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Geol 2312 Igneous and Metamorphic Petrology. Lecture 27 Metamorphism of Pelitic Sediments. April 10, 2009. Metamorphic Zones vs. Metamorphic Facies Pelitic Sediments - Mafic Igneous Rocks. Pelitic Sedimentary Rocks Mudstone, Shale, Graywacke.

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Geol 2312 Igneous and Metamorphic Petrology


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Geol 2312 Igneous and Metamorphic Petrology

Lecture 27

Metamorphism of Pelitic Sediments

April 10, 2009

pelitic sedimentary rocks mudstone shale graywacke
Pelitic Sedimentary RocksMudstone, 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

petrogenetic grids for metapelites kfmash k 2 o feo mgo al 2 o 3 sio 2 h 2 o
Petrogenetic Grids for Metapelites“KFMASH” - K2O-FeO-MgO-Al2O3-SiO2-H2O

Purple: Mg

Orange: Fe

orogenic metamorphism chlorite biotite zone
Orogenic MetamorphismChlorite-Biotite Zone

Biotite-in

Isograd

Tie-line Flip

Chl + Kfs = Bt + Ms (+ Qtz + H2O)

slide7

Orogenic MetamorphismBiotite Zone

Progressive Metamorphism through the Biotite Zone

Muscovite diminishes relative to Biotite and Al-rich Chlorite

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Orogenic MetamorphismBiotite-Garnet Zone

Garnet-in

Isograd

Lower Biotite Zone

Garnet Zone

Upper Biotite Zone

Tie-line Flip

Chloritoid + Biotite (+ Qtz)

= Garnet + Fe-Chlorite (+ H2O)

Occurs at lower T for Fe-rich

compositions

orogenic metamorphism garnet staurolite zone
Orogenic MetamorphismGarnet-Staurolite Zone

Staurolite-in

Isograd

Cld+Ky = St + Chl

(+ Qtz + H2O)

Cld (+Qtz) = Grt + Chl

+ St (+ Qtz + H2O)

Grt + Chl (+ Ms) =

St + Bt (+ Qtz + H2O)

Retreat of Chl SS

orogenic metamorphism staurolite zone
Orogenic MetamorphismStaurolite Zone

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

orogenic metamorphism staurolite kyanite sillimanite zone
Orogenic MetamorphismStaurolite-Kyanite-Sillimanite Zone

Kyanite-in

Isograd

Sillimanite-in

Isograd

thermal metamorphism of metapelites
Thermal Metamorphism of Metapelites

Albite-Epidote hornfels facies

Hornblende hornfels facies

Pyroxene

hornfels facies

migmatites
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.