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Igneous Rocks and Plate Tectonics

Igneous Rocks and Plate Tectonics. Mantle Melting. Raw material: Lherzolite (Ol + Cpx + Opx) First to melt is Clinopyroxene Magma is rich in Clinopyroxene + Ca-Plagioclase = Tholeiite Basalt Residual = Harzburgite (Ol + Opx) Further melting removes Orthopyroxene

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Igneous Rocks and Plate Tectonics

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  1. Igneous Rocks and Plate Tectonics

  2. Mantle Melting • Raw material: Lherzolite (Ol + Cpx + Opx) • First to melt is Clinopyroxene • Magma is rich in Clinopyroxene + Ca-Plagioclase = Tholeiite Basalt • Residual = Harzburgite (Ol + Opx) • Further melting removes Orthopyroxene • Residual = Dunite (Ol only)

  3. Tholeiite Basalt • Olivine + Augite + Fe-Ti-Cr Oxides • Greek Tholos: Shield • More or less directly from Mantle • Mid-Ocean Ridge (Spreading Center) • Oceanic Islands (Hot Spots) • Oceanic Volcanic Plateaus • Continental Flood Basalts • Earliest Continental Margin • Earliest Continental Rifting

  4. Tholeiite Evolution

  5. Continental Margins

  6. Subduction Zones • Descending slab doesn’t melt • Dewatering melts mantle wedge • Young island arcs erupt tholeiite basalt • Older arcs erupt basalt-andesite-rhyolite • Oldest arcs have high-K varieties • Trends reflect: • Removal of ferromagnesian minerals • Accumulation of silica and alkalis • Contamination with crustal materials

  7. Extreme Rocks • Anorthosites • Carbonatites • Kimberlites • Diatremes

  8. Anorthosites • >90% Plagioclase; Plot as Gabbro • Lunar • Fractionation of Lunar Magma Ocean • Archean • Proterozoic

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