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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
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
Rhyolite
Trachyte
Latite
Intermediate
Granodiorite
Diorite
Dacite
Andesite
Mafic
Gabbro
Basalt
Ultramafic
Peridotite
10
15
20
20
25
50
95
Composition of Igneous Rocks
Classification of Igneous Rocks
Figure 2-1a. 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.
(a)
Classification of
Phaneritic
Igneous Rocks
Plutonic rocks
The rock must contain a total of
at least 10% of the minerals below.
Renormalize to 100%
Q
Quartzolite
90
90
Quartz-rich
Granitoid
60
60
Granodiorite
Granite
Alkali Fs.
Quartz Syenite
Alkali Fs.
Syenite
20
20
Quartz
Monzonite
Quartz
Syenite
5
10
A
Syenite
(Foid)-bearing
Syenite
35
Monzonite
(Foid)-bearing
Monzonite
Quartz
Monzodiorite
65
Monzodiorite
(Foid)-bearing
Monzodiorite
10
(Foid)-bearing
Alkali Fs. Syenite
(Foid)
Monzosyenite
Figure 2-2. A classification of the phaneritic igneous
rocks. a. Phaneritic rocks with more than 10% (quartz +
feldspar + feldspathoids). After IUGS.
(Foid)
Monzodiorite
60
60
(Foid)olites
F
Qtz. Diorite/
Qtz. Gabbro
5 Diorite/Gabbro/
90
Anorthosite
P
10 (Foid)-bearing
Diorite/Gabbro
Classification of Igneous Rocks
Plagioclase
Anorthosite
Figure 2-2. A classification of the phaneritic
igneous rocks. b. Gabbroic rocks. c. Ultramafic
rocks. After IUGS.
90
lite
cto
Tro
Ga
bb
ro
Gabbroic
rocks
Olivine
gabbro
Olivine
Ultramafic
rocks
Dunite
90
Peridotites
Plagioclase-bearing ultramafic rocks
Pyroxene
Lherzolite
Olivine
(b)
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
(c)
10
Orthopyroxene
Websterite
Clinopyroxenite
Clinopyroxene
Q
Classification of
Aphanitic
Igneous Rocks
60
Volcanic rocks
60
Rhyolite
Dacite
20
20
Trachyte
Latite
35
A
10
(foid)-bearing
Trachyte
Andesite/Basalt
65
(foid)-bearing
Latite
Phonolite
(foid)-bearing
Andesite/Basalt
10
Tephrite
Figure 2-3. A classification and nomenclature
of volcanic rocks. After IUGS.
60
60
(Foid)ites
F
P
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/~vcgeo005
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…

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