Igneous
Petrology
PETROGRAPHY
The description and systematic classification of rocks, aided by
the microscopic examination of thin sections.
PETROLOGY
The study of the origin, occurrence, structure and history of rocks,
much broader process/study than petrography.
PETROGENESIS
A branch of petrology dealing with the origin and formation of
rocks. Involves a combination of mineralogical, chemical and
field data.
Petrologic, petrographic, and petrogenetic studies can be applied
to igneous, metamorphic or sedimentary rocks.
The Earth’s Interior
Crust:
Oceanic crust
Thin: 10 km
Relatively uniform stratigraphy
= ophiolite suite:
•
•
•
•
•
Sediments
pillow basalt
sheeted dikes
more massive gabbro
ultramafic (mantle)
Continental Crust
Thicker: 20-90 km average ~35 km
Highly variable composition
– Average ~ granodiorite
The Earth’s Interior
Mantle:
Peridotite (ultramafic)
Upper to 410 km (olivine  spinel)
 Low Velocity Layer 60-220 km
Transition Zone as velocity increases ~ rapidly
 660 spinel  perovskite-type

SiIV  SiVI
Lower Mantle has more gradual
velocity increase
Figure 1-2. Major subdivisions of the Earth.
Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
The Earth’s Interior
Core:
Fe-Ni metallic alloy
Outer Core is liquid

No S-waves
Inner Core is solid
Figure 1-2. Major subdivisions of the Earth.
Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
Figure 1-3. Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left,
rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford.
NOMENCLATURE AND CLASSIFICATION
-Formation of minerals in an igneous rocks is controlled by
the chemical composition of the magma and the physicalchemical conditions present during crystallization.
- Mineralogical composition and texture are used to describe,
name and classify rocks.
- Both overall chemistry ( whole-rock chemistry) and the
chemistry of constitute minerals offer clues to igneous rock
origins.
- Studies of rock chemistry reveal where magmas form and
how they are modified before they solidify.
- the problem in rock classification is the selection of a basis
for classification.
- proposed classifications use texture, mineralogy, chemistry,
geographic location and rock associations.
-Systems of nomenclature and classification may reflect:
genetic, textural, chemical or mineralogical features.
GENETIC
basic system which classifies rocks on the basis of where
they form.
plutonic - at depth
hypabyssal - intermediate depth
volcanic - on the Earth's surface.
This system is not very practical, but it serves as a first
approximation, it tells nothing about mineralogy, chemistry of
the rocks and can not distinguish basalt from rhyolite.
-TEXTURAL
relies on the grain size of individual minerals in the rock.
aphanitic - fine grained < 1 mm
phaneritic - medium grained 1 to 5 mm
coarse grained (pegmatitic) > 5 mm
This system has the same shortcomings as a genetic
classification, however specific textures present may aid in
classification, e.g., phenocryst, ophitic, coronas, but these are
not indicative of a specific environment of formation or a
specific lithology.
-CHEMICAL
This type of classification requires a complete
chemical analysis of the rock
A chemical classification system has been
proposed for volcanic rocks and a comparable
scheme for plutonic rocks is not available.
This leaves us with a system based on mineralogy.
MINERALOGICAL
The one gaining application is the result of several
years work by the IUGS Subcommission on the
Classification of Igneous Rocks or Streckeissen
Classification.
CLASSIFICATION SYSTEMS
Several aspects which historically have played and continue to
play a role in the classification of igneous rocks should also be
considered.
GRADATION IN SILICA CONTENT
- referred to as acid or basic, implying a range of silica content.
Acidic > 66 wt% SiO2
Granites ~ 72 wt% SiO2, granodiorites ~ 68 wt% SiO2
Intermediate - 52 to 66 wt% SiO2
Andesite 57 wt% SiO2
Basic - 45 to 52 Wt% SiO2
Basalts range from 48 to 50 wt%
Ultrabasic - < 45 wt% SiO2
peridotites 41 to 42 wt% SiO2
COLOUR GRADATION
Felsic rocks are light coloured, contain felsic
minerals (e.g. qtz, feldspar, feldspathoids) which are
themselves light in colour and have a low density
which contribute to the pale colour of the rock.
Mafic Rocks are denser and dark coloured, the
result of containing mafic minerals (pyroxene,
amphibole, olivine, biotite). These minerals
contribute to the green, brown and black colour of
these rocks.
Chemistry of Igneous rocks
-Modern chemical analyses of igneous rocks
generally include a major elements analyses and
minor or trace elements analyses.
- Earth is composed almost entirely of 15 elements,
12 of which are the dominant elements of the crust.
