1. Geol 2312 Igneous and Metamorphic Petrology
Lecture 21
Metamorphism of Pelitic Sediments
April 11, 2016

2. Metamorphic Zones vs. Metamorphic Facies Pelitic Sediments - Mafic Igneous Rocks

3. Pelitic Sedimentary Rocks Mudstone, Shale, Graywacke
Pelitic sediments are composed of predominantly Al-K-rich clays and micas (>50%), with quartz, feldspar, and other minerals.
Because clays are very sensitive to variations in temperature and pressure, they undergo extensive changes in mineralogy during progressive metamorphism
Shale
Basalt
49.0
1.5
15.0
9.0
11.0
0.4
10.0
2.5
0.9
0.5
Graywacke

4. Chemographic Diagrams for Metapelites AKF and AFM – Greenschist Facies
Pelitic Rocks
Mafic Rocks

5. Petrogenetic Grids for Metapelites “KFMASH” - K2O-FeO-MgO-Al2O3-SiO2-H2O
Purple: Mg
Orange: Fe

6. Orogenic Metamorphism Chlorite-Biotite Zone
Biotite-in
Isograd
Tie-line Flip
2) Chl + Kfs = Bt + Ms (+ Qtz + H2O)

7. Orogenic Metamorphism Biotite Zone
Progressive Metamorphism through the Biotite Zone
Muscovite diminishes relative to Biotite and Al-rich Chlorite

8. Orogenic Metamorphism Biotite-Garnet Zone
Garnet-in
Isograd
Lower Biotite Zone
Upper Biotite Zone
Garnet Zone
Tie-line Flip
4) Chloritoid + Biotite (+ Qtz)
= Garnet + Fe-Chlorite (+ H2O)

9. Orogenic Metamorphism Garnet-Staurolite Zone
Staurolite-in
Isograd
5) Cld+Ky = St + Chl
(+ Qtz + H2O)
6) Cld (+Qtz) = Grt + Chl
+ St (+ Qtz + H2O)
7) Grt + Chl (+ Ms) =
St + Bt (+ Qtz + H2O)
Retreat of Chl SS

10. Orogenic Metamorphism Staurolite Zone = Amphibolite Facies
Importance of Composition
A) Gt+Chl+Bt  Gt+Bt+St
B) Gt+Chl+St  Gt+Bt+St
C) Gt+Chl+St  Chl+Bt+St
D) Gt+Chl+Bt  Chl+Bt+St
Staurolite
Stability Field

11. Orogenic Metamorphism Staurolite-Kyanite-Sillimanite Zone
Kyanite-in
Isograd
Sillimanite-in
Isograd

12. Thermal Metamorphism of Metapelites
Albite-Epidote hornfels facies
Hornblende hornfels facies
Pyroxene
hornfels facies

13. High Pressure Metamorphism of Pelitic Sediments

14. Partial Melting of Pelitic SedimentsNa

15. Felsic Melt Generation by Dehydration Melting of Muscovite and Biotite
Reactions are not balanced and commonly leave out Ms and Qtz, which are considered to be present in excess.

16. Migmatites a. Breccia structure in agmatite. b. Net-like structure.
c. Raft-like structure.
d. Vein structure.
e. Stromatic, or layered, structure.
f. Dilation structure in a boudinaged layer.
g. Schleiren structure.
h. Nebulitic structure.
From Mehnert (1968) Migmatites and the Origin of Granitic Rocks. Elsevier.