Classification of igneous rocks

1
Classification of igneous rocks

• Chapter 2

2
How to classify

• Composition
• Chemistry
• Mineralogy
• Texture/fabric

3
Classification based on texture/fabric

• Phaneritic
• Minerals large to see with naked eye
• Cooling history?
• Aphanitic
• Minerals need microscope to see
• Porphyritic 2 different grains sizes
• Cooling history?
• Phenocrysts larger grains
• Groundmass or matrix smaller grains

4
Classification based on texture/fabric

• Glassy
• Rocks with glass
• Volcaniclastic
• Minerals and rock fragments that are broken in
  pieces

5
Classification based on field relations

• Intrusive
• Usually phaneritic
• Extrusive
• Aphanitic and glassy
• Many are porphyritic
• Some are volcaniclastic

6
Classification based on mineralogy

• Felsic
• Feldspar and silica
• Light colored rocks
• Mafic
• Magnesium and iron
• Dark colored rocks
• Ultramafic
• Especially rich in Mg and Fe

7
Classification based on mineralogy

• Mode of each mineral in rock
• Must add up to 100
• Quick ID of major minerals
• Choose which diagram to name your rock

8
Phaneritic rock with gt10 felsic minerals
Q
.
Quartzolite
90
90
Quartz-rich
Granitoid
60
60

• QAPF diagram
• Anorthosite gt90 plag
• Gabbro gt35 mafic mineras
• Diorite lt35 mafic minerals

Grano-
Tonalite
Granite
Alkali Feldspar Granite
diorite
Alkali Fs.
Qtz. Diorite/
20
20
Quartz Syenite
Qtz. Gabbro
Quartz
Quartz
Quartz
Monzonite
Syenite
Monzodiorite
Alkali Fs.
5
5
Diorite/Gabbro/
Syenite
Syenite
Monzonite
Monzodiorite
Anorthosite
35
90
10
65
A
P
(Foid)-bearing
(Foid)-bearing
(Foid)-bearing
Syenite
Monzonite
Monzodiorite
(Foid)-bearing
10
10
Diorite/Gabbro
(Foid)-bearing
Alkali Fs.
Syenite
(Foid)
(Foid) Syenite
(Foid)
Monzosyenite
Monzodiorite
(Foid) Gabbro
60
60
(Foid)olites
F
9
Phaneritic rock with gt10 felsic minerals
Q
.
Quartzolite
90
90
Quartz-rich
Granitoid
60
60

• Other names
• Porphyry porphyritic rock
• Aplite uniformly fine-granite phaneritic light
  colored granite (sugary)
• Pegmatite phaneritic with highly variable grain
  size (can be large)

Grano-
Tonalite
Granite
Alkali Feldspar Granite
diorite
Alkali Fs.
Qtz. Diorite/
20
20
Quartz Syenite
Qtz. Gabbro
Quartz
Quartz
Quartz
Monzonite
Syenite
Monzodiorite
Alkali Fs.
5
5
Diorite/Gabbro/
Syenite
Syenite
Monzonite
Monzodiorite
Anorthosite
35
90
10
65
A
P
(Foid)-bearing
(Foid)-bearing
(Foid)-bearing
Syenite
Monzonite
Monzodiorite
(Foid)-bearing
10
10
Diorite/Gabbro
(Foid)-bearing
Alkali Fs.
Syenite
(Foid)
(Foid) Syenite
(Foid)
Monzosyenite
Monzodiorite
(Foid) Gabbro
60
60
(Foid)olites
F
10
Phaneritic mafic and ultramafic rocks
(c)
11
Classification of Igneous Rocks

• Mafic rocks plag, pyroxene, olivine
• Norite gabbro with more opx than cpx
• Top and bottom fields at 10 tick marks
• Side fields at 5 tick marks

(c)
12
Classification of igneous rocks

• Ultramafic rocks opx, cpx, olivine
• If cant tell opx vs. cpx use general names of
  pyroxenite and peridotite

13
Classification of Igneous Rocks

• Aphanitic rocks
• Based from phenocrysts
• Need to normalize A and P
• Not the most definite way to name rock

Figure 2-3. A classification and nomenclature of
volcanic rocks. After IUGS.
14
Classification of 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.
15
Classification of Igneous Rocks

• Rock with 70 X, 20 Y, 10 Z
• Plot mineral composition of triangle diagram

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.
16
Classification of Igneous Rocks

• Figure out of each mineral
• Reduce to 3 minerals of choice and normalize
• In rock
• X9
• Y2.6
• Z1.3

Figure 2-1b. 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.
17
Classification of Igneous Rocks

• In rock
• X9
• Y2.6
• Z1.3
• Of these 3
• X70
• Y20
• Z10
• XYZ
• X/(XYZ)

Figure 2-1b. 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.
18
Classification of Igneous Rocks

• Of these 3
• X70
• Y20
• Z10
• X/(XYZ) 70
• Now normalize Y and Z
• Z/(ZY) and Y/(ZY)
• 20/30 (67) for Y
• 10/30 (33) for Z

Figure 2-1b. 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.
19
Naming aphanitic rocks based on chemistry
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.
20
Rock classification based on chemistry

• Silica concentration principal oxide
• Silicic rocks with SiO2
• gt66 wt silica is felsic
• 52-66 wt silica intermediate
• 45-52 wt silica is mafic
• gt45 wt silica is ultramafic
• Also
• Magnesian, aluminous, alkaline (Na2O and K2O)

21
Naming rocks

• Use Fig. 2.8 and 2.10 to name
• Example 45 quartz, 25 k-spar, 10 plag, 15
  biotite, 5 hornblende
• 56 quartz, 31 k-spar, 13 plag
• 30 plag, 70 k-spar
• Granite
• Hornblende, biotite granite
• Modifying minerals listed in increasing abundance