian Pelitic Staurolite ZoneBarrovian Pelitic Staurolite Zone•• AlAl--rich: rich: Ga+St+BiGa+St+Bi•• FeFe--rich: rich: Chl+St+BiChl+St+Bi

Barrovian Pelitic Kyanite ZoneBarrovian Pelitic Kyanite Zone

•• This is the first occurrence in pelitic rocks This is the first occurrence in pelitic rocks of a aluminosilicate phase (T=550C)of a aluminosilicate phase (T=550C)

•• Staurolite is lost to leave Staurolite is lost to leave Ky+Bi+GaKy+Bi+Ga

Ky

Ga

BiMs

Q

Barrovian Pelitic Sillimanite ZoneBarrovian Pelitic Sillimanite Zone

•• Highest grade Highest grade attained without attained without loss of schistose loss of schistose texturetexture

•• Sill+Ga+BiSill+Ga+Bi

Figure 22-9A : Sillimanite as fibrolite from a Sill+Ga+Bi+Ms+Q schist

Sill (fibrolite)

Barrovian Pelitic Barrovian Pelitic Sillimanite+OrthoclaseSillimanite+Orthoclase zonezone•• Represents loss of Represents loss of

muscovite by the muscovite by the ractionractionMs+QMs+Q = Sill+Ksp+H2O= Sill+Ksp+H2O

•• The production of KThe production of K--feldspar and loss of feldspar and loss of muscovite usually produces muscovite usually produces a gneissic texture from the a gneissic texture from the former schistformer schist

Figure 22-9B: Coarse prismatic sillimanite (diamond shaped cross-section)from a Q+Bi+Ga+Sill+Ksp gneiss

Sill

Ga Bi

Buchan (LowBuchan (Low--P) Pelitic Mineral P) Pelitic Mineral AssemblagesAssemblages•• The Buchan zone represents The Buchan zone represents

geothermal gradients between geothermal gradients between HornfelsHornfels (contact) and (contact) and Barrovian gradients (i.e. higher Barrovian gradients (i.e. higher T at equivalent P compared to T at equivalent P compared to Barrovian)Barrovian)

•• Key mineral phases are Key mineral phases are Andalusite and CordieriteAndalusite and Cordierite

Figure 22-14A,B,C: AFM diagrams for the Buchan metamorphic gradient

MigmatiticMigmatitic (Partially Melted) (Partially Melted) Metamorphic RocksMetamorphic Rocks•• The upper end of metamorphism is The upper end of metamorphism is

melting melting •• Melting does not occur equally in the Melting does not occur equally in the

variable composition of variable composition of metasediments therefore the result metasediments therefore the result is is migmatitemigmatite (mixed rock)(mixed rock)

Figure 22-16: Melting curve with important metamorphic reactions

Figure 22-15: migmatite exposure

Eutectic Melting in Eutectic Melting in ““WetWet”” QQ--PlPl--KspKspsystemsystem

•• The eutectic of the QThe eutectic of the Q--PlPl--KspKsp system is close to system is close to the bulk composition of pelitic and arkosic the bulk composition of pelitic and arkosic sandstone favoring melt productionsandstone favoring melt production

•• Resulting melts are SResulting melts are S--type granite, and leave type granite, and leave behind granulite behind granulite Ga+Cpx+OpxGa+Cpx+Opx rocks rocks

Figure 22-17: Q-Pl-Ksp igneous system.

