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- 1. Textural classification of igneousrocks Phaneritic: crystals visible with naked eye Plutonic or intrusive rocks Aphanitic: crystal too small for naked eye Volcanic or extrusive rocks Porphyritic: two different, dominant grain sizes Large xtals = phenocrysts; small xtals = groundmass Fragmental: composed of disagregated igneous material Pyroclastic rocks
- 2. Textural classification of igneousrocks Pegmatitic: very large xtals (cm to 10s of cm); i.e., slowly cooled Forms veins or layers within plutonic body Glassy: non-crystalline; cools very fast (e.g., obsidian) Volcanic rocks Vesicular: vesicles (holes, pores, cavities) form as gases expand Volcanic rocks
- 3. Compositional terms for igneousrocks Felsic: feldspar + silica ~55-70% silica, K-feldspar > 1/3 of feldspars present light-colored silicate minerals — Continental crust Intermediate: between felsic and mafic ~55-65% silica, plag > 2/3 of feldspars present Na-rich plag predominates over Ca-rich plag Mafic: magnesium + ferric iron ~45-50% silica; Ca-rich plag dominant feldspar dark silicate minerals — Oceanic crust Ultramafic: >90% mafic minerals, silica < 45%, few or no feldspars Mantle-derived
- 4. Classification of commonigneous rocks Composition Phaneritic Aphanitic Color index (% dark minerals) Felsic Granite Syenite Monzonite Rhyolite Trachyte Latite 10 15 20 Intermediate Granodiorite Diorite Dacite Andesite 20 25 Mafic Gabbro Basalt 50 Ultramafic Peridotite 95
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- 6. Classification of IgneousRocks 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.
- 7. Classification of Phaneritic Igneous Rocks Figure2-2. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS. The rock must contain a total of at least 10% of the minerals below. Renormalize to 100% (a) Quartz-rich Granitoid 90 90 60 60 20 20 Alkali Fs. Quartz Syenite Quartz Syenite Quartz Monzonite Quartz Monzodiorite Syenite Monzonite Monzodiorite (Foid)-bearing Syenite 5 10 35 65 (Foid)-bearing Monzonite (Foid)-bearing Monzodiorite 90 Alkali Fs. Syenite (Foid)-bearing Alkali Fs. Syenite 10 (Foid) Monzosyenite (Foid) Monzodiorite Qtz. Diorite/ Qtz. Gabbro 5 10 Diorite/Gabbro/ Anorthosite (Foid)-bearing Diorite/Gabbro 60 (Foid)olites Quartzolite Granite Grano- diorite Q A P F 60 Plutonic rocks
- 8. Classification of IgneousRocks Figure 2-2. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. Plagioclase Olivine Pyroxene G a b b r o T r o c t o l i t e Olivine gabbro Plagioclase-bearing ultramafic rocks 90 (b) Anorthosite Olivine Clinopyroxene Orthopyroxene Lherzolite Websterite Orthopyroxenite Clinopyroxenite Olivine Websterite Peridotites Pyroxenites 90 40 10 10 Dunite (c) Gabbroic rocks Ultramafic rocks
- 9. Classification of Aphanitic Igneous Rocks Figure2-3. A classification and nomenclature of volcanic rocks. After IUGS. (foid)-bearing Trachyte (foid)-bearing Latite (foid)-bearing Andesite/Basalt (Foid)ites 10 60 60 35 65 10 20 20 60 60 F A P Q Rhyolite Dacite Trachyte Latite Andesite/Basalt Phonolite Tephrite Volcanic rocks
- 10. Classification of IgneousRocks 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.
- 11. Classification of IgneousRocks 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. Pyroclastic rocks
- 12. Igneous Textures Figure 3-5.a. Compositionally zoned hornblende phenocryst with pronounced color variation visible in plane-polarized light. Field width 1 mm. b. Zoned plagioclase twinned on the carlsbad law. Andesite, Crater Lake, OR. Field width 0.3 mm. © John Winter and Prentice Hall.
- 13. Figure 3-6. Examplesof plagioclase zoning profiles determined by microprobe point traverses. a. Repeated sharp reversals attributed to magma mixing, followed by normal cooling increments. b. Smaller and irregular oscillations caused by local disequilibrium crystallization. c. Complex oscillations due to combinations of magma mixing and local disequilibrium. From Shelley (1993). Igneous and Metamorphic Rocks Under the Microscope. © Chapman and Hall. London.
- 14. Figure 3-18. a.Carlsbad twin in orthoclase. Wispy perthitic exsolution is also evident. Granite, St. Cloud MN. Field widths ~1 mm. © John Winter and Prentice Hall. Figure 3-18. b. Very straight multiple albite twins in plagioclase, set in felsitic groundmass. Rhyolite, Chaffee, CO. Field widths ~1 mm. © John Winter and Prentice Hall.
- 15. Figure 3-18. (c-d)Tartan twins in microcline. Field widths ~1 mm. © John Winter and Prentice Hall.
- 16. Figure 3-19. Polysyntheticdeformation twins in plagioclase. Note how they concentrate in areas of deformation, such as at the maximum curvature of the bent cleavages, and taper away toward undeformed areas. Gabbro, Wollaston, Ontario. Width 1 mm. © John Winter and Prentice Hall.
- 17. Figure 3-21. Myrmekiteformed in plagioclase at the boundary with K-feldspar. Photographs courtesy © L. Collins. http://www.csun.edu/~vcgeo005
- 18. Michel-Levy method for determiningfeldspar composition 1 2 3 Rotate clockwise… Rotate counterclockwise… Using albite twins In XPL, find uniform extinction in N-S direction Angle between CW and CCW measurement should be within a few degrees; measure 5-10 grains and take highest angle.