AFM Diagram 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

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

different diagrams are separated by metamorphic reactions Review and 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

2. 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?

kya sill 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 rx could further restrict P&T kya sill and

Metapelites Metapelites = metamorphosed mudstones and shales Distinguishing chemical characteristics: high Al2O3 and K2O, and low CaO Reflect the high clay and mica content of the original sediment and lead to the dominance of muscovite and quartz throughout most of the range of metamorphism High proportion of micas ® common development of foliated rocks, such as slates, phyllites, and mica schists the petrogenesis of pelites is represented well in AKF and AFM diagrams

Barrovian Zones in Metapelites Chlorite zone Biotite zone Garnet zone Staurolite zone Kyanite zone Sillimanite zone K-feldspar zone

Variable P-T Conditions in a Convergent Plate Setting Low P, high T (contact) high P, low T (“blueschist”) high P and T (regional) Barrovian Barrovian Series rocks typical of regional metamorphism at mid- to lower crust in mountain belts

Barrovian Series metapelites have kyanite The molar volume of cordierite is also quite high, indicating that it too is a low-pressure mineral The geothermal gradient in this northern district was higher than in Barrow’s area, and rocks at any equivalent temperature must have been at a lower pressure This lower P/T variation has been called Buchan-type metamorphism. It too is relatively common Miyashiro (1961), from his work in the Abukuma Plateau of Japan, called such a low P/T variant Abukuma-type Both terms are common in the literature, and mean essentially the same thing

Barrovian Buchan Dutchess Co. Trip

Barrovian Zones in Metapelites Chlorite zone Biotite zone Garnet zone Staurolite zone Kyanite zone Sillimanite zone K-feldspar zone

P-T grid Barrovian Series

Metamorphic “zones” based on metapelites can give relatively high resolution P-T estimates Greenschist Amphibolite Granulite Kfs Zone Sil Zone Ky Zone Barrovian Series St Zone Grt Zone Bt Zone Chl Zone P-T grid

Chlorite Zone lower greenschist facies 300 – 400°C

Biotite Isograd tie-line flip (discontinuous) reaction type Chl + Kfs  Bt + phengitic Ms 400 – 425°C

“P-T” grid  = Biotite Isograd

Chapter 28: Metapelites Biotite Zone middle to upper greenschist Continuous reactions (over a range of P-T) involving solid solution gradual expansion of Ms-Bt-Chl triangle to include more pelite compositions middle to upper greenschist 400 – 500°C P,T increasing

Garnet Isograd Part 1 tie-line flip (discontinuous) reaction type Cld + Bt  Grt + Chl ~500°C

Chapter 28: Metapelites Garnet Isograd Part 2 P,T increasing Continuous reaction type (over a range of P-T) involving solid solution Chl + Bt  Grt + Mg-rich Chl + Mg-rich Bt P,T increasing This is the garnet isograd for almost all common metapelites 525 – 555°C

“P-T” grid  = Garnet Isograd

Staurolite Isograd Chapter 28: Metapelites Part 1 terminal point reaction type (“chloritoid-out”; disappearance of chloritoid) Cld  Grt + Chl + St ~550°C

Chapter 28: Metapelites Staurolite Isograd Part 2 tie-line flip (discontinuous) rxn Chl + Grt  St + Bt This is the staurolite isograd for almost all common metapelites 550 – 600°C

“P-T” grid  = Staurolite Isograd

Kyanite Isograd Part 1 tie-line flip (discontinous) rxn St + Chl  Ky + Bt ~625°C

Chapter 28: Metapelites Kyanite Isograd Part 2 terminal point reaction type (“staurolite-out”; disappearance of staurolite) Ky St  Grt + Bt + Ky This is the kyanite isograd for almost all common metapelites 625-675°C

“P-T” grid  = Kyanite Isograd

Chapter 28: Metapelites Sillimanite Isograd polymorphic transition Ky Ky  Sil 650 - 700°C

“P-T” grid  = Sillmanite Isograd

K-feldspar Isograd (“2nd sillimanite isograd”) Chapter 28: Metapelites breakdown of muscovite; dehydration reaction Ms + Qtz  Sil + Kfs + H2O ~750°C liberated H2O may cause partial melting Kfs

“P-T” grid 11 = K-feldspar Isograd

Granulite Facies Breakdown of biotite; dehydration reactions presence of cordierite and/or Opx (depending on P) Bt + Sil  Grt + Crd + H2O Bt + Qtz  Opx + Kfs + H2O >750 - 800° C liberated H2O may cause partial melting

Migmatites migmatite = “mixed rock”; part igneous, part metamorphic Breakdown of muscovite and biotite at high grades may cause partial melting

Metamorphic “zones” based on metapelites can give relatively high resolution P-T estimates Greenschist Amphibolite Granulite Kfs Zone Sil Zone Ky Zone St Zone Grt Zone Bt Zone Chl Zone