Metamorphism of Calcareous Metamorphism of Calcareous (Carbonate) Rocks (Carbonate) Rocks

•• Protolith would be limestone or dolostoneProtolith would be limestone or dolostone•• Metamorphic fluid will contain significant Metamorphic fluid will contain significant

amounts of CO2amounts of CO2•• Impure limestone and dolostone Impure limestone and dolostone protolithsprotoliths

contain more chemical components and contain more chemical components and therefore produce more stable minerals at therefore produce more stable minerals at a given Pa given P--T condition T condition

Typical Mineral Assemblages in Typical Mineral Assemblages in Calcareous Calcareous ProtolithsProtoliths•• Very low gradeVery low grade

–– Pure marbles: Pure marbles: Cc+Do+QtzCc+Do+Qtz–– Impure marbles: Impure marbles: Cc+Chl+Ab+QtzCc+Chl+Ab+Qtz

•• Low gradeLow grade–– Pure marbles: Pure marbles: Cc+Do+Q+TlcCc+Do+Q+Tlc–– Impure marbles: Impure marbles: Cc+Chl+Ms+Ab+QCc+Chl+Ms+Ab+Q

•• Medium GradeMedium Grade–– Pure Marbles: Pure Marbles: Cc+Do+Q+Cpx+TremCc+Do+Q+Cpx+Trem–– Impure Marbles: Impure Marbles: Cc+Bi+Pl+CaCc+Bi+Pl+Ca--Amph+Sph+Q+EpAmph+Sph+Q+Ep

•• High GradeHigh Grade–– Pure Marbles: Pure Marbles: Cc+Do+Ol+WoCc+Do+Ol+Wo–– Impure Marbles: CaImpure Marbles: Ca--Amph+Cpx+Ep+CaAmph+Cpx+Ep+Ca--Pl+Sph+Q+CcPl+Sph+Q+Cc

•• Very High/ContactVery High/Contact–– Pure Marbles: Pure Marbles: Cc+Cpx+Ol+WoCc+Cpx+Ol+Wo–– Impure Marbles: CaImpure Marbles: Ca--Ga+CaGa+Ca--Pl+Cpx+Sph+QPl+Cpx+Sph+Q

CaCa--MgMg--SiSi Ternary: Low to Medium Ternary: Low to Medium GradeGrade

•• Dark gray= Dark gray= pure pure marble; light marble; light gray = gray = siliceous siliceous marblemarble

Figure 23-5: Increasing grade from A to D.

CaCa--MgMg--SiSi Ternary: Medium to High Ternary: Medium to High GradeGrade

Figure 23-7: Grade increasing from A to D.

Exam Summary for Metamorphic Exam Summary for Metamorphic Rock AssociationsRock Associations

•• Know the definitions of the ternary classification Know the definitions of the ternary classification diagrams (ACF, AKF, AFM).diagrams (ACF, AKF, AFM).

•• A in ACF = Al2O3 + Fe2O3 A in ACF = Al2O3 + Fe2O3 –– (Na2O + K2O) (Na2O + K2O) •• C in ACF = CaO C in ACF = CaO –– 3.3 P2O53.3 P2O5•• F in ACF = FeO + MgO + MnOF in ACF = FeO + MgO + MnO•• Know how to plot minerals on ACF, etc., for Know how to plot minerals on ACF, etc., for

example Anorthite (CaAl2Si2O8) = 1C + 1A + 0F example Anorthite (CaAl2Si2O8) = 1C + 1A + 0F = 50%C + 50% A + 0%F = 50%C + 50% A + 0%F

Exam Summary continued Exam Summary continued ……

•• Know what mineral assemblages are best for a particular Know what mineral assemblages are best for a particular ternary (AFM is good for Feternary (AFM is good for Fe--Mg solid solutions like Mg solid solutions like Garnet and Biotite)Garnet and Biotite)

•• Be able to determine the stable mineral assemblage on a Be able to determine the stable mineral assemblage on a ternary given a bulk rock compositionternary given a bulk rock composition

•• Be able to explain the meaning of Be able to explain the meaning of ““crossingcrossing”” tie lines tie lines (i.e. sample is in disequilibrium or it happens to be (i.e. sample is in disequilibrium or it happens to be located on a univariant curve)located on a univariant curve)

•• Be able to predict mineral reactions or replacement Be able to predict mineral reactions or replacement textures based on a bulk composition on 2 different textures based on a bulk composition on 2 different ternary diagrams representing a change in grade.ternary diagrams representing a change in grade.