UVM GEOL 110 - Lecture 23 - Igneous Rocks

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Plate Tectonic - Igneous GenesisIgneous MineralsSlide 3Slide 4CompositionSilica and Aluminum ContentClassification of Igneous RocksIgneous RocksIgneous TexturesSlide 10Textures ISlide 12Typically Volcanic TexturesClassification based on Field RelationsNames of Igneous RocksVisual Estimation of Modal AbundanceClassification based on Modal MineralogySlide 18Slide 19Classification of Phaneritic Igneous RocksAphanitic rocksSlide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Plate Tectonic - Igneous GenesisPlate Tectonic - Igneous Genesis 1.1. Mid-ocean RidgesMid-ocean Ridges2.2. Intracontinental RiftsIntracontinental Rifts3. 3. Island ArcsIsland Arcs4.4. Active ContinentalActive ContinentalMarginsMargins 5.5. Back-arc BasinsBack-arc Basins6.6. Ocean Island BasaltsOcean Island Basalts7.7. Miscellaneous Intra-Miscellaneous Intra-Continental ActivityContinental Activitykimberlites, carbonatites, kimberlites, carbonatites, anorthosites...anorthosites...Igneous MineralsIgneous MineralsQuartz, Feldspars (plagioclase and alkaline), Quartz, Feldspars (plagioclase and alkaline), Olivines, Pyroxenes, AmphibolesOlivines, Pyroxenes, AmphibolesAccessory Minerals – mostly in small quantities Accessory Minerals – mostly in small quantities or in ‘special’ rocksor in ‘special’ rocksMagnetite (FeMagnetite (Fe33OO44))Ilmenite (FeTiOIlmenite (FeTiO33))Apatite (CaApatite (Ca55(PO(PO44))33(OH,F,Cl)(OH,F,Cl)Zircon (ZrSiOZircon (ZrSiO44))Titanite (CaTiSiOTitanite (CaTiSiO55))Pyrite (FeSPyrite (FeS22))Fluorite (CaFFluorite (CaF22))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 Fe-rich), UltramaficAlso ID’d based on alkalic (K and Na) or alkaline (Ca-rich)Liquid hotMAGMACa2+Na+Mg2+Fe2+Si4+Si4+Si4+O2-O2-O2-O2-O2-O2-O2-O2-O2-O2-rockcoolingMg2+Fe2+Mg2+CompositionCompositionFrom Magma we saw how a crystal’s From Magma we saw how a crystal’s composition can change on crystallization composition can change on crystallization  different elemental composition from melt on different elemental composition from melt on partial crystallizationpartial crystallizationSilica and Aluminum ContentSilica 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 RocksClassification of Igneous RocksFigure 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 RocksIgneous RocksTextures Textures  aphanitic, phaneritic, aphanitic, phaneritic, porphyritic, pegmatitic, vesicular, glass, and porphyritic, pegmatitic, vesicular, glass, and pyroclastic pyroclastic Compositions Compositions  Silicic, Intermediate, Silicic, Intermediate, Mafic, UltramaficMafic, UltramaficIgneous TexturesIgneous TexturesFigure 3-1.Figure 3-1. Idealized rates of crystal Idealized rates of crystal nucleation and growth as a function nucleation and growth as a function of temperature below the melting of temperature below the melting point. Slow cooling results in only point. Slow cooling results in only minor undercooling (Tminor undercooling (Taa), so that ), so that rapid growth and slow nucleation rapid growth and slow nucleation produce fewer coarse-grained produce fewer coarse-grained crystals. Rapid cooling permits more crystals. Rapid cooling permits more undercooling (Tundercooling (Tbb), so that slower ), so that slower growth and rapid nucleation produce growth and rapid nucleation produce many fine-grained crystals. Very many fine-grained crystals. Very rapid cooling involves little if any rapid cooling involves little if any nucleation or growth (Tnucleation or growth (Tcc) producing ) producing a glass.a glass.Figure 3-17Figure 3-17.. “ “Ostwald ripeningOstwald ripening” in a monomineralic material. Grain boundaries with significant negative ” in a monomineralic material. Grain boundaries with significant negative curvature (concave inward) migrate toward their center of curvature, thus eliminating smaller grains and curvature (concave inward) migrate toward their center of curvature, thus eliminating smaller grains and establishing a uniformly coarse-grained equilibrium texture with 120establishing a uniformly coarse-grained equilibrium texture with 120oo grain intersections (polygonal mosaic). grain intersections (polygonal mosaic). © John Winter and Prentice Hall© John Winter and Prentice HallTextures ITextures IAphaniticAphanitic - fine grain size (< 1 mm); result of - fine grain size (< 1 mm); result of quick cooling quick cooling Rhyolite, Basalt, RhyoliteRhyolite, Basalt, Rhyolite, , AndesiteAndesite PhaneriticPhaneritic - coarse grain size; visible grains (1- - coarse grain size; visible grains (1-10 mm); result of slow cooling 10 mm); result of slow cooling GraniteGranite, , Diorite,Diorite, GabbroGabbro PegmatiticPegmatitic - very large crystals (many over 2 - very large crystals (many over 2 cm) cm) Granite pegmatiteGranite pegmatite or or pegmatitic granitepegmatitic granitePorphyriticPorphyritic- Mixture of grain sizes caused by mixed cooling history; - Mixture of grain sizes caused by mixed cooling history; slow cooling first, followed by a period of somewhat faster cooling. slow cooling first, followed by a period of somewhat faster cooling. Terms for the textural components: Terms for the textural components: PhenocrystsPhenocrysts - the large crystals - the large crystals GroundmassGroundmass or or matrix matrix - the finer crystals surrounding the large - the finer crystals surrounding the large crystals. The groundmass may


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UVM GEOL 110 - Lecture 23 - Igneous Rocks

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