- The crustal elements, considered to be the major
elements, in order of decreasing abundance, are O,
Si, Al, Fe, Ca, Na, Mg, K, Ti, H, P and Mn.
Composition of Earth shells
Elements wt%
Crust
Mantle
Core
Continental
Oceanic
Upper
Lower
O
41.2
43.7
44.7
43.7
Si
28
22
21.1
22.5
Al
14.3
7.5
1.9
1.6
Fe
4.7
8.5
5.6
9.8
Ca
3.9
7.1
1.4
1.7
K
2.3
0.33
0.08
0.11
Na
2.2
1.6
0.15
0.84
Mg
1.9
7.6
24.7
18.8
Ti
0.4
1.1
0.12
0.08
C
0.3
H
0.2
Mn
0.07
0.15
0.07
0.33
Ni
Cr
0.51
Outer
Inner
10--15
80--85
80
5
20
The chemical composition of rocks is determined by analyzing a powder
of the rock.
Routine geochemical analysis of geologic materials can be
carried out using either or a combination of the following two
techiques:
X-ray Fluoresence Spectroscopy (XRF) to determine both major
and trace elements
Atomic Absorbtion Spectrometry (AAS) to determine both major
and trace elements
The composition of an igneous rock is dependant on:
Composition of the source material
Depth of melting
Tectonic environment where crystallization occurs. e.g. rifting vs.
subduction
Secondary alteration
These are the 13 major oxide
components which are reported as
weight percent (wt%).
Because these are reported as a
percentage the total should sum to
100 %, ideally, however acceptable
totals lie in the range 98.5 to 101
wt%.
Rare Earth Elements (REE or
lathanides, atomic number
57 to 71), are reported in
ppm or mg/g. The REE are
important for petrogenetic
studies, because as a
group the REE behave
coherently.
SATURATION CONCEPT
Used in reference to the SiO2 and Al2O3 which are the two most abundant
components of igneous rocks.
SiO2 Saturation
SiO2 Saturation
Minerals present in igneous rocks can be divided into two groups:
Those which are compatible with quartz or primary SiO2 mineral (tridymite,
cristobalite) these minerals are saturated with respect to Si, e.g feldspars,
pyroxenes.
Those which never occur with a primary silica mineral. These are
undersaturated minerals, e.g. Mg-rich olivine, nepheline.
The occurrence of quartz with an undersaturated mineral causes a reaction
between the two minerals to form a saturated mineral.
2SiO2 + NaAlSiO4 ===> NaAlSi3O8
Qtz + Ne ===> Albite
SiO2 + Mg2SiO4 ===> 2MgSiO3
Qtz + Ol ===> En
Rock Classification (Silica saturation)
Oversaturated - contains primary silica mineral
Saturated - contains neither quartz nor an unsaturated
mineral
Unsaturated - contains unsaturated minerals
Al2O3 Saturation
Four subdivisions of rocks independant of silica saturation,
based on the molecular proportions of Al2O3, Na2O, K2O
and CaO applied mainly to granitic lithologies.
Peraluminous - Al2O3 > (Na2O + K2O + CaO)
Metaluminous - Al2O3 < (Na2O + K2O + CaO) but Al2O3 >
(Na2O + K2O)
Subaluminous - Al2O3 = (Na2O + K2O)
Peralkaline - Al2O3 < (Na2O + K2O)
VARIATION DIAGRAMS
A main objective of any research program
on igneous rocks is to describe and
display chemical variations for simplicity
and to facilitate condensing information.
The best way to simplify and condense
analytical data is by graphical means.
Harker Diagrams
The oldest method is the variation diagram or Harker diagram which dates
from 1909, and plots oxides of elements against SiO2.
Bivariate (x-y) diagrams
22
Oxides ( K2O, Na2O,
CaO,MgO, Al2O3) plotted Al2O3
17
against Silica (SiO2) form
linear arrays.
A set of such plots is called a
Harker diagram.
12
10
MgO
5
0
15
FeO* 10
With increasing Silica the
following trends are
evident:
FeO, MgO and CaO decrease
in abundance.
10
5
CaO
5
0
0
4
6
K2O and Na2O increase.
Al2O3 does not exhibit a
strong variation.
3
Na2O
4
2
2
0
45
1
50
55
60
SiO2
65
70
75 45
50
55
60
SiO2
65
70
0
75
K2O
wt %
ppm
Note
magnitude
of trace
element
changes
ppm
Trace Elements
Figure 9-1. Harker Diagram for Crater Lake. From data
compiled by Rick Conrey. From Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice Hall.
