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Concept of Index Minerals 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) 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 Note that this subdivides the diagram into 5 sub-triangles 2-phase tie line What is the stable mineral assemblage in (A)? x-xy-x2z 3-phase field (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 in which some minerals exhibit solid solution 2-phase tie lines 3-phase field if protolith chemistry = (f), What is the stable mineral assemblage? 2-phase tie lines 3-phase field if protolith chemistry = (f), What is the stable mineral assemblage? Graphical representation of the chemistry of mineral assemblages in metamorphic rocks 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 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)?? 1) Plag + Alm + Hbl 2) Plag + Hbl 3) Plag + Hbl + Diop 4) Hbl + Alm Different protoliths have different assemblages at specific P-T conditions metacarbonates metapelites metabasalts 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 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”? Sil + St + Bt + Qtz + Ms Basic 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 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 Ky H to J Al2SiO5 in nearby rocks could further restrict P&T Sil And eclogite blueschist H to J greenschist granulite amphibolite