Textural classification of igneous rocks Phaneritic crystals visible

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 10 s 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

Compositional terms for igneous rocks 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

Classification of common igneous rocks Composition Phaneritic Aphanitic Color index (% dark minerals) Felsic Granite Syenite Monzonite Intermediate Granodiorite Rhyolite Trachyte Latite Diorite Dacite Andesite Mafic Gabbro Basalt Ultramafic Peridotite 10 15 20 20 25 50 95

Composition of Igneous Rocks

Classification of Igneous Rocks Figure 2 -1 a. 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.

The rock must contain a total of at least 10% of the minerals below. Renormalize to 100% 90 90 Quartz-rich Granitoid 60 par Gra n ite 60 lds Fe ali lite Granodiorite Granite na Plutonic rocks Quartzolite To Classification of Phaneritic Igneous Rocks Q Alkali Fs. Quartz Syenite Alkali Fs. Syenite Alk (a) 20 20 5 10 A Quartz Monzonite Quartz Syenite Monzonite (Foid)-bearing Syenite Monzonite 35 Quartz Monzodiorite 65 Monzodiorite (Foid)-bearing Monzodiorite 10 bro Ga b (Foid) Monzodiorite (Fo e it en Sy (Foid) Monzosyenite id) (Foid)-bearing Alkali Fs. Syenite Figure 2 -2. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS. 60 60 (Foid)olites F Qtz. Diorite/ Qtz. Gabbro 5 Diorite/Gabbro/ 90 Anorthosite P 10 (Foid)-bearing Diorite/Gabbro

Classification of Igneous Rocks Gabbroic rocks Figure 2 -2. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. Olivine Peridotites Ha Lherzolite urg 90 hr rzb Dunite We ite Ultramafic rocks 40 (c) Pyroxenites Olivine Websterite Orthopyroxenite 10 10 Orthopyroxene Websterite Clinopyroxene

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

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.

Classification of Igneous Rocks Pyroclastic 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, 40 -43. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev. , 1, 287 -298.

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 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.

Figure 3 -18. (c-d) Tartan twins in microcline. Field widths ~1 mm. © John Winter and Prentice Hall.

Figure 3 -19. 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 3 -21. Myrmekite formed in plagioclase at the boundary with K-feldspar. Photographs courtesy © L. Collins. http: //www. csun. edu/~vcgeo 005

Michel-Levy method for determining feldspar composition In XPL, find uniform extinction in N-S direction 3 1 Using albite twins Angle between CW and CCW measurement should be within a few degrees; measure 5 -10 grains and take highest angle. Rotate counterclockwise… 2 Rotate clockwise…