Triangular Variation Diagrams
Triangular Variation Diagrams
These diagrams visually present the variation in 3
chemical parameters. Two are commonly used:
AFM - Mainly for Mafic Rocks
A = Na2O + K2O
F = FeO (+Fe2O3)
M = MgO
Plotted as either molecular or weight percent values.
Na2O - K2O - CaO - Mainly for Felsic Rocks
Uses either the molecular or weight percent values for the
three oxides listed.
Data may be plotted as weight percent oxide or atomic
percent of the cations. The disadvantage to this is that
the absolute values of the analyses are not readliy
determined.
Table 9-6 A brief summary of some particularly useful trace elements in igneous petrology
Element
Use as a petrogenetic indicator
Ni, Co, Cr Highly compatible elements. Ni (and Co) are concentrated in olivine, and Cr in spinel and
clinopyroxene. High concentrations indicate a mantle source.
V, Ti
Both show strong fractionation into Fe-Ti oxides (ilmenite or titanomagnetite). If they behave
differently, Ti probably fractionates into an accessory phase, such as sphene or rutile.
Zr, Hf
Very incompatible elements that do not substitute into major silicate phases (although they may
replace Ti in sphene or rutile).
Ba, Rb
Incompatible element that substitutes for K in K-feldspar, micas, or hornblende. Rb substitutes
less readily in hornblende than K-spar and micas, such that the K/Ba ratio may distinguish these
phases.
Sr
Substitutes for Ca in plagioclase (but not in pyroxene), and, to a lesser extent, for K in Kfeldspar. Behaves as a compatible element at low pressure where plagioclase forms early, but
as an incompatible at higher pressure where plagioclase is no longer stable.
REE
Garnet accommodates the HREE more than the LREE, and orthopyroxene and hornblende do
so to a lesser degree. Sphene and plagioclase accommodates more LREE. Eu 2+ is strongly
partitioned into plagioclase.
Y
Commonly incompatible (like HREE). Strongly partitioned into garnet and amphibole. Sphene
and apatite also concentrate Y, so the presence of these as accessories could have a
significant effect.
Table 9-6. After Green (1980). Tectonophys., 63, 367-385. From Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
Trace elements as a tool to
determine paleotectonic
environment
• Useful for rocks in mobile belts that are no
longer recognizably in their original setting
• Can trace elements be discriminators of
igneous environment?
• Approach is empirical on modern
occurrences
• Concentrate on elements that are
immobile during low/medium grade
Table 18-4. A
Classification of
Granitoid Rocks Based
on Tectonic Setting.
After Pitcher (1983) in
K. J. Hsü (ed.),
Mountain Building
Processes, Academic
Press, London; Pitcher
(1993), The Nature and
Origin of Granite,
Blackie, London; and
Barbarin (1990) Geol.
Journal, 25, 227-238.
Winter (2001) An
Introduction to Igneous
and Metamorphic
Petrology. Prentice Hall.
SiO2 is generally chosen because it is the most
abundant oxide in igneous rocks and exhibits a wide
variation in composition. This type of graphical
presentation is useful for large quantities of analytical
data and yields an approximation of inter-element
variations for a group of samples.
No genetic link can be inferred from Harker diagrams,
i.e. that the lowest SiO2 content present on the
diagram represents the original or first liquid, for the
group of samples presented, from which all other
liquids were derived.
• Harker diagrams: SiO2 vs. oxide.
• The meaning of geochemical trends: can be
interpreted as magmatic
“evolution” from “primitive” to “differenciated”
rocks. More or less implicitly
assumes fractional crystallization.
• The nature of the phases crystallizing can be
inferred from the shape of the
trends. Ex.: decreasing Fe, Mg = precipitation
of mafic minerals.
Magmatic series: reflect first order differences between rock
groups.
• TAS diagram separates alkali and sub-alkali series
• Sub-alkali series are further separated on the basis of their
Fe-Mg contens
(AFM diagram) into tholeitic and calc-alkaline
In addition, important role of the relative proportions of Al2O3
and CaO-Na2O-K2O
• A>CNK: Peraluminous rocks. Have Al-rich minerals such as
biotite,
muscovite, garnet, cordierite…
• A<CNK:
o .. and A>NK: Metaluminous. No particular minerals, mafics
are
pyroxene, amphibole, biotite
o .. and A<NK: peralkaline rocks. Alklai-rich minerals such as
alkali
amphiboles and pyroxenes.
