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Geol 2312 Igneous and Metamorphic Petrology

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Presentation on theme: "Geol 2312 Igneous and Metamorphic Petrology"— Presentation transcript:

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 Sediments
Na

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.


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