Pelites

1. Pelites Chapter 14 and 18

2. Mudstones and shales • Accumulate in wedge of sed. Off continental shelf • Boring muds: interesting metamorphosed • Clays sensitive to P&T changes

3. High Al2O3 and K2O • Low CaO

4. Barrow’s Area • Seen in regional met. • Famous locale in Scotland studied by George Barrow(1912)

5. Sandstone, limestone, basalt, shales • Shales show best systematic change • Mapped metamorphic zones (index minerals)

6. Sequence of min. with increasing met. grade • Chlorite • Biotite • Garnet • Staurolite • Kyanite • Sillimanite • Known as Barrovian sequence

7. Near Aberdeen in Buchan district: • chlorite • biotite • cordierite • andalusite • sillimanite • Buchan sequence

8. An isograd (in this classical sense) represents the first appearance of a particular metamorphic index mineral in the field as one progresses up metamorphic grade • When one crosses an isograd, such as the biotite isograd, one enters the biotite zone • Zones have the same name as the isograd that forms the low-grade boundary of that zone • Since classic isograds are based on the first appearance of a mineral, and not its disappearance, an index mineral may still be stable in higher grade zones

9. Fig. 18.13: Barrovian zones and met. facies

10. AKF and AFM diagrams • Chlorite zone, lower greenschist facies • 350-450°C

11. Greenschist facies ~430°Ca: below the biotite isogradb: above the biotite isograd • Reaction?Continuous or discontinuous? • Affects most pelites?

12. Note Ms-Bt-Chl triangle migration as muscovite solid solution drops • A: less Al-rich • C: more Al-rich

13. AFM diagrams of biotite and upper biotite zones, greenschist facies

14. Garnet zone, early amphibolite facies, ~530°C • What rxn?

15. Staurolite zone ~570-610°Camphibolite facies • Rxn?

16. Which phases lost and gained in comps. A, B, C, D, E?

17. Kyanite zone, amphibolite facies630°C • Rxn?

18. Sillimanite isogradupper amphibolite facies 690°C • Degenerative rxn • St = sillimanite + biotite + garnet

19. Second sillimanite isograd >750°C early granulite facies • Muscovite + quartz = Kspar + Al2SiO5 + water

20. Second sillimanite isograd >750°C early granulite facies • Does this reaction happen often in pelites?

21. Migmatites form when pelites start melting

22. Complex migmatite veins

23. Granulite faces ~790°C • Sillimanite + Bt = Garnet + cordierite

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

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

27. H to J kya sill Al2SiO5 in nearby rx 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

28. eclogite blueschist epid amphib greenschist amphibolite granulite 3. Correlation with mafic rocks What is the equivalent metamorphic facies? epidote amphibolite What assemblage would be present in interlayered mafic rocks? epid/czo + hbl+ plag

29. 4. “Too many AFM minerals”: Interpreting complex assemblages staur gar chl bio What are the possible interpretations??

30. d) Other possibilities… ? …look for textural evidence of rxn …chl replacing Grt, St, or Bt; don’t plot chl …no textural evidence of rxn …observations lead to interpretations a) Reaction frozen in, V=1 b) Extra component, e.g. Mn, Ca in Grt or Zn in St c) Retrograde mineral, e.g. chlorite

31. P-T-t paths for Barrovian sequence • 6 garnets analyzed for thermobarometry • 6 separate P-T paths recording garnet growth • If curves nested: different crustal depths during crustal thickening

32. P-T-t paths for Barrovian sequence • Large amplitude folding, variable uplift rates, irregular distribution of T and P

33. P-T-t path for Buchan sequence • How do you explain the Buchan sequence?

34. Return flow of country rock around rising plutons • Wall rock heats, wallrock is pushed down, everything cools (no crustal thickening)

35. Lower crust: what is it doing? (circle in fertile source rock)