Alkali vs. Silica diagram for Hawaiian volcanics:
Seems to be two distinct groupings: alkaline and subalkaline
12
10
Alkaline
8
6
4
2
Subalkaline
35
40
45
50
%SiO
55
60
65
AFM diagram: can further subdivide the subalkaline
magma series into a tholeiitic and a calc-alkaline series
Figure 8-14. AFM diagram showing the distinction
between selected tholeiitic rocks from Iceland, the MidAtlantic Ridge, the Columbia River Basalts, and Hawaii
(solid circles) plus the calc-alkaline rocks of the Cascade
volcanics (open circles). From Irving and Baragar (1971).
After Irvine and Baragar (1971). Can. J. Earth Sci., 8,
523-548.
1. Tholeitic series
Fe-rich, alkali poor.
Metaluminous
Px/Hb/Bt-bearing basalts, andesites, dacites, rhyolites (BADR)
Tholeitic series are common in oceanic ridges, intraplate-volcanoes ± convergent
margins. They correspond to melting by decrease of pressure.
2. Calc-alkaline series
Moderately alkaline, more magnesian
Metaluminous to peraluminous
BADR, that can feature ms/gt/cd in the more differenciated terms
Calc-alkaline series are mostly found in convergent margins. They correspond to
melting by adding water to the source (and therefore “shifting” the solidus towards
lower temperatures).
3. Alkaline series
Alkali rich, Fe-rich
Metaluminous to peralkaline
Evolution towards trachytes (moderaltely alkaline series) or phonolites (very
alkaline series), that can feature riebeckite, aegyrine, etc.
Alkaline series are found in intra-plate situations ± convergent margins. They
correspond to melting by increase of temperature.
Fractionation Indices
To obtain a genetic link between
analyses of a given suite of samples
fractionation indices were developed.
These indices attempt to the results of
chemical analyses from an individual
igneous suite into their correct
evolutionary order. These indices are not
realistic but several come close to such
an order.
MgO Index
This is used for basaltic rocks. Positive correlations are
produced for Na2O, K2O, and P2O5 indicating enrichment
in these oxides with successive liquids. Negative
correlations result for CaO.
Mg-Fe Ratios
Again used for basaltic rocks. These involve a ratio of Mg to Fe:
MgO/MgO+FeO (ferrous)
MgO/MgO+FeO+Fe2O3 (ferric)
Mg/Mg+Fe (uses atomic proportions of the cations).
Normative Ab/Ab+An
Based on the values of Na2O and CaO. Only good for rocks
which crystallize plagioclase, not effected by mafic mineral
formation. Generally applied to granites.
The above three indices are only good for specific lithologies,
and thus have a restricted application.
Two fractionation indices, based on complex equations have
been suggested for more comprehensive use.
Solidification Index (Kuno, 1959)
SI = 100 MgO/(MgO+FeO+Fe2O3+Na2O+K2O)
For basalts this is similar to Mg/Fe ratios due to the relatively
poor alkali content. As fractionation progresses the
residual liquids become enriched in alkaliis, thus Na2O
and K2O contents offset the Mg-Fe index. For mafic rocks
SI is high, for felsic rocks SI is low.
Differentiation Index (Thornton and tuttle, 1960)
DI = normative Q+Or+Ab+Ne+Ks+Lc
This is based on the normative analyis results. For mafic
rocks DI will be low, because in normative calculation
these minerals are minor. Felsic rocks DI will be high
because these minerals are abundant in the norm.
Alkali vs. Silica diagram for Hawaiian volcanics:
Seems to be two distinct groupings: alkaline and subalkaline
12
10
Alkaline
8
6
4
2
Subalkaline
35
40
45
50
%SiO
55
60
65
AFM diagram: can further subdivide the subalkaline
magma series into a tholeiitic and a calc-alkaline series
Figure 8-14. AFM diagram showing the distinction
between selected tholeiitic rocks from Iceland, the MidAtlantic Ridge, the Columbia River Basalts, and Hawaii
(solid circles) plus the calc-alkaline rocks of the Cascade
volcanics (open circles). From Irving and Baragar (1971).
After Irvine and Baragar (1971). Can. J. Earth Sci., 8,
523-548.
Tholeiitic
B-A
A
D
R
Calc-alkaline
biotite
muscovite
cordierite
andalusite
garnet
pyroxene
hornblende
biotite
aegirine
riebeckite
arfvedsonite
CaO
CaO
moles
CaO
K2O
K2O
Al2O3
K2O
Na2O
Peraluminous
Al2O3
Al2O3
Na2O
Metaluminous
Na2O
Peralkaline
Figure 18-2. Alumina saturation classes based on the molar proportions of Al2O3/(CaO+Na2O+K2O) (“A/CNK”) after
Shand (1927). Common non-quartzo-feldspathic minerals for each type are included. After Clarke (1992). Granitoid
Rocks. Chapman Hall.
