GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 2
Classification of Igneous Rock
or
How many ways can you skin a rock?
Jan. 23, 2009
GOALS OF A ROCK CLASSIFICATION SCHEME
Practical – especially for field use
 Descriptive – minimal genetic terms
 Systematic – allows for easy information retrieval
 Hierarchical – allows for greater levels of detail
 Natural – defines boundaries that agree with nature

Most common schemes based on mode (vol. %) of
“essential” primary minerals ...
(When observation of primary minerals is not possible (e.g.,
too fine, altered) chemistry is commonly used)
...with modification by texture and structural features
and occurrence of “accessory” and minor minerals
“ESSENTIAL” MINERALS
Basic
Spinel
LIGHT MINERALS
•Leucocratic
•Felsic Minerals
Synonymous
Terms
Synonymous –
Terms
DARK MINERALS
•Melanocratic
•Mafic Minerals
Color Index-CI
=% Dark Minerals
Acid
A SIMPLE (BUT LIMITING)
CLASSIFICATION SCHEME
CLASSIFICATION OF PHANERITIC MAFIC IGNEOUS
ROCKS RECOMMENDED BY THE IUGS*
gabbro
troctolite
Ol gabbro
*International Union of Geological Sciences
PLOTTING ON TERNARY DIAGRAMS
Rock A
Mineral X – 59.5%
Mineral Y – 17%
Mineral Z – 8.5%
Mineral A – 12%
Mineral B – 3%
Normalization to XYZ only
X+Y+Z = 85%
Xn = 59.5/85 = 70%
Yn = 17/85 = 20%
Zn = 8.5/85 =10%
CLASSIFICATION OF ULTRAMAFIC IGNEOUS
ROCKS RECOMMENDED BY IUGS
Olivine
Dunite
90
Peridotites
Lherzolite
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
Websterite
10
Orthopyroxene
Clinopyroxenite
Clinopyroxene
MAFIC ROCK CLASSIFICATION SCHEMES
APPLIED TO THE DULUTH COMPLEX
Streckeisen (1976)
Phinney (1972a)
LeMaitre (1989)
Davidson (1969a)
Severson and Hauck (1990)
ELEMENTS OF A PRACTICAL CLASSIFICATION
SCHEME FOR DULUTH COMPLEX MAFIC ROCKS
- uses all five major essential mineral phases (Pl, Ol, Cpx, Opx, and
Feox)
- defines modal boundaries that bracket natural modal populations
and cotectic proportions determined from experimental data
- uses simple mafic mineral ratios (3:1 or 1:1) which are easy to
estimate in the field.
From Miller (1986)
PREFERRED MODAL CLASSIFICATION
MODIFIERS – FOR A COMPLETE ROCK
DESCRIPTION
Alteration
Foliation/Layering
Absolute Grain Size
Bulk Rock Texture –based on pyroxene habit
Accessory/Minor Minerals
For example:
Serpentinized, modally layered, medium-grained,
subophitic, biotitic, oxide-bearing TROCTOLITE
Why PYROXENE-BASED BULK-ROCK TEXTURES?
Ophitic-Subophitic
Ophitic
Subophitic
Intergranular
Pl
Ol
CLASSIFICATION OF INTERMEDIATE
AND FELSIC ROCKS
Intermediate
(“ferro-”)
Felsic
Root
Rock
Name
From Streckeisen
(1976)
Attach mafic mineral
prefix
to root rock name for
intermediate rocks...
or “ferro” if uncertain of
mafic mineral phase
THE MAFIC TO INTERMEDIATE TRANSITION



Typically based on average An content (> or < An50)
Hyndman (1972) suggests also considering:
 rock associations (diorite with more granitoid rocks, gabbro with
more mafic rocks)
 mafic mineral assemblage (diorite = hornblende or biotite 
pyroxene; gabbro = pyroxene  olivine  hornblende)
 plagioclase color (diorite whitish or nearly so; gabbro - greenish
gray to gray)
Blatt and Tracy (1995) suggest using diorite for rock that contains
hornblende over pyroxene and that contain less than 35 modal
percent high-temperature mafic minerals (olivine, pyroxene).
Problems
Can’t “see” AN content in hand samples
Even DC felsic rocks are dominated by pyx over hb and bio
Suggested Field Criteria for DC
Intermediate
Dark Mineral Habit
Felsic Mesostasis
prismatic, subprismatic
>5 %
Mafic
granular to poikilitic
<5 %
Q
CLASSIFICATION OF
VOLCANIC ROCKS
RECOMMENDED BY
THE IUGS
60
60
Rhyolite
Dacite
20
20
Basalt/Andesite Distinctions
Trachyte
SiO2
Basalt
< 52%
Andesite
> 52%
CI
> 35%
< 35%
Foid Minerals
Nepheline
Leucite
Kalsilite
Analcime
Sodalite
Latite
35
A
10
(foid)-bearing
Trachyte
Andesite/Basalt
65
(foid)-bearing
Latite
Phonolite
(foid)-bearing
Andesite/Basalt
10
Tephrite
60
60
(Foid)ites
F
P
CHEMISTRY-BASED TAS CLASSIFICATION OF
APHANITIC OR METAMORPHOSED IGNEOUS ROCKS
CIPW
NORMATIVE
CALCULATIONS
PSEUDOMINERALOGY
Calculated from
whole rock
geochemical
analyses;
distributes major
elements among
rock-forming
minerals
CLASSIFICATION OF PYROCLASTIC ROCKS