Basis for Classification

• Minerals present in the rock Rock Classification – Best for coarse-grained rocks – Useful for field work Chapter 2, pp. 17-26 • Chemical Composition – Works for fine-grained rock – Expensive and takes time

Mineralogical Classification Monomineralic Rocks

• Color Index = % of dark minerals • Plagioclase Anorthosite • Felsic < 35% mafic minerals • Olivine Dunite • Mafic = 35% – 90% mafic minerals • Augite Clinopyroxenite • Ultramafic > 90% mafic mineral • Hypersthene Orthopyroxenite

QAPF Chemical Classification Diagram • CIPW norm • Useful for most – Calculated minerals from Common rocks chemical analysis • Saturation concept – Si saturation • Recalculate the • Acid to basic minerals to – Al saturation 100% QAP or • Harker-Peacock index FAP – Alkalies vs calcium

1 Silica Saturation Aluminum Saturation

Acid SiO2 > 66 % Based on the feldspar ratio 1:1:3 (NaAlSi3O8) Intermediate SiO2 52 to 66 %

Basic SiO2 45 to 52 % Peraluminous Al2O3 > (CaO + Na2O + K2O)

Ultrabasic SiO2 < 52 % Peralkaline (Na2O + K2O) > Al2O3

Classification 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 Figure 2-1b. Method #2 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall. diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

Feldspar Classification Pyroxene Classification

2 Classification of Igneous Rocks Basalt Plagioclase Basalt Anorthosite Figure 2-2. A classification of the phaneritic 90 igneous rocks. b. Gabbroic rocks. c. Ultramafic Classification rocks. After IUGS.

o T Phases: r r b o c b t Olivine a o l G it Olivine e Dunite Augite (Cpx) gabbro 90 Hypersthene (Hy) Olivine (Oliv) W Peridotites e e t h i r g l it Plagioclase (Plag) r e Plagioclase-bearing ultramafic rocks u b Lherzolite z r a Nepheline (Ne) Pyroxene Olivine H (b) 40

Olivine Websterite Orthopyroxenite Pyroxenites 10

Websterite (c) 10 Clinopyroxenite Orthopyroxene Clinopyroxene

Q (a) The rock must contain a total of Q at least 10% of the minerals below. Quartzolite Renormalize to 100% 90 90 Classification of Quartz-rich Granitoid

60 60 Igneous Rocks 60 60

e Classification of it n T a r o G n

r a Grano- l a Granite it p e s diorite Rhyolite Dacite ld e Igneous Rocks F li a lk Alkali Fs. A 20 20 Qtz. Diorite/ 20 20 Quartz Syenite Quartz Quartz Quartz Qtz. Gabbro Alkali Fs. Syenite Monzonite Monzodiorite 5 Trachyte Latite Andesite/Basalt Syenite 5 Diorite/Gabbro/ 10Syenite 35 Monzonite 65 Monzodiorite 90 Anorthosite 35 65 A (Foid)-bearing (Foid)-bearing (Foid)-bearing P A (foid)-bearing (foid)-bearing (foid)-bearing P Syenite Monzonite Monzodiorite Trachyte Latite Andesite/Basalt 10 10 10 (Foid)-bearing 10 Diorite/Gabbro ( (Foid)-bearing F o Alkali Fs. Syenite id ) o S r y (Foid) (Foid) b e b Phonolite Tephrite n Monzosyenite Monzodiorite a i G t e ) id Figure 2-3. A classification and nomenclature o F ( of volcanic rocks. After IUGS.

60 Figure 2-2. A classification of the phaneritic igneous 60 60 60 rocks. a. Phaneritic rocks with more than 10% (quartz + (Foid)olites (Foid)ites feldspar + feldspathoids). After IUGS.

F F

Basalt Discriminator Basalt Trends

3 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.

Si Activity α • SiO2 • Based on s set of reactions • Limits three main magma series

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