Lect2 Igneous Classification - University of Texas at … of Igneous Rocks Figure 2.2c. A...
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Classification of Igneous Rocks
Classification I: texture
Classification II: field relations
Classification III: chemical compositionsp
Classification IV: textural-mineralogicalg
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Classification I: textureClassification I: texture
Phaneritic: rocks with mineral grains that are largePhaneritic: rocks with mineral grains that are large enough to be identified by eye. Slowly cooled intrusive rocks.Aphanitic: rocks with grain too small to be identified by eye. Rapidly solidified extruded magma and marginal facies of shallow intrusions.Porphyritic- bimodal grain size distributionGlassy- no crystals formed
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Granite Hand SpecimenGranite Hand Specimen
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Photomicrograph Photomicrograph -- Phaneritic TexturePhaneritic Texture
Interlocking grains
Photo credit: C.C. Plumme
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Rhyolite Hand SpecimenRhyolite Hand Specimen
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Classification II: Field Relations
Extrusive or volcanic rocks: typically aphanitic or glassy. Many varieties are porphyritic and some have fragmentalMany 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 phaneriticIntrusive 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.reflecting slow cooling and exsolution is widespread.
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Classification III: Chemical compositionsSilica percentage (% SiO2)
>66%, felsic, granite/rhyolite52-66%, intermediate, diorite/andesite45-52%, mafic, gabboro/basalt<45%, ultramafic, peridotite/dunite/komatiite
Silica vs. alkaline elements (SiO2 Vs. Na2O+K2O)CalcicC l lk liCalc-alkalicAlkalic-calcicAlkalic
Aluminum saturation (Al2O3 Vs. Na2O+K2O+CaO)PeraluminousMetaaluminousMetaaluminousPeralkaline
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Whole Rock Major and Trace Element Composition of B lt ( l t 7 f l ti l th d )Basalt (see lecture 7 for analytical methods)
What controls chemical compostion of a rock?
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Classification IV: textual + mineralogical
Textual : phaneritic vs. Aphanitic Mi l i lMineralogical:
EssentialCharacterizing accessory mineral (>5%)g y ( )Minor accessory mineral (<5%)
Classification IV:Felsic: >10% quatz, feldspar or feldspathoidGabbroic: plag+opx+cpx+ol+hbd > 95%Gabbroic: plag+opx+cpx+ol+hbd > 95%Ultramafic systems: ol+opx+cpx+hdb >95%
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Classification of Igneous Rocks
Figure 2.1a. Method #1 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
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Classification of I R kIgneous Rocks
Figure 2.2a. A classification of the phaneritic igneous rocks: Phaneritic rocks with more than 10% (quartz + f ld + f ld th id ) Aft IUGSfeldspar + feldspathoids). After IUGS.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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Classification of Igneous Rocks
Figure 2.2b. A classification of the phaneritic igneous rocks: Gabbroic rocks. After IUGS.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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Classification of Igneous Rocks
Figure 2.2c. A classification of the phaneritic igneous rocks: Ultramafic rocks. After IUGS.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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Classification of Igneous Rocks
Figure 2.3. A classification and nomenclature of volcanic rocks After IUGSof volcanic rocks. After IUGS.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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Classification of Igneous Rocks
Figure 2.5. Classification of the pyroclastic rocks. After Fisher (1966) Earth Sci. Rev., 1, 287-298.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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The Petrology of the Silicate Earth
Oceanic crust: basalt+gabboroOceanic crust: basalt+gabboro
Continental crust: graniteContinental crust: granite
M tl id titMantle: peridotite
A l di i / d iArc lavas: diorite/andesite
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Summary
Four classification schemesDifference between these rocks
Continental crustal rocksContinental crustal rocksOceanic crustal rocksMantle rocksMantle rocksArc lavas
Able to read/report chemical composition of a rock