Concept of Index Minerals Chlorite, biotite, garnet, kyanite, sillimanite Only exist over a narrow P-T range

Geologic Mapping of Metamorphic Terranes Index minerals are mapped into “zones” with equivalent P-T conditions Boundaries between zones are called “isograds” (lines of equal P-T)

Chemographic Diagrams Biotite Isograd Chlorite + K-feldspar  Biotite + Muscovite (phengitic) 400 – 425°C Chemographic Diagrams

Chemographic Diagrams Graphical representation of the chemistry of mineral assemblages in metamorphic rocks Plot the following “minerals” on an “XYZ” diagram xz, xyz, and yz2 Suppose you had a small area of a metamorphic terrane in which the rocks correspond to a hypothetical 3-component system with variable proportions of the components x-y-z The rocks in the area are found to contain 6 minerals with the fixed compositions x, y, z, xz, xyz, and yz2

x-xy-x2z Note that this subdivides the diagram into 5 sub-triangles 2-phase tie line 3-phase field What is the stable mineral assemblage in (A)? x-xy-x2z A diagram like this is a compatibility diagram, a type of phase diagram commonly employed by metamorphic petrologists Any point within the diagram represents a specific bulk rock composition The diagram determines the corresponding mineral assemblage that develops at equilibrium For example, a point within the sub-triangle (E), the corresponding mineral assemblage corresponds to the corners = y - z - xyz Any rock with a bulk composition plotting within triangle (E) will develop that same mineral assemblage

(A) through (E) might represent the protolith bulk chemistry 2-phase tie line 3-phase field What is the stable mineral assemblage if protolith chemistry = (B)? xy-x2z-xyz A diagram like this is a compatibility diagram, a type of phase diagram commonly employed by metamorphic petrologists Any point within the diagram represents a specific bulk rock composition The diagram determines the corresponding mineral assemblage that develops at equilibrium For example, a point within the sub-triangle (E), the corresponding mineral assemblage corresponds to the corners = y - z - xyz Any rock with a bulk composition plotting within triangle (E) will develop that same mineral assemblage

A diagram in which some minerals exhibit solid solution 2-phase tie lines 3-phase field Minerals x(y,z) and x2(y,z) show limited solid solution of components y and z on one type of lattice site. Mineral x(y,z) allows more y in the lattice than does mineral x2(y,z) Minerals (xyz)ss and zss (the subscript denotes solid solution) show limited solid solution of all three components Click Suppose a bulk rock composition is in the shaded field of the mineral (xyz)ss phi = 1, but the system is not degenerate Due to the variable nature of the composition of the phase, C must still equal 3 and the phase rule tells us that F = C - phi + 2 = 4 Thus P, T, and any 2 of the 3 components in the phase are independently variable The shaded area of mineral (xyz)ss is thus an area (compositionally divariant), and our single-phase rock can have any composition within the shaded solid solution limits phi < C in this case because of the solid solution and compositional variance

if protolith chemistry = (f), What is the stable mineral assemblage? 2-phase tie lines 3-phase field As in the previous example, there are two coexisting phases (xyz)ss and zss The composition of the two minerals that correspond to bulk rock composition (f) are indicated by the two shaded dots at the ends of the tie-line through (f) phi = 2 and C is still 3, so F = 3 - 2 + 2 = 3 Since P and T are independently variable, that means the composition of each phase is univariant, and must vary along the lines where the bundles of tie- lines end Although the composition of (xyz)ss can vary anywhere in the shaded area, the composition of (xyz)ss that coexists with Zss is constrained to the edge of the area facing z A degree of freedom is thus lost as a phase is gained Likewise the composition of zss that coexists with (xyz)ss is constrained to a portion of the edge of the shaded zss area

if protolith chemistry = (f), What is the stable mineral assemblage? In such situations phi = 3, and C = 3, as predicted by the mineralogical phase rule Since F = 2 and corresponds to P and T the phase rule tells us that all of the compositional variables for each phase are fixed

Graphical representation of the chemistry of mineral assemblages in metamorphic rocks Fig. 24-4 illustrates the positions of several common metamorphic minerals on the ACF diagram. Note: this diagram is presented only to show you where a number of important phases plot. It is not specific to a P-T range and therefore is not a true compatibility diagram, and has no petrological significance

