Lecture 2 Classification of Igneous Rocks

Lecture 2 Classification of Igneous Rocks - Classification...

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Unformatted text preview: Classification of Igneous Rocks GEOL 320 -- Petrology Igneous Rock Classification q Preferred method for describing any rock type (igneous, sedimentary, metamorphic) is based on: x x texture composition TEXTURE q Criteria for Igneous Rocks Criteria phaneritic crystals that compose the rock are readily visible with the naked eye Granite (Siim Sepp, 2005) TEXTURE q Criteria for Igneous Rocks Criteria aphanitic crystals that compose the rock are too small to be seen with the naked eye Rhyolite (Cyril Moore Science Center) TEXTURE q x x Criteria for Igneous Rocks porphyritic igneous rocks with 2 dominant grain sizes that vary by a significant amount phenocrysts larger crystals groundmass finer crystals Pophyritic trachyte TEXTURE q Criteria for Igneous Rocks fragmental rock composed of pieces of disaggregated material, deposited and later amalgamated; fragments themselves may include pieces of pre-existing (predominately igneous) rock, crystal fragments, or glass Tuff Breccia, Inyo Co., CA TEXTURE q Criteria for Igneous Rocks vitreous igneous rock consisting of amorphous or glassy material; indicative of relatively rapid cooling Obsidian from Lake County, Oregon COMPOSITION Igneous Rock Classification COMPOSITION Reading and Plotting Data on Ternary Diagrams 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. COMPOSITION Reading and Plotting Data on Ternary Diagrams 70% X A Z 67% Y Figure 2-1b. Method #2 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. COMPOSITION Determining the Modal Mineralogy Point-counting Geochemical analysis Thin section photomicrograph of basalt (a) COMPOSITION The rock must contain a total of at least 10% of the minerals below. Renormalize to 100% Q Quartzolite 90 90 Alkali Fs. Quartz Syenite Alkali Fs. Syenite 5 Alk ali F eld s Classification of Igneous Rocks 20 Quartz-rich Granitoid 60 60 ran ite lite na To par G Granite Granodiorite 20 Quartz Syenite 10 Quartz Monzonite 35 Quartz Monzodiorite 65 Qtz. Diorite/ Qtz. Gabbro 5 Diorite/Gabbro/ A 10 Syenite (Foid)-bearing Syenite (Fo 60 Monzonite (Foid)-bearing Monzonite Monzodiorite 90 (Foid)-bearing Monzodiorite P Anorthosite (Foid)-bearing Alkali Fs. Syenite 10 (Foid)-bearing Diorite/Gabbro 60 (Foid)olites Figure 2-2. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS. F (Fo id) G (Foid) Monzosyenite (Foid) Monzodiorite ab bro id) it en Sy e COMPOSITION Classification of Igneous Rocks Plagioclase 90 Anorthosite Figure 2-2. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. Ga bb ro lite cto Tro bu rgi te Olivine Dunite 90 Olivine gabbro Peridotites lite hr We Plagioclase-bearing ultramafic rocks (b) (c) Olivine 40 Ha rz Pyroxene Lherzolite Orthopyroxenite Olivine Websterite 10 Pyroxenites Orthopyroxene 10 Websterite Clinopyroxenite Clinopyroxene Q COMPOSITION Classification of Igneous Rocks 20 60 60 Rhyolite Dacite 20 Trachyte Latite 35 Andesite/Basalt 65 A 10 (foid)-bearing Trachyte (foid)-bearing Latite (foid)-bearing Andesite/Basalt P 10 Phonolite Tephrite 60 60 (Foid)ites Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS. F COMPOSITION 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. COMPOSITION Classification of Igneous Rocks Glass Ash (<2mm) Tuff Lapilli Tuff Lapilli (2-64mm) Lapillistone 30 (a) 30 Vitric Tuff Lapilli-Tuff Breccia 70 Pyroclastic Breccia or Agglomerate 70 (b) Lithic Tuff Crystal Tuff Rock Fragments Crystals Blocks and Bombs (> 64 mm) Figure 2-5. Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298. ...
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