Textural classification of igneous rocks 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

Textural classification of igneous rocks 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 Volcanic rocks

Compositional terms for igneous rocks Felsic: feldspar + silica ~55-70% silica, K-feldspar > 1/3 of feldspars present ~55-70% silica, K-feldspar > 1/3 of feldspars present light-colored silicate minerals — Continental crust light-colored silicate minerals — Continental crust Intermediate: between felsic and mafic ~55-65% silica, plag > 2/3 of feldspars present ~55-65% silica, plag > 2/3 of feldspars present Na-rich plag predominates over Ca-rich plag Na-rich plag predominates over Ca-rich plag Mafic: magnesium + ferric iron ~45-50% silica; Ca-rich plag dominant feldspar ~45-50% silica; Ca-rich plag dominant feldspar dark silicate minerals — Oceanic crust dark silicate minerals — Oceanic crust Ultramafic: >90% mafic minerals, silica 90% mafic minerals, silica < 45%, few or no feldspars Mantle-derived Mantle-derived

Classification of common igneous rocks CompositionPhaneriticAphanitic Color index (% dark minerals) Felsic Granite Syenite Monzonite Rhyolite Trachyte Latite Intermediate Granodiorite Diorite Dacite Andesite Mafic GabbroBasalt 50 Ultramafic Peridotite 95

Composition of Igneous Rocks

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.

Classification of Phaneritic Igneous Rocks Figure 2-2. 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 Alkali Fs. Quartz Syenite Quartz Syenite Quartz Monzonite Quartz Monzodiorite Syenite Monzonite Monzodiorite (Foid)-bearing Syenite (Foid)-bearing Monzonite (Foid)-bearing Monzodiorite 90 Alkali Fs. Syenite (Foid)-bearing Alkali Fs.Syenite 10 (Foid) Monzosyenite (Foid) Syenite (Foid) Monzodiorite (Foid) Gabbro Qtz. Diorite/ Qtz. Gabbro 5 10 Diorite/Gabbro/ Anorthosite (Foid)-bearing Diorite/Gabbro 60 (Foid)olites Quartzolite Granite Grano- diorite Tonalite Alkali Feldspar Granite Q A P F 60 Plutonic rocks

Classification of Igneous Rocks Figure 2-2. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS.OlivineClinopyroxene Orthopyroxene Lherzolite Harzburgite Wehrlite Websterite Orthopyroxenite Clinopyroxenite Olivine Websterite Peridotites Pyroxenites Dunite (c) Gabbroic rocks Ultramafic rocks

Classification of Aphanitic Igneous Rocks Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS. Volcanic rocks

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, Oxford University Press.

Classification of Igneous Rocks 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, b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, Pyroclastic rocks

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.

Figure 3-6. Examples of 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.

Figure 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 b. Very straight multiple albite twins in plagioclase, set in felsitic groundmass. Rhyolite, Chaffee, CO. Field widths ~1 mm. © John Winter and Prentice Hall.

Figure (c-d) Tartan twins in microcline. Field widths ~1 mm. © John Winter and Prentice Hall.

Figure Polysynthetic deformation 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.

Figure Myrmekite formed in plagioclase at the boundary with K-feldspar. Photographs courtesy © L. Collins.

Michel-Levy method for determining feldspar composition 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.