Alkaline
Calc-alkaline
Tholeitic
Series
Alkaline
Subalkaline
Calcalkaline
Tholeitic
Alkali
content
High
Fe-Mg
Al
Fe-rich
Metaluminous
to peralkaline
Low to
moderate
Mg-rich
Metaluminous
to peraluminous
Low
Fe-rich
Metaluminous
A world-wide survey suggests that there may be
some important differences between the three series
Characteristic
Plate Margin
Series
Convergent Divergent
Alkaline
yes
Tholeiitic
yes
yes
Calc-alkaline
yes
Within Plate
Oceanic Continental
yes
yes
yes
yes
After Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer
- B. Normalization and spidergrams
1. What is “normalization”, and why do it?
Abundance of elements varies greatly in the Earth:
• Different families of elements are more or less present
• Even within a family, nucleosynthesis results in huge variations
2. Spidergrams
Spidergrams allow to
• See many elements at a time
• Compare elements with large differences of absolute abundance (log scale!)
• To some degree, make petrogenetic interpretations
Making a spidergram
• For each sample, arrange elements in order of increasing compatibility (i.e.,
the more incompatible at the left). (technically, this implies a different order
for each different source!).
• Plot the normalized value of each elements (log scale!)
• Link the dots
• Look at the “anomalies”!
Some classical spidergrams:
• REE diagrams (n’ed to chondrites or PRIMA=PRImitive MAntle in general)
• Multi-element diagrams for incompatible elements (N’ed to
PRIMA/chondrites, or to MORBs)
• PGE diagrams
• Transition metal diagrams
MODAL ANALYSIS
Two types of analysis are useful when examining Igneous Rocks:
Modal analysis - requires only a thin section,
Normative analysis - requires a chemical analysis.
MODAL ANALYSIS
Produces an accurate representation of the distribution and volume
percent of the mineral within a thin section. Three methods of
analysis are used:
Measure the surface area of mineral grains of the same mineral,
relative to the total surface area of the thin section.
Measure the intercepts of each mineral along a series of lines.
POINT COUNT - Count each mineral occurrence along a series of
traverse line across a given thin section. For a statistically valid
result > 2000 individual points must be counted.
The number of grains counted, the spacing between points and
successive traverse lines is dependant on the mean grain size of
the sample.
Advantages
One can compare rocks from different areas if you
only have a thin section, no chemical analysis is
required, using a petrographic microscope.
Gives the maximum and minimum grain sizes.
Disadvantages
Meaningless if the sample has a preferred orientation
of one or more minerals.
Porphyritic rocks are difficult to count.
Total area of sample must be sufficiently larger than
the max. diameter of the smallest grain size.
NORMATIVE ANALYSIS OR NORM
Normative analysis is defined as the calculation of a theoretical assemblage
of standard minerals for a rock based, on the whole rock chemical
composition as determined by analytical techniques. The original
purpose for the norm was essentially taxonomic. An elaborate
classification scheme based on the normative mineral percentages was
proposed. The classification groups together rocks of similar bulk
composition irrespective of their mineralogy. Various types of NORMs
have been proposed - CIPW, Niggli, Barth. Each of theses proposals has
its own specific advantages and/or disadvantages.
The CIPW norm, originally proposed in 1919, was proposed as a means of
comparing and classifying all igneosu rocks for which chemical analyses
wers available. The NORM takes it's name from the four authors who
proposed it - Cross, Iddings, Pirsson and Washington. This NORM was
very elegant and based on a number of simplifications:
The magma crystallizes under anhydrous conditions so that no hydrous
minerals (hornblende, biotite) are formed.
The feromagnesium minerals are assumed to be free of Al2O3.
The Fe/Mg ratio for all feromagnesium minerals is assumed to be the same.
Several minerals are assumed to be incompatible, thus nepheline and/or
olivine never appear with quartz in the norm.
Since the CIPW NORM was introduced in 1919 several other normative
calculations have been suggested, e.g. Niggli norm, Barth mesonorm.
The latter is used commonly when examining granitic rocks.
Plate Tectonic - Igneous
Genesis
1. Mid-ocean Ridges
2. Intracontinental
Rifts
3. Island Arcs
4. Active Continental
Margins
5. Back-arc Basins
6. Ocean Island Basalts
7. Miscellaneous IntraContinental Activity

kimberlites, carbonatites,
anorthosites...