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AFM Diagram 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 low P-T Below the isograd Bulk rock composition A+BC+ D At the isograd Above the isograd high P-T This is called a tie-line flip, and results in new mineral assemblages in the next metamorphic zone Review and P-T Application AFM basics each diagram represents stable assemblages at fixed P & T different assemblages = different bulk X different diagrams are separated by metamorphic reactions 2. Getting P-T constraints chl gar bio Example: Over what P-T range is the assemblage Gar+Chl+Bio stable? Step 1: find AFM range for assemblage I J Where in P-T space does this assemblage occur? H Step 2: use AFM labels to find P-T field This is the only part of P-T space where gar+chl+bio can coexist kya H to J Al2SiO5 in nearby rx could further restrict P&T 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 Series rocks typical of regional metamorphism at mid- to lower crust in mountain belts Barrovian Series metapelites have kyanite Barrovian Dutchess Co. Trip Buchan Barrovian Zones in Metapelites •Chlorite zone •Biotite zone •Garnet zone •Staurolite zone •Kyanite zone •Sillimanite zone •K-feldspar zone P-T grid Metamorphic “zones” based on metapelites can give relatively high resolution P-T estimates Amphibolite Greenschist Ky Zone Grt Zone Chl Zone St Zone Bt Zone P-T grid Granulite Sil Zone Kfs Zone 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 • Continuous reactions (over a range of P-T) Metapelites BiotiteChapter Zone 28: involving solid solution • middle to upper greenschist • gradual expansion of Ms-Bt-Chl triangle to include more pelite compositions • 400 – 500°C P,T increasing Part 1 Garnet Isograd • tie-line flip (discontinuous) reaction type • Cld + Bt Grt + Chl • ~500°C Chapter 28: Metapelites Garnet Isograd P,T increasing Part 2 • Continuous reaction type (over a range of P-T) involving solid solution • Chl + Bt Grt + Mg-rich Chl + Mg-rich Bt • This is the garnet isograd for almost all common metapelites • 525 – 555°C “P-T” grid = Garnet Isograd Part 1 Isograd Chapter 28:Staurolite Metapelites • terminal point reaction type (“chloritoid-out”; disappearance of chloritoid) • Cld Grt + Chl + St • ~550°C Staurolite Isograd Part 2Chapter 28: Metapelites • 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 Part 1 • tie-line flip (discontinous) rxn • St + Chl Ky + Bt • ~625°C Kyanite Isograd Part 2 Isograd Chapter 28:Kyanite Metapelites • terminal point reaction type (“staurolite-out”; disappearance of staurolite) • St Grt + Bt + Ky • This is the kyanite isograd for almost all common metapelites • 625-675°C Ky “P-T” grid = Kyanite Isograd Isograd ChapterSillimanite 28: Metapelites • polymorphic transition • Ky Sil • 650 - 700°C Ky Sil “P-T” grid = Sillmanite Isograd K-feldspar Isograd (“2 sillimanite isograd”) Chapter 28: Metapelites nd • 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 • >750 - 800° C • liberated H2O may cause partial melting Bt + Qtz Opx + Kfs + H2O 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 Amphibolite Greenschist Ky Zone Grt Zone Chl Zone Bt Zone St Zone Granulite Sil Zone Kfs Zone