2/3/2017

1

Classification of Igneous Rocks

Texture - largely volcanic vs. plutonic

• Volcanic, shallow intrusivemust have significant aphanitic or glassy fraction

• Other intrusivephaneritic

Composition – mineralogy, bulk chemistry (color)

2

Fig 4.8 Understanding Earth

Composition

Felsic Intermediate Mafic Ultramafic

Extrusive (glassy, aphanitic, porphyritic, fragmental, vesicular, etc.)

Rhyolite Dacite Andesite Basalt Komatiite (rare)

Te

xtu

re

Intrusive (phaneritic, pegmatitic)

Granite Granodiorite Diorite Gabbro Peridotite

Simplified classification

more silica; less iron & magnesium

less silica; more iron & magnesium

lighter darker

3 Major MineralsSimplified View

More silica (SiO2), sodium, potassium

More iron, magnesium, calcium

Lighter color Darker color

mod

ifie

d fr

om F

ig 4

.6 U

nder

stan

ding

Ear

th

Rhyolite or Granite

Dacite or Granodiorite

Andesite or Diorite

Basalt or Gabbro

Komatiite or Peridotite

4

Textural Classification of Pyroclastic Rocks

Figure 2.5. Classification of the pyroclastic rocks. After Fisher (1966) Earth Sci. Rev., 1, 287-298.

5

Note: The compositional classification still applies and is typically used in

conjunction with the textural classification.

Ternary Diagrams

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.

6

2/3/2017

2

Figure 2.2a. A classification of the phaneritic igneous rocks: Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS.

Figure 2.3. A classification and nomenclature of volcanic rocks. After IUGS.

IUGS Classifications7

Phaneritic Volcanic

Figure 2.2a. A classification of the phaneritic igneous rocks: Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS.

8

Phaneritic Classification

Further sub-divided

on later slides

9Phaneritic Classification Quartz-bearing10

Phaneritic Classification

Feldspathoid-bearing

5%

Mafic & Ultramafic, Phaneritic Rock Classification

Figure 2.2b. A classification of the phaneritic igneous rocks: Gabbroic rocks. After IUGS.

11

Bottom of diagram

further sub-divided on next slide

Figure 2.2c. A classification of the phaneritic igneous rocks: Ultramafic rocks. After IUGS.

12

Mafic & Ultramafic, Phaneritic Rock ClassificationUnderstanding Earth

2/3/2017

3

13

Classification of Volcanic Rocks

Figure 2.3. A classification and nomenclature of volcanic rocks. After IUGS.

14

Classification of Volcanic

Rocks

Figure 2.4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Maitre (2002) . Igneous Rocks: A Classification and Glossary of Terms. Cambridge University Press.

15

Lamprophyres

a group of potassium-rich mafic to ultramafic rocks

16

1996 - Classification of lamprophyres, lamproites, kimberlites, and the kalsilitic, melilitic, and leucitic rockshttp://canmin.geoscienceworld.org/content/34/2/1752005 - Integrating Ultramafic Lamprophyres into the IUGS Classification of Igneous Rocks: Rationale and Implications https://academic.oup.com/petrology/article/46/9/1893/1385496/Integrating-Ultramafic-Lamprophyres-into-the-IUGS

Average compositions (% by weight) and liquidus temperatures of different magmas

Composition: Ultramafic Mafic Intermediate Felsic

Volc. rock name: Komatiite (rare) Basalt Andesite Rhyolite

SiO2 47.9 50.9 58.9 73.7

TiO2 0.4 1.7 0.9 0.2

Al2O3 4.1 14.6 17.1 14.3

FeO 9.7 14.6 6.2 2.1

MgO 27.5 4.8 3.8 0.3

CaO 7.5 8.7 5.3 1.4

Na2O 0.2 3.1 4.3 5.2

K2O 0.02 0.8 1.2 4.1

Temp (°C) ~1600 ~1200 ~1000 ~900

Eruption temperatures can be a little lower than liquidus temperatures.

17 18Excel Plot of Some Bulk Rock Analyses

2/3/2017

4

“Normative” mineral abundances

A standardized, idealized calculation based on a bulk chemical analysis

Uses pure end-member mineral compositions (no solid-solutions)Sometimes used to categorize rock (e.g. “normative olivine”)

CIPW Norm19

http://minerva.union.edu/hollochk/c_petrology/norms.htm

20

Brownlow's Geochemistry