Plate Tectonic Igneous Genesis 1 Mid ocean Ridges 2 Intracontinental Rifts 3 Island Arcs 4 Active Continental Margins 5 Back arc Basins 6 Ocean Island Basalts 7 Miscellaneous IntraContinental Activity kimberlites carbonatites anorthosites Igneous Minerals Quartz Feldspars plagioclase and alkaline Olivines Pyroxenes Amphiboles Accessory Minerals mostly in small quantities or in special rocks Magnetite Fe3O4 Ilmenite FeTiO3 Apatite Ca5 PO4 3 OH F Cl Zircon ZrSiO4 Titanite CaTiSiO5 Pyrite FeS2 Fluorite CaF2 Ca 2 O2Si4 O2 Mg2 Na Fe2 O Liquid hot O2MAGMA 2O O2 Si4 O2 O2O2 Si4 O2 Minerals which form are thus a function of melt composition and how fast it cools re equilibration governed by the stability of those minerals and how quickly they may or may not react with the melt during crystallization General Compositions Silicic Si rich Sialic Si and Al rich Intermediate Mafic Mg and Ferich Ultramafic Also ID d based on alkalic K and Na or alkaline Ca rich Mg2 2 cooling rock Mg2 Fe2 Composition From Magma we saw how a crystal s composition can change on crystallization different elemental composition from melt on partial crystallization Silica and Aluminum Content Silica Oversaturated if it contains Quartz Undersaturated if it has silica deficient minerals like the feldspathoids ex nepheline Aluminum Peraluminous if it has a great excess of aluminum after feldspars form more Al left over for Al rich phases like corumndum garnet kyanite etc Peralkaline So little Al left after feldspars form only Al deficient minerals like aegerine type of pyroxene and riebekite sodic amphibole Classification of Igneous Rocks Figure 2 4 A chemical classification of volcanics based on total alkalis vs silica After Le Bas et al 1986 J Petrol 27 745 750 Oxford University Press Igneous Rocks Textures aphanitic phaneritic porphyritic pegmatitic vesicular glass and pyroclastic Compositions Silicic Intermediate Mafic Ultramafic Igneous Textures Figure 3 1 Idealized rates of crystal nucleation and growth as a function of temperature below the melting point Slow cooling results in only minor undercooling Ta so that rapid growth and slow nucleation produce fewer coarse grained crystals Rapid cooling permits more undercooling Tb so that slower growth and rapid nucleation produce many fine grained crystals Very rapid cooling involves little if any nucleation or growth Tc producing a glass Figure 3 17 Ostwald ripening in a monomineralic material Grain boundaries with significant negative curvature concave inward migrate toward their center of curvature thus eliminating smaller grains and establishing a uniformly coarse grained equilibrium texture with 120 o grain intersections polygonal mosaic John Winter and Prentice Hall Textures I Aphanitic fine grain size 1 mm result of quick cooling Phaneritic coarse grain size visible grains 110 mm result of slow cooling Rhyolite Basalt Rhyolite Andesite Granite Diorite Gabbro Pegmatitic very large crystals many over 2 cm Granite pegmatite or pegmatitic granite Porphyritic Mixture of grain sizes caused by mixed cooling history slow cooling first followed by a period of somewhat faster cooling Terms for the textural components Phenocrysts the large crystals Groundmass or matrix the finer crystals surrounding the large crystals The groundmass may be either aphanitic or phaneritic Types of porphyritic textures Porphyritic aphanitic Porphyritic phaneritic Origin mixed grain sizes and hence cooling rates imply upward movement of magma from a deeper hotter location of extremely slow cooling to either a much shallower cooler location with fast cooling porphyriticaphanitic or a somewhat shallower slightly cooler location with continued fairly slow cooling porphyritic phaneritic Typically Volcanic Textures Glassy instantaneous cooling Obsidian volcanic glass Vesicular contains tiny holes called vesicles which formed due to gas bubbles in the lava or magma Very porous May resemble a sponge Commonly low density may float on water Vesicular Basalt Pumice Scoria Pyroclastic or Fragmental pieces of rock and ash come out of a volcano and get welded together by heat May resemble rhyolite or andesite but close examination shows pieces of fine grained rock fragments in it May also resemble a sedimentary conglomerate or breccia except that rock fragments are all fine grained igneous or vesicular Tuff made of volcanic ash Volcanic breccia contains fragments of fine grained igneous rocks that are larger than ash Classification based on Field Relations Extrusive or volcanic rocks typically aphanitic or glassy Many varieties are porphyritic and some have fragmental volcaniclastic fabric High T disordered fsp is common e g sanadine Also see leucite tridymite and cristobalite Intrusive or plutonic rocks typically phaneritic Monomineralic rocks of plagioclase olivine or pyroxene are well known but rare Amphiboles and biotites are commonly altered to chlorite Muscovite found in some granites but rarely in volcanic rocks Perthitic fsp reflecting slow cooling and exsolution is widespread Names of Igneous Rocks Texture Composition name Set up diagrams many ternary ones again you remember how these work to represent composition changes for rocks of a certain texture Composition can be related to specific minerals or even physical characteristics of mineral grains Modal Composition of minerals comprising a rock Visual Estimation of Modal Abundance Classification based on Modal Mineralogy Felsic rocks mnemonic based on feldspar and silica Also applies to rocks containing abundant feldspathoids such as nepheline GRANITE Mafic rocks mnemonic based on magnesium and ferrous ferric Synonymous with ferromagnesian which refers to biotite amphibole pyroxene olivine and Fe Ti oxides BASALT Ultramafic rocks very rich in Mg and Fe Generally have little feldspar PERIDOTITE Silicic rocks dominated by quartz and alkali fsp Sometimes refered to as sialic Si Al Classification of Igneous Rocks granite granodiorite Classification of Phaneritic Igneous Rocks Q Quartzolite 90 90 Quartz rich Granitoid 60 Gra n Alka li Fel dsp ar Alkali Fs Syenite 20 20 Quartz Monzonite Quartz Syenite 5 10 A Granodiorite Granite lite na To Alkali Fs Quartz Syenite Syenite Foid bearing Syenite 35 Monzonite Foid bearing Monzonite Quartz Monzodiorite 65 Monzodiorite Foid bearing Monzodiorite Figure 2 2 A classification of the phaneritic igneous rocks a Phaneritic rocks with more than 10 quartz feldspar feldspathoids After IUGS Foid Monzodiorite Ga Foid