Transcript Slide 1
Northern cordillera Eocene Plutonic Rocks The Granite Problem? 18th century: Plutonists versus igneous/metamorphic Early 20th century: Magmatists sedimentary versus igneous Now: Mantle Neptunists Transformists metasomatic versus Crystal fractionation Crust Melting Granite Plutons in Niger Archean Granite – Greestone Terranes granitoid dome greenstone belt granitoid dome granitoid dome granitoid dome Classification Liquidus Projection Melt Inclusion Data Are andesite magmas mixtures of mantle-derived basaltic magmas and rhyolitic partial melts of the continental crust? Eocene Sr - Nd Mantle Ocean Continent crust crust SiO2 TiO2 Al2O3 MgO FeO CaO Na2O K2O Total 45.2 0.7 3.5 37.5 8.5 3.1 0.6 0.1 99.2 49.4 1.4 15.4 7.6 10.1 12.5 2.6 0.3 99.3 60.3 1.0 15.6 3.9 7.2 5.8 3.2 2.5 99.5 Spectrum of Igneous liquids Cations normalized to 100 cations Si Ti Al Mg Fe Ca Na K O 38.5 0.5 3.6 47.6 6.0 2.8 0.9 0.1 140.2 46.1 1.0 16.9 10.6 7.9 12.5 4.7 0.5 153.0 56.4 0.7 17.2 5.4 5.6 5.8 5.8 3.0 161.3 Mineralogy (oxygen units, XFe3+ = 0.10) Quartz Feldspar Clinopyroxene Orthopyroxene Olivine Oxides 0.0 13.2 6.7 18.3 59.9 1.8 0.0 57.3 25.7 4.1 9.9 3.0 13.0 64.3 5.9 14.7 0.0 2.0 Oceanic crust - MORB basalt Continental crust - granite p e Francis, Igneous Petrology EPSC 423A, 2011 Continental Crustal Granitoids Second Stage Melting of Basalt The majority of crustal granitoids are, however, thought to be liquids produced at the eutectic point e by the second stage melting of silicasaturated basaltic/gabbroic mafic crust, consisting largely of pyroxene and plagioclase. Francis, Igneous Petrology EPSC 423A, 2011 e MASH Mixing - Assimilation - Storage - Homogenization Feature A -Type Rock Types : granite Mafic Minerals: Accessories: Na-amphibole Na-pyroxene fluorite topaz tourmaline I - Type S - Type tonalite to granodiorite to granite to granite Oxide: ilmenite magnetite muscovite biotite garnet Al-silicates cordierite corundum ilmenite Feldspar : perthite K-spar & Plag subsolvus K-spar and plag subsolvus hypersolvus (Na+K)/Al: >1 per-alkaline (Na+K+Ca×2)/Al: >> 1 HFSE elements: (Zr,Nb, REE) Restite/ Enclaves: Source Regions: Tectonic Setting: hornblende biotite granodiorite to <1 << 1 >1 meta-aluminous <1 per-aluminous high low low few amphibolite granulitic lower crust with halogen volatile flux extensional within-plate extensional mafic igneous rocks mantle-derived magmas (M) compressional volcanic arcs, collisions biotite-cordierite sillimanite-garnet gneiss sedimentary rocks pelite greywacke compressional continental interplate Granitoids Classified According Interpretted to Source Region