- Slides: 33
Classification of Igneous Rock
Goals of a Rock Classification Scheme l l l Practical – especially for field use Descriptive – minimal genetic terms Systematic – allows for easy information retrieval Hierarchical – allows for greater levels of detail Natural – defines boundaries that agree with nature Most common schemes based on mode (vol. %) of “essential” primary minerals. . . (When observation of primary minerals is not possible (e. g. , too fine, altered) chemistry is commonly used). . . with modification by texture and structural features and occurrence of “accessory” and minor minerals
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
Determination of Modal Parameters
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.
Classification of Igneous Rocks Figure 2. 1 b. 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.
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 nite 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 ro Ga bb (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 Figure 2. 2 a. A classification of the phaneritic igneous rocks: Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS.
Classification of Aphanitic Igneous Rocks Volcanic rocks Figure 2 -3. A classification and nomenclature of volcanic rocks. After IUGS.
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 Lherzolite bu rgi 90 hr Ha rz Dunite We te Ultramafic rocks 40 (c) Pyroxenites Olivine Websterite Orthopyroxenite 10 10 Orthopyroxene Websterite Clinopyroxene
Classification of Igneous Rocks Figure 2. 2 b. A classification of the phaneritic igneous rocks: Gabbroic rocks. After IUGS.
pl opx cpx pl ol pl classification of ultramafic rocks (Streckeinsen, 1973) and Le. Maitre (1989)
Classification of Igneous Rocks Figure 2. 2 c. A classification of the phaneritic igneous rocks: Ultramafic rocks. After IUGS.
Textures of Igneous Rocks (1): Allotriomorphic-granular (gabbroic) texture is a feature of phaneritic rocks in which all of the minerals have anhedral shapes; it is characteristic of some mafic and ultramafic rocks, such as dunite and pyroxenite. Hypidiomorphic-granular (granitic) texture: igneous texture in which most of the mineral grains are subhedral. Typical of granite, granodiorite, quartz monzonite, etc. Porphyritic texture: rocks in which larger grains are contained in a finer grained matrix. May occur in either intrusive or extrusive rocks, but it is most common in extrusive rocks such as basalt, andesite, dacite, and rhyolite. Intergranular texture: Plagioclase laths with interstitial pyroxene grains that are smaller than the plagioclase; commonly found in basalts.
Textures of Igneous Rocks (2): Intersertal texture: Small feldspars with glass or altered glass interstitial to the feldspars; common texture of basalts. Ophitic texture: Pyroxene grains partially or completely surround plagioclase laths; common texture of gabbros and basalts. Trachytic texture: Subparallel feldspars formed during flow in volcanic rocks. Spinifex texture: Interlacing olivine or pyroxene, a texture formed by quenching in komatiites.
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 -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…
Descriptive Terminology Pyroxene-based (augite) bulk textural terminology l ophitic – multiple lath-shaped crystals of plagioclase totally enclosed in crystals of pyroxene l subophitic – multiple lath-shaped crystals of plagioclase partially enclosed in crystals of pyroxene l intergranular – generally equigranular euhedral to anhedral primary minerals (need not be augite and plagioclase), none enclosing the others Terms for other primary non-granular phases l poikilitic – one phase completely envelops many other more granular phases (e. g. , plagioclase-poikilitic) l subpoikilitic – one phase partially envelops other more granular phases (e. g. , olivine-subpoikilitic)
Mineral Textures Poikilitic (Ophitic) Subpoikilitic Subhe Ameboidal Anhedral Granula Euhedral Granular
Absolute and Relative Grain Size Terminology Generalized scale: Fine (<1 mm)Medium (1 -5 mm) Coarse (5 -12 mm) Very Coarse/Pegmatitic (>12 mm) Detailed scale: Very Fine (<0. 2 mm) Fine (0. 2 -0. 8 mm) Medium Fine (0. 8 -1. 5 mm) Medium (1. 5 -3 mm) Medium Coarse (3 -7 mm) Coarse (7 -12 mm) Very Coarse (12 -30 mm) Pegmatitic (>30 mm) equigranular – generally equal sizes for all granular phases seriate – gradational range in grain size of all granular phases hiatial – bimodal range in grain size of all granular phases porphyritic – bimodal range in grain size of one (or rarely two) pr modifiers based on contrast in grain size– weakly, modera
Planar Features Alignment of elongate or tabular mineral phases (igneous lamination, fluxion structure, foliation) Scale - % aligned within 10 of a common plane u u u non-foliated (<25%) (decussate) poorly foliated (25 -50%) moderately foliated (50 -75%) well foliated (75 -90%) very well foliated (>90%) Layering (not typically observed at the scale of a thin section) Type: modal, isomodal, graded modal, grain size, textural, phase, combination Contrast (or demarcation): strong, moderate, weak, subtle Frequency: single layer, rhythmic, intermittent, irregular Scale: centimeter, decimeter, inconsistent, variable