Lecture 2 Classification of Igneous Rocks

Friday, January 28th, 2005

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.

1 Classification of Igneous Rocks

Ratio of Y to Z = 100Y(Y + Z) = 100*20(20 + 10) = 67

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.

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

Quartz-rich Granitoid Classification of 60 60 Tonalite

Granite Grano- Igneous Rocks diorite

Alkali Feldspar Granite Alkali Fs. 20 20 Qtz. Diorite/ Quartz Syenite Quartz Quartz Quartz Qtz. Gabbro Foids are feldspathoids Alkali Fs. Syenite Monzonite Monzodiorite 5 5 Diorite/Gabbro/ Syenite Syenite Monzodiorite such as nepheline and 10 35 Monzonite 65 90 Anorthosite A (Foid)-bearing (Foid)-bearing (Foid)-bearing P leucite Syenite Monzonite Monzodiorite 10 10 (Foid)-bearing Diorite/Gabbro (Foid)-bearing (Foid) Syenite Alkali Fs. Syenite (Foid) (Foid) Monzosyenite Monzodiorite

(Foid) Gabbro

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

F

2 Let’s try an example

Modal analysis of a rock gives the following mineral proportions Quartz = 26.9 K-feldspar = 23.7 Plagioclase = 39.8 Biotite = 5.3 Hornblende = 4.3 Re-summing Qtz = 29.8 Kspar = 26.2 Plag = 44.0

100P/(P + A) = 63 Rock is a granite!

How about another? Modal analysis of a rock gives the following mineral proportions Quartz = 8.5 K-feldspar = 5.1 Plagioclase = 59.6 Biotite = 16.1 Hornblende = 10.9 Re-summing Qtz = 11.6 Kspar = 7.0 Plag = 81.4

100P/(P + A) = 92 Rock could be Qtz diorite or Qtz gabbro

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

Troctolite Olivine

Gabbro Olivine Dunite gabbro 90

Wehrlite Peridotites

Plagioclase-bearing ultramafic rocks Lherzolite Pyroxene Olivine Harzburgite (b) 40

Orthopyroxenite Olivine Websterite Pyroxenites 10

Websterite (c) 10 Clinopyroxenite Orthopyroxene Clinopyroxene

Q Classification of

Igneous Rocks 60 60

Rhyolite Dacite

20 20

Trachyte Latite Andesite/Basalt 35 65 A (foid)-bearing (foid)-bearing (foid)-bearing P Trachyte Latite Andesite/Basalt 10 10

Phonolite Tephrite Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS.

60 60

(Foid)ites

F

4 Classification of Igneous Rocks

Alkaline -alkaline or calc Tholeiitic

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.

An actual example from the Hawaii Scientific Drilling Project

The dashed line separates alkalic from tholeiitic basalts

Total alkali = SiO2 x 0.37 + 14.43

5 We can calculate an alkalinity index from the previous equation. It measures how far a sample plots above or below the alkali – tholeiite line. In other words, how alkaline or tholeiitic it is.

Alkalinity Index = (Na2O + K2O) – (SiO2 x 0.37 + 14.43)

This example shows the alkalinity index with depth in rocks from the Hawaii Scientific Drilling Project

Classification of Igneous 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.

6 Some Useful Textural Terms

● Grain Size – determined by cooling rate of a magma (also influenced by volatile content). Fine < 1 mm Medium 1 – 5 mm Course > 5 mm

● Aphanitic – a rock that is so fine-grained that individual minerals cannot be distinguished without a microscope.

● Phaneritic – all essential minerals are clearly visible

● Porphyritic – a rock containing large phenocrysts in a finer grained groundmass

● Grain Shapes euhedral – grains with well-developed crystal faces subhedral – grains with a few well-developed crystal faces anhedral – grains that have no distinct shape

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