Concept of Index Minerals

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

2. 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)

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

4. 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

5. 2-phase tie line 3-phase field What is the stable mineral assemblage in (A)? Note that this subdivides the diagram into 5 sub-triangles x-xy-x2z

6. 2-phase tie line 3-phase field What is the stable mineral assemblage if protolith chemistry = (B)? (A) through (E) might represent the protolith bulk chemistry xy-x2z-xyz

7. 2-phase tie lines 3-phase field A diagram in which some minerals exhibit solid solution

8. if protolith chemistry = (f), What is the stable mineral assemblage? 2-phase tie lines 3-phase field

9. if protolith chemistry = (f), What is the stable mineral assemblage?

10. Graphical representation of the chemistry of mineral assemblages in metamorphic rocks

11. 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

13. Green area is bulk composition of metabasaltic (mafic) rocks A typical ACF compatibility diagram, referring to a specific P-T condition (the kyanite zone in the Scottish Highlands) What are the common assemblages for kyanite zone (amphibolite facies)?? • Plag + Alm + Hbl • Plag + Hbl • Plag + Hbl + Diop • Hbl + Alm

14. metacarbonates Different protoliths have different assemblages at specific P-T conditions metabasalts metapelites

15. 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

16. Below the isograd Bulk rock composition low P-T A + B  C + 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

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

18. Muscovite and quartz must be present in the assemblage AFM Diagram What is the assemblage if protolith is “x”? Sil + St + Bt + Qtz + Ms

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

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

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

22. H to J Ky Sil Al2SiO5 in nearby rocks could further restrict P&T And 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

23. eclogite blueschist H to J greenschist granulite amphibolite