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Pelites. Chapter 14 and 18. Mudstones and shales Accumulate in wedge of sed. Off continental shelf Boring muds: interesting metamorphosed Clays sensitive to P&T changes. High Al 2 O 3 and K 2 O Low CaO. Barrow’s Area. Seen in regional met.

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pelites

Pelites

Chapter 14 and 18

slide2
Mudstones and shales
  • Accumulate in wedge of sed. Off continental shelf
  • Boring muds: interesting metamorphosed
  • Clays sensitive to P&T changes
slide4

Barrow’s Area

  • Seen in regional met.
  • Famous locale in Scotland studied by George Barrow(1912)
slide5
Sandstone, limestone, basalt, shales
  • Shales show best systematic change
  • Mapped metamorphic zones (index minerals)
sequence of min with increasing met grade
Sequence of min. with increasing met. grade
  • Chlorite
  • Biotite
  • Garnet
  • Staurolite
  • Kyanite
  • Sillimanite
  • Known as Barrovian sequence
slide7
Near Aberdeen in Buchan district:
    • chlorite
    • biotite
    • cordierite
    • andalusite
    • sillimanite
  • Buchan sequence
slide8
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
akf and afm diagrams
AKF and AFM diagrams
  • Chlorite zone, lower greenschist facies
  • 350-450°C
greenschist facies 430 c a below the biotite isograd b above the biotite isograd
Greenschist facies ~430°Ca: below the biotite isogradb: above the biotite isograd
  • Reaction?Continuous or discontinuous?
  • Affects most pelites?
slide12
Note Ms-Bt-Chl triangle migration as muscovite solid solution drops
  • A: less Al-rich
  • C: more Al-rich
sillimanite isograd upper amphibolite facies 690 c
Sillimanite isogradupper amphibolite facies 690°C
  • Degenerative rxn
  • St = sillimanite + biotite + garnet
second sillimanite isograd 750 c early granulite facies
Second sillimanite isograd >750°C early granulite facies
  • Muscovite + quartz = Kspar + Al2SiO5 + water
second sillimanite isograd 750 c early granulite facies20
Second sillimanite isograd >750°C early granulite facies
  • Does this reaction happen often in pelites?
granulite faces 790 c
Granulite faces ~790°C
  • Sillimanite + Bt = Garnet + cordierite
slide25

2. Getting P-T constraints

chl

gar

bio

Example:

Over what P-T range is the

assemblage Gar+Chl+Bio stable?

slide26

H

I

J

Step 1: find AFM range for assemblage

Where in P-T space does this assemblage occur?

slide27

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

slide28

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

slide29

4. “Too many AFM minerals”:

Interpreting complex assemblages

staur

gar

chl

bio

What are the possible interpretations??

slide30

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

p t t paths for barrovian sequence
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
p t t paths for barrovian sequence32
P-T-t paths for Barrovian sequence
  • Large amplitude folding, variable uplift rates, irregular distribution of T and P
p t t path for buchan sequence
P-T-t path for Buchan sequence
  • How do you explain the Buchan sequence?
slide34
Return flow of country rock around rising plutons
  • Wall rock heats, wallrock is pushed down, everything cools (no crustal thickening)