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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 physical-

chemical 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 Outer Inner

O 41.2 43.7 44.7 43.7 10--15

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 80--85 80

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 5 20

Cr 0.51

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%.

A typical rock analysis

For Major Oxides.

Oxide Wt. %

SiO2 49.20

TiO2 1.84

Al2O3 15.74

Fe2O3 3.79

FeO 7.13

MnO 0.20

MgO 6.73

CaO 9.47

Na2O 2.91

K2O 1.10

H2O+ 0.95

(O)

Total 99.06

Table 8-3. Chemical analyses of some

representative igneous rocks

Peridotite Basalt Andesite Rhyolite Phonolite

SiO2 42.26 49.20 57.94 72.82 56.19

TiO2 0.63 1.84 0.87 0.28 0.62

Al2O3 4.23 15.74 17.02 13.27 19.04

Fe2O3 3.61 3.79 3.27 1.48 2.79

FeO 6.58 7.13 4.04 1.11 2.03

MnO 0.41 0.20 0.14 0.06 0.17

MgO 31.24 6.73 3.33 0.39 1.07

CaO 5.05 9.47 6.79 1.14 2.72

Na2O 0.49 2.91 3.48 3.55 7.79

K2O 0.34 1.10 1.62 4.30 5.24

H2O+ 3.91 0.95 0.83 1.10 1.57

Total 98.75 99.06 99.3 99.50 99.23

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.

12

17

22

Al 2 O 3

0

5

10

MgO

0

5

10 FeO*

0

2

4

6

Na 2 O

0

5

10

15

CaO

45 50 55 60 65 70 75

0

1

2

3

4

K 2 O

SiO 2

45 50 55 60 65 70 75

SiO 2

Bivariate (x-y) diagrams

Oxides ( K2O, Na2O, CaO,MgO, Al2O3) plotted

against Silica (SiO2) form linear arrays.

A set of such plots is called a

Harker diagram.

With increasing Silica the following trends are

evident: FeO, MgO and CaO decrease

in abundance.

K2O and Na2O increase.

Al2O3 does not exhibit a

strong variation.

wt %

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.

Note

magnitude

of trace

element

changes

Trace Elements

ppm

ppm

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.

Ternary Variation Diagrams

Example: AFM

diagram

(alkalis-FeO*-MgO)

FeO + Fe2O3

K2O + Na2O MgO

Triangular variation

diagrams show only

the ratios of various

oxides or elements,

rater than their

actual values.

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 K-

feldspar. 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

metamorphism

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, cordieriteQ

• 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.

12

10

8

6

4

2

35 40 45 50 55 60 65

%SiO2

Alkaline

Subalkaline

Alkali vs. Silica diagram for Hawaiian volcanics:

Seems to be two distinct groupings: alkaline and subalkaline

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 Mid-

Atlantic 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.

12

10

8

6

4

2

35 40 45 50 55 60 65

%SiO2

Alkaline

Subalkaline

Alkali vs. Silica diagram for Hawaiian volcanics:

Seems to be two distinct groupings: alkaline and subalkaline

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 Mid-

Atlantic 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.

Calc-alkaline

Tholeiitic

B-A

A

D

R

Al2O3K2O

CaO

Al2O3

K2O

CaO

Al2O3

CaO

biotitemuscovitecordieriteandalusitegarnet

pyroxenehornblendebiotite

aegirineriebeckitearfvedsonite

Peraluminous Metaluminous Peralkaline

mole

s

Na2ONa

2O

K2O

Na2O

CaO

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 Alkali

content

Fe-Mg Al

Alkaline High Fe-rich

Metaluminous

to peralkaline

Sub-

alkaline

Calc-

alkaline

Low to

moderate

Mg-rich Metaluminous

to per-

aluminous

Tholeitic Low Fe-rich Metaluminous

Characteristic

Series Convergent Divergent Oceanic Continental

Alkaline yes yes yes

Tholeiitic yes yes yes yes

Calc-alkaline yes

Plate Margin Within Plate

A world-wide survey suggests that there may be

some important differences between the three series

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 Intra-

Continental Activity � kimberlites, carbonatites,

anorthosites...