Transcript G 2312 I M
GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY Lecture 21 Metamorphism of Pelitic Sediments April 11, 2016 METAMORPHIC ZONES VS. METAMORPHIC FACIES PELITIC SEDIMENTS - MAFIC IGNEOUS ROCKS 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 Table 28-1. Chemical Compositions* of Shales and Metapelites SiO2 TiO2 Al 2O3 MgO FeO MnO CaO Na 2O K2O P2O5 1 64.7 0.80 17.0 2.82 5.69 0.25 3.50 1.13 3.96 0.15 2 64.0 0.81 18.1 2.85 7.03 0.10 1.54 1.64 3.86 0.15 3 61.5 0.87 18.6 3.81 10.0 4 65.9 0.92 19.1 2.30 6.86 0.81 1.46 3.02 0.17 0.85 3.88 5 56.3 1.05 20.2 3.23 8.38 0.18 1.59 1.86 4.15 Total 100.00 100.08 100.07 99.98 96.94 Basalt 49.0 1.5 15.0 9.0 11.0 0.4 10.0 2.5 0.9 0.5 * Reported on a volatile-free basis (normalized to 100%) to aid comparison. 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 -grade pelitic rocks, Littleton Fm, N.H. (Shaw , 1956). 5. Ave. of Graywacke CHEMOGRAPHIC DIAGRAMS FOR METAPELITES AKF AND AFM – GREENSCHIST FACIES Pelitic Rocks Mafic Rocks PETROGENETIC GRIDS FOR METAPELITES “KFMASH” - K2O-FEO-MGO-AL2O3-SIO2-H2O Purple: Mg Orange: Fe OROGENIC METAMORPHISM CHLORITE-BIOTITE ZONE Biotite-in Isograd Tie-line Flip 2) Chl + Kfs = Bt + Ms (+ Qtz + H2O) OROGENIC METAMORPHISM BIOTITE ZONE Progressive Metamorphism through the Biotite Zone Muscovite diminishes relative to Biotite and Al-rich Chlorite 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) 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 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 OROGENIC METAMORPHISM STAUROLITE-KYANITE-SILLIMANITE ZONE Kyanite-in Isograd Sillimanite-in Isograd THERMAL METAMORPHISM OF METAPELITES Albite-Epidote hornfels facies Hornblende hornfels facies Pyroxene hornfels facies HIGH PRESSURE METAMORPHISM OF PELITIC SEDIMENTS PARTIAL MELTING OF PELITIC SEDIMENTS Na Felsic Melt Generation by Dehydration Melting of Muscovite and Biotite 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.