GY303 Igneous & Metamorphic Petrology

Metamorphic Rock Associations

Outline of this Presentation

• Graphical Representation of Metamorphic Reactions and Mineral Assemblages.

• Types of Metamorphic Reactions.

• Metamorphism of Mafic and Ultramafic Igneous Rocks.

• Metamorphism of Aluminous Clastic Sedimentary Rocks.

• Metamorphism of Calcareous (carbonate) Rocks.

Multi-component Systems

• Ternary phase diagrams are the most useful.

• Quaternary systems are used but only as “projected” from an apex to a ternary “face”.

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

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

Example of Ternary Metamorphic Phase Diagram

• Anhydrous CaO-Al2O3-SiO2 (CO2 saturated) system.• Most likely bulk compositions would fall into a divariant field

consisting of 3 phases.

Winkler (1968) Figure 19-6: CaO-Al2O3-SiO2 Ternary

Bulk composition =An + Q + Aluminosilicate

Eskola’s ACF Ternary

• Eskola (Father of Metamorphic Petrology).

• ACF: (all components in molecular proportions):

– A = Al2O3 + Fe2O3 – (Na2O + K2O)

– C = CaO – 3.3 P2O5

– F = FeO + MgO + MnO

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

• Assumes that quartz is present in rock.

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

proportions:• A = Al2O3 + Fe2O3 – (Na2O +

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

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

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

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

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

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

Oxide Sample

Wt %

M.W. Oxide

Mol. Prop. Oxide

SiO2 64.72 60.08 1.0771

Al2O3 15.69 101.96 0.1539

Fe2O3 0.30 159.64 0.0019

FeO 1.70 71.85 0.0237

MnO 0.30 70.94 0.0042

MgO 2.70 40.30 0.0670

CaO 11.99 56.08 0.2138

Na2O 1.80 61.98 0.0290

K2O 0.02 94.20 0.0002

P2O5 0.21 141.94 0.0015

ACF Minerals

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

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

• Chlorite and Garnet overlap in composition- which is present depends on metamorphic grade.

Mineral Molecular Formula

A% C% F%

Anorthite CaAl2Si2O8

(1CaO + 1Al2O3 + 0FeO)

50 50 0

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

25 75 0

Alumino-silicate

Al2SiO5

(1Al2O3 + 0CaO + 0FeO)

100 0 0

Diopside Ca(Mg,Fe)Si2O6 (1CaO + 1(MgO,FeO) + 0Al2O3

0 50 50

A’KF Ternary

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

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

• Minerals: Muscovite, Biotite, K-Feld, Aluminosilicate, Staurolite, Cordierite, Garnet, Chlorite.

ACF/A’KF Diagram

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

Winkler (1968) Figure 19-7 : ACF & A’KF Ternary graphs

AKFM Tetrahedron

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

• Assumes Q+Ms are always present in the metamorphic rock.

• Good for visualizing the variability of Chlorite, Biotite, Garnet and Staurolite solid solution (Fe-Mg).

AKFM Projection (From Ms)

• Sample and mineral compositions are projected from Ms to the AFM plane.

• Ms and Qtz are assumed to be present in the rock as is H2O fluid.

AFM Ternary

• Good for Fe-Mg solid solution phases such as garnet, clinopyroxene, biotite and chlorite.

Ternary “Tie-Lines”

• Define mineral phases in equilibrium with bulk composition.

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

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

• Example (x) is in equilibrium with Ga+St+Ms+Bi in the A’KF indicating dis-equlibrium or that the composition is on a univariant curve.

Metamorphic Reactions on Ternary Phase Diagrams

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

• An example would be the loss of staurolite in a schist due to the reaction:

St + Ms + Q = Ga + Bi + Sil + H2O

Metamorphic Reactions cont.

• Figure 20-5 (A) & (B) (Winkler, 1968): Compositions x, y, z contain different stable assemblages at lower staurolite grade.

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

Types of Metamorphic Reactions

• Most are dehydration reactions:

– Garnet + chlorite + muscovite = staurolite + biotite + quartz + H2O

• Decarbonization reactions are common in calcareous protoliths:

– CaCO3 + SiO2 = CaSiO3 (wollastonite) + CO2

Metamorphism of Mafic & Ultramafic Rocks

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

• Mafic Low Grade: Chl + Ab-Pl + Act + Ep

• Mafic Med. Grade: Pl + HBl + Ga

• Mafic High Grade: Cpx + Opx + Garnet + Hbl

Ultramafic Protoliths

• Low grade: Chl + Ol + Talc + Trem

• Med grade: Chl + Ol + Anth + Trem

• High grade: Ol + Opx + Cpx + Spinel

ACF Ternary for Mafic Compositions

• Lower Greenschist Facies (Barrovian).

• Chl+Ep+Ct common.

ACF Ternary for Mafic Compositions/Lower Amphibolite Facies• Plagioclase is dominantly Ab.• Greenstones fall in Pl + Ep + Mg-Chl ternary.

ACF for Mafic compositions/ Middle Grade Barrovian Type

• Middle Amphibolite Facies.

• Mafic basalt/gabbro = Pl + Hbl + Ga.

