1INTRUSIVE IGNEOUS ROCKSChapter 4Polymerization of Silica•Each Si atom is surrounded by 4 oxygens.•Polymerization is the degree to which the oxygens are shared with other Si atoms.•Polymerization is the number of Si-O-Si bonds.–Olivine has none: isolated SiO4groups–Pyroxene has one third: SiO3chains–Mica has has 40% shared: Si4O10sheets–Quartz and feldspar have 100% (Si,Al)O2framework.Polymerization of SilicaTetrahedronOlivinePyroxenePolymerization of SilicaMica (Sheet)Quartz (Framework)MAGMA•Magmais the term for any molten silicate material, whether below the surface or on top.•Volcanic rocksare erupted on the surface.– Volcanic rocks are fine-grained (<1 mm)•Intrusive igneous rockscrystallize from magma below the surface.– Intrusive igneous rocks are coarse-grained (> 1mm)–Pegmatitesare very coarse grained (> 1cm) Intrusive Igneous Rocks•Composition•Mineralogy•Geologic Setting–Mantle–Oceanic Crust–Continental Crust•Igneous Fractionation–How the chemistry evolves2Igneous Rock Compositions•Rock compositions are described in weight percents of oxides.•The principal variation in igneous rock compositions is silica (SiO2) content.•The degree of polymerization of silica increases with silica content. (both in crystals and melt).Igneous Rock Compositions•Igneous rocks vary in composition from –ultramafic (~40 wt%) (peridotite)–mafic (48-55%) (gabbro) –intermediate (55-65%) (diorite)–silicic (65-75 wt %) (granite)•The mantle is peridotite (ultramafic).•The ocean basins gabbro (mafic). •The continents are granite (silicic)Igneous Rock Names and Compositions• Intrusive Rock Name• Peridotite• Gabbro• Diorite• Granite• Composition Name• Ultramafic•Mafic• Intermediate• Silicic (felsic)• Volcanic Rock Name• (Komatiite)• Basalt• Andesite• RhyoliteIgneous Rock Mineralogy• Peridotite (Ultramafic)– Olivine ((Mg,Fe)2SiO4)– Pyroxene ((Mg,Fe,Ca)SiO3)– Garnet (Mg3Al2Si3O12) or– Spinel (MgAl2O4)• Gabbro (Mafic)– Feldspar (CaAl2Si2O8)– Pyroxene ((Mg,Fe,Ca)SiO3)– Olivine ((Mg,Fe)2SiO4)• Diorite (Intermediate)– Feldspar (CaAl2Si2O8) – Feldspar (NaAlSi3O8)– Pyroxene ((Mg,Fe,Ca)SiO3)– Mica K(Mg,Fe,)3(Si,Al)2O10(OH)2Igneous Rock Mineralogy• Granite (Silicic)– Quartz (SiO2)– Feldspar (NaAlSi3O8) – Feldspar (KAlSi3O8)– Mica (biotite) (K(Mg,Fe,)3(Si,Al)4O10(OH)2– Mica (muscovite)– (KAl2(Si,Al)4O10(OH)2Geologic Settings••Where?Where?•Mantle•Oceanic•Subduction Zones•Continental Mass•Compositions•Ultramafic•Mafic•Intermediate•Silicic (felsic)3Geologic Settings:MantleMid-ocean ridges Oceanic IslandsIsland arcs Continental There appear to be 13 major plates that cover the globe. Igneous Fractionation:Peridotite-basalt-granite •Partial Melting–Rocks have a melting interval of several hundred degrees C.–First melt is rich in silica, water and trace elements.•Fractional Crystallization–First-formed crystals low in silica settle to the bottom.4Igneous Fractionation: Layered Mafic IntrusionIntrusive forms•Pluton: Any large discordant intrusion at depth–Batholith: large > 100 km2–Stock: small < 100 km2•Dike: Tabular non-conformable•Sill: Tabular conformableDikes:Tabular discordantDikes:Tabular discordantSills:Tabular conformable5Small Pluton: StockIntrusive forms•Pluton: Any large discordant intrusion at depth–Batholith: large > 100 km2–Stock: small < 100 km2•Dike: Tabular non-conformable•Sill: Tabular conformableTerms•Polymerization•Magma•Pegmatitie•Igneous Fractionation•Partial melting•Fractional
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