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 ROCKSMUDSTONE, SHALE, GRAYWACKE

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

1 2 3 4 5SiO2 64.7 64.0 61.5 65.9 56.3

TiO2 0.80 0.81 0.87 0.92 1.05

Al2O3 17.0 18.1 18.6 19.1 20.2MgO 2.82 2.85 3.81 2.30 3.23FeO 5.69 7.03 10.0 6.86 8.38MnO 0.25 0.10 0.18CaO 3.50 1.54 0.81 0.17 1.59Na2O 1.13 1.64 1.46 0.85 1.86

K2O 3.96 3.86 3.02 3.88 4.15

P2O5 0.15 0.15

Total 100.00 100.08 100.07 99.98 96.94 * Reported on a volatile-free basis (normalized to 100%) to aid comparison.

Table 28-1. Chemical Compositions* of Shalesand Metapelites

1. "North American Shale Composite". Gromet et al. (1984). 2. Average of ~100 published shale and slate analyses (Ague, 1991).3. Ave. pelite-pelagic clay (Carmichael, 1989). 4. Ave. of low -gradepelitic rocks, Littleton Fm, N.H. (Shaw , 1956). 5. Ave. of

Basalt49.01.5

15.09.0

11.00.4

10.02.50.90.5

CHEMOGRAPHIC DIAGRAMS FOR METAPELITESAKF AND AFM

Mafic Rocks

Pelitic Rocks

PETROGENETIC GRIDS FOR METAPELITES

“KFMASH” - K2O-FEO-MGO-AL2O3-SIO2-H2O

Purple: MgOrange: Fe

OROGENIC METAMORPHISMCHLORITE-BIOTITE ZONE

Tie-line Flip

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

Biotite-inIsograd

OROGENIC METAMORPHISMBIOTITE ZONE

Progressive Metamorphism through the Biotite Zone

Muscovite diminishes relative to Biotite and Al-rich Chlorite

OROGENIC METAMORPHISMBIOTITE-GARNET ZONE

Lower Biotite Zone Upper Biotite Zone Garnet Zone

Tie-line Flip

Chloritoid + Biotite (+ Qtz) = Garnet + Fe-Chlorite (+ H2O)

Occurs at lower T for Fe-rich compositions

Garnet-inIsograd

OROGENIC METAMORPHISMGARNET-STAUROLITE ZONE

Staurolite-inIsograd

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 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

StauroliteStability Field

OROGENIC METAMORPHISMSTAUROLITE-KYANITE-SILLIMANITE

ZONE Kyanite-inIsograd

Sillimanite-inIsograd

THERMAL METAMORPHIS

M OF METAPELITES

Albite-Epidote hornfels facies

Hornblende hornfels facies

Pyroxenehornfels facies

HIGH PRESSURE METAMORPHISMOF PELITIC SEDIMENTS

PARTIAL MELTING OF PELITIC SEDIMENTS

Na

MIGMATITESa. 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.