Polymerization of Silica • Each Si atom is surrounded by 4 Chapter 4 oxygens. • Polymerization is the degree to which the oxygens are shared with other Si atoms. INTRUSIVE IGNEOUS • Polymerization is the number of Si-O-Si bonds. ROCKS – Olivine has none: isolated SiO4 groups – Pyroxene has one third: SiO3 chains – Mica has has 40% shared: Si4O10 sheets – Quartz and feldspar have 100% (Si,Al)O2 framework. Polymerization of Silica Polymerization of Silica Pyroxene Olivine Mica (Sheet) Tetrahedron Quartz (Framework) MAGMA Intrusive Igneous Rocks • Magma is the term for any molten silicate • Composition material, whether below the surface or on top. • Mineralogy • Volcanic rocks are erupted on the surface. • Geologic Setting – Volcanic rocks are fine-grained (<1 mm) – Mantle • Intrusive igneous rocks crystallize – Oceanic Crust from magma below the surface. – Continental Crust – Intrusive igneous rocks are coarse-grained (> 1mm) • Igneous Fractionation – Pegmatites are very coarse grained (> 1cm) – How the chemistry evolves 1 Igneous Rock Compositions Igneous Rock Compositions • Rock compositions are described • Igneous rocks vary in composition in weight percents of oxides. from • The principal variation in igneous – ultramafic (~40 wt%) (peridotite) rock compositions is silica (SiO2) – mafic (48-55%) (gabbro) content. – intermediate (55-65%) (diorite) • The degree of polymerization of – silicic (65-75 wt %) (granite) silica increases with silica content. • The mantle is peridotite (ultramafic). (both in crystals and melt). • The ocean basins gabbro (mafic). • The continents are granite (silicic) Igneous Rock Names Igneous Rock Mineralogy and Compositions • Peridotite (Ultramafic) • Gabbro (Mafic) • Composition • Intrusive • Volcanic Name Rock Name Rock Name – Olivine ((Mg,Fe)2SiO4) – Feldspar – Pyroxene (CaAl2Si2O8) • Ultramafic • Peridotite • (Komatiite) ((Mg,Fe,Ca)SiO3) – Pyroxene •Mafic • Gabbro • Basalt – Garnet (Mg3Al2Si3O12) or ((Mg,Fe,Ca)SiO3) – Spinel (MgAl O ) • Intermediate • Diorite • Andesite 2 4 – Olivine • Silicic (felsic) • Granite • Rhyolite ((Mg,Fe)2SiO4) Igneous Rock Mineralogy Geologic Settings • Diorite (Intermediate) • Granite (Silicic) – Feldspar (CaAl2Si2O8) – Quartz (SiO2) • Compositions • Where? – Feldspar (NaAlSi3O8) – Feldspar (NaAlSi3O8) • Ultramafic • Mantle – Pyroxene ((Mg,Fe,Ca)SiO3) – Feldspar (KAlSi3O8) • Mafic • Oceanic – Mica – Mica (biotite) • Intermediate • Subduction Zones K(Mg,Fe,)3(Si,Al)2O10(OH)2 (K(Mg,Fe,)3(Si,Al)4O10(OH)2 – Mica (muscovite) • Silicic (felsic) • Continental Mass – (KAl2(Si,Al)4O10(OH)2 2 Geologic Settings: There appear to be 13 major plates that cover the globe. Mantle Mid-ocean ridges Oceanic Islands Island arcs Continental 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. 3 Igneous Fractionation: Layered Mafic Intrusion Dikes: Intrusive forms Tabular discordant • Pluton: Any large discordant intrusion at depth – Batholith: large > 100 km2 – Stock: small < 100 km2 • Dike: Tabular non-conformable • Sill: Tabular conformable Dikes: Sills: Tabular discordant Tabular conformable 4 Small Pluton: Stock Intrusive forms • Pluton: Any large discordant intrusion at depth – Batholith: large > 100 km2 – Stock: small < 100 km2 • Dike: Tabular non-conformable • Sill: Tabular conformable Terms • Polymerization • Silicic • Magma • Peridotite • Pegmatitie • Gabbro • Igneous • Diorite Fractionation • Granite • Partial melting • Pluton • Fractional • Stock Crystallization • Batholith • Ultramafic • Dike • Mafic • Sill • Intermediate 5.