Chemographic Diagrams for Metamorphic Rocks Most rocks/minerals contain the major elements: SiO2, Al2O3, K2O, CaO, Na2O, FeO, MgO, MnO and H2O such that number of components in the system is large. Three components is the maximum number that we can easily deal with in 2-D (ie. a triangular diagram) Some simplifying methods: (lumping of components) A = Al2O3 + Fe2O3 - Na2O - K2O C = CaO - 3.3 P2O5 F = FeO + MgO + MnO All 9 is clearly too complex. Must simplify if we want to display the system in a convenient graphical way Four components requires 3-D tetrahedra, and we lose even a semi- quantitative sense of depth in the diagram More than four components is much too complex to be useful

A typical ACF compatibility diagram, referring to a specific P-T condition (the kyanite zone in the Scottish Highlands) Green area is bulk composition of metabasaltic (mafic) rocks What are the common assemblages for kyanite zone (amphibolite facies)?? Plag + Alm + Hbl Plag + Hbl Plag + Hbl + Diop Hbl + Alm Plot all phases and connect coexisting ones with tie-lines The composition of most mafic rocks fall in the hornblende-plagioclase field or the hornblende- plagioclase-garnet triangle, and thus most metabasaltic rocks occur as amphibolites or garnet amphibolites in this zone More aluminous rocks develop kyanite and/or muscovite and not hornblende More calcic rocks lose Ca-free garnet, and contain diopside, grossularite, or even calcite (if CO2) We again see how the diagram allows us to interpret the relationship between the chemical composition of a rock and the equilibrium mineral assemblage

Different protoliths have different assemblages at specific P-T conditions metacarbonates metabasalts metapelites Plot all phases and connect coexisting ones with tie-lines The composition of most mafic rocks fall in the hornblende-plagioclase field or the hornblende- plagioclase-garnet triangle, and thus most metabasaltic rocks occur as amphibolites or garnet amphibolites in this zone More aluminous rocks develop kyanite and/or muscovite and not hornblende More calcic rocks lose Ca-free garnet, and contain diopside, grossularite, or even calcite (if CO2) We again see how the diagram allows us to interpret the relationship between the chemical composition of a rock and the equilibrium mineral assemblage

At different P-T conditions, the diagrams change Other minerals become stable Different arrangements of the same minerals (different tie-lines connect different coexisting phases) Use to graphically show important isograd reactions low P-T high P-T

A + B  C + D Below the isograd Bulk rock composition At the isograd low P-T A + B  C + D At the isograd Above the isograd Note that phi = 4 at the isograd with crossing tie-lines Then have new groupings : A + C + D or B + C + D No new minerals become stable- simply different associations The groupings follow from the reaction: If A > B then B consumed first, and A remains with new C & D -> A + C + D C + D cannot coexist below the isograd, and A + B cannot coexist above it If a chemographic diagram is a projection, the approach still works, but you will have to balance the reaction with other components For example, if the previous diagram is projected from quartz, SiO2 will have to be added to one side of the A + B = C + D reaction to balance it properly high P-T This is called a tie-line flip, and results in new mineral assemblages in the next metamorphic zone

Chemographic Diagrams Biotite Isograd Chlorite + K-feldspar  Biotite + Muscovite (phengitic) 400 – 425°C Chemographic Diagrams

AFM Diagram Muscovite and quartz must be present in the assemblage What is the assemblage if protolith is “x”? Due to extensive Mg-Fe solid solution in biotite and garnet, much of the area is dominated by 2-phase fields with tie- lines (really 4-phase when we include the Qtz and Mu projection phases) Although we can easily plot ideal mineral formulas on the ACF and AKF diagrams, in order for a real mafic phase to be plotted on an AFM diagram we must know Mg/(Fe+Mg), which can only be determined by chemical analysis of the minerals, generally performed using the electron microprobe If analyses are unavailable, we can approximate the correct positions on the basis of typical relative Mg/(Fe+Mg), based on our knowledge of numerous analyses of these minerals available in the literature. From these we know that Mg-enrichment occurs typically in the order: cordierite > chlorite > biotite > staurolite > garnet Sil + St + Bt + Qtz + Ms

different diagrams are separated by metamorphic reactions Basic P-T Application AFM basics each diagram represents stable assemblages at fixed P & T different diagrams are separated by metamorphic reactions different assemblages = different bulk X

Getting P-T constraints chl gar bio Example: Over what P-T range is the assemblage Gar+Chl+Bio stable?

H I J Step 1: find AFM range for assemblage Where in P-T space does this assemblage occur?

Ky Sil And Step 2: use AFM labels to find P-T field H to J This is the only part of P-T space where gar+chl+bio can coexist H to J Al2SiO5 in nearby rocks could further restrict P&T Ky Sil And

blueschist greenschist granulite amphibolite eclogite H to J