ACF at Upper Amphibolite Facies/ Mafic compositions

• Upper amphibolite facies/ Granulite facies.

• Mafic basalt/gabbro = Hbl + Pl + Cpx.

ACF Granulite Facies for Mafic Rocks

• Hornblende and other hydrated mineral phases become unstable.

• Opx may form in MgO+FeO rich compositions.

Metamorphism of Aluminous Clastic Rocks (Metapelites)

• Most sedimentary rocks are clastic, and most clastic sediments are aluminous shale, claystone or mudstones.

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

Metamorphism of Pelites: Barrovian Facies

• Pelites are the most likely crustal rock to become metamorphosed.

• Pelites are chemically reactive.

• A’KF and AFM ternary diagrams are most effective in displaying mineral assemblages.

Barrovian Isograds• Each isograd indicates increasing grade (T)

Example: Garnet isograd

• Isograd represents changing stability area on phase diagram.

• Chlorite becomes unstable at higher T, reacts to form Ga.

Staurolite “Out” Reaction

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

Staurolite “Out” PT phase diagram

• St+Musc+Qtz = Sill+Bt+Grt

Barrovian Pelitic Chlorite Zone

• Pelitic rocks (Aluminous shale protolith).

• Fine-grained slates, phyllites.

• Qtz+Ms(phengite)+Chl+Ab.

Barrovian Pelitic Biotite Zone

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

• Qtz+Chl+Bi+Ab.

• Note that the biotite producing reaction may consume chlorite, but not all chlorite will be consumed and some remains in equilibrium with biotite.

Barrovian Pelitic Garnet Zone

• 1st appearance of small almandine (Fe) garnets in schist.

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

• Qtz+Ms+Bi+Ga+Ab.

• Appearance of garnet is typically around 450C.

• As more garnet is produced chlorite is consumed entirely as grade increases (T).

“Chlorite out, Garnet in” Reaction• Note that the Ga+Chl+Bi 3-phase

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

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

Barrovian Pelitic Staurolite Zone

• Al-rich: Ga+St+Bi.

• Fe-rich: Chl+St+Bi.

Barrovian Pelitic Kyanite Zone

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

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

Ky

Ga

BiMs

Q

Barrovian Pelitic Sillimanite Zone

• Highest grade attained without loss of schistose texture.

• Sill+Ga+Bi.

Figure 22-9A (Winkler, 1968) : Sillimanite as fibrolite from a Sill+Ga+Bi+Ms+Q schist

Sill (fibrolite)

Barrovian PeliticSillimanite+Orthoclase zone

• Represents loss of muscovite by the ractionMs+Q = Sill+Ksp+H2O.

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

Figure 22-9B (Winkler, 1968): Coarse prismatic sillimanite (diamond shaped cross-section) from a Q+Bi+Ga+Sill+Ksp gneiss.

Sill

Ga Bi

Buchan (Low-P) Pelitic Mineral Assemblages

• The Buchan zone represents geothermal gradients between Hornfels (contact) and Barrovian gradients (i.e. higher T at equivalent P compared to Barrovian).

• Key mineral phases are Andalusite and Cordierite.

Figure 22-14A,B,C (Winkler, 1968): AFM diagrams for the Buchan metamorphic gradient.

Migmatitic (Partially Melted) Metamorphic Rocks

• The upper end of metamorphism is melting.

• Melting does not occur equally in the variable composition of metasediments therefore the result is migmatite (mixed rock).

Figure 22-16 (Winkler, 1968): Melting curve with important metamorphic reactions.

Figure 22-15 (Winkler, 1968): migmatite exposure.

Eutectic Melting in “Wet” Q-Pl-Kspsystem

• The eutectic of the Q-Pl-Ksp system is close to the bulk composition of pelitic and arkosicsandstone favoring melt production.

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

Figure 22-17 (Winkler, 1968) : Q-Pl-Ksp igneous system.

Metamorphism of Calcareous (Carbonate) Rocks

• Protolith would be limestone or dolostone.

• Metamorphic fluid will contain significant amounts of CO2.

• Impure limestone and dolostone protoliths contain more chemical components and therefore produce more stable minerals at a given P-T condition.

Typical Mineral Assemblages in Calcareous Protoliths

• Very low grade– Pure marbles: Cc+Do+Qtz– Impure marbles: Cc+Chl+Ab+Qtz

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

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

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

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

Ca-Mg-Si Ternary: Low to Medium Grade

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

Figure 23-5 (Winkler, 1968): Increasing grade from A to D.

Ca-Mg-Si Ternary: Medium to High Grade

Figure 23-7 (Winkler, 1968): Grade increasing from A to D.

Exam Summary for Metamorphic Rock Associations

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

• A in ACF = Al2O3 + Fe2O3 – (Na2O + K2O).

• C in ACF = CaO – 3.3 P2O5.

• F in ACF = FeO + MgO + MnO.

• Know how to plot minerals on ACF, etc., for example Anorthite (CaAl2Si2O8) = 1C + 1A + 0F = 50%C + 50% A + 0%F.

Exam Summary continued …

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

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

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

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