Igneous petrology.....Masoom

IGNEOUS PETROLOGY 2013

Masoom Shani Page 1

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. It 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

It consists of two parts:


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Oceanic crust

Thin: 10 km

Relatively uniform stratigraphy = ophiolite suite:

1. Sediments

2. pillow basalt

3. sheeted dikes

4. more massive gabbro

5. ultramafic (mantle)

Continental Crust

Thicker: 20-90 km, average ~35 km

Highly variable composition

Average composition is granodiorite

Mantle

Peridotite (ultramafic)

Upper to 410 km (olivine spinel)

Low Velocity Layer 60-220 km

Figure 1-2. Major subdivisions of the Earth

Transition Zone as velocity increases ~ rapidly

660 spinel perovskite-type

SiIV

SiVI

Lower Mantle has more gradual velocity increase

Core

Fe-Ni metallic alloy

Outer Core is liquid i.e. No S-waves


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Inner Core is solid

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 rock 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 CLASSIFICATION

Basic system which classifies rocks on the basis of where they form.

Plutonic - at depth


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Hypabyssal - intermediate depth

Volcanic - on the Earth's surface.

But this system is not very practical, but it serves as a first approximation, it tells nothing

about mineralogy, chemistry of the rocks and cannot distinguish basalt from rhyolite.

TEXTURAL CLASSIFICATION

It 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

Glassy- no crystal structure

Porphyritic Texture-Two stages of cooling, i.e. slow cooling to grow a few large crystals,

followed by rapid cooling to grow many smaller crystals could result in a porphyritic

texture, a texture with two or more distinct sizes of grains. In a porphyritic texture, the

larger grains are called phenocryst and the material surrounding the phenocryst is

called groundmass or matrix.

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 CLASSIFICATION

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 CLASSIFICATION

The one gaining application is the result of several years’ work by the IUGS Sub-

commission 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

It referred to as acid or basic, implying a range of silica content.

1. Acidic > 66 wt% SiO2

e.g! Granites ~ 72 wt% SiO2, granodiorite ~ 68 wt% SiO2

2. Intermediate - 52 to 66 wt% SiO2

e.g! Andesite 57 wt% SiO2


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3. Basic - 45 to 52 wt% SiO2

e.g! Basalts range from 48 to 50 wt%

4. Ultrabasic - < 45 wt% SiO2

e.g! Peridotite 41 to 42 wt% SiO2

COLOUR GRADATION

Felsic rocks are light coloured, contain felsic minerals (e.g. qtz, feldspar, and

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, and 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

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 techniques:


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1. X-ray Fluorescence Spectroscopy (XRF) to determine both major and trace elements

2. Atomic Absorption Spectrometry (AAS) to determine both major and trace elements

The composition of an igneous rock is dependent on:

A. Composition of the source material. B. Depth of melting

C. Tectonic environment where crystallization occurs. e.g; rifting vs. subduction

D. Secondary alteration

SATURATION CONCEPT

It is used in reference to the SiO2 and Al2O3 which are the two most abundant components of

igneous rocks.

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

Shand (1927) proposed the following list of minerals, subdivided on the basis of silica saturation

and/or undersaturation, i.e. those that coexist with quartz (+Q) and those that do not coexist with

quartz (-Q).

Saturated (+Q) Undersaturated (-Q)

All feldspars leucite

All pyroxenes nepheline

All amphiboles sodalite

Fayalite (Fe-rich olivine) analcite etc.

Undersaturated and saturated minerals can coexist stably under magmatic conditions, but

quartz, tridymite and cristobalite can only coexist stably with saturated minerals. For

http://www.brocku.ca/earthsciences/people/gfinn/petrology/sio2sat.htm


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example Q + ne is an impossible igneous assemblage, as is Q + Ol (Mg - rich) (see

reactions above), but Q + Ol (Fe- rich) is stable.

Rock Classification (Silica saturation)

1. Oversaturated - contains primary silica mineral.

2. Saturated - contains neither quartz nor an unsaturated mineral.

3. Unsaturated - contains unsaturated minerals.

Al2O3 Saturation

Four subdivisions of rocks independent of silica saturation, based on the molecular proportions

of Al2O3, Na2O, K2O and CaO applied mainly to granitic lithologies.

Per-aluminous

If the conc. of alumina is greater than the concentration of the sum of Na2O, K2O and CaO, then

it is known as “Per-aluminous”.

i.e; Al2O3 > (Na2O + K2O + CaO)

Metaluminous

If the conc. of alumina is less than the concentration of the sum of Na2O, K2O and CaO but the

conc. of alumina is greater than the sum of the conc. of Na2O and K2O, then it is known as

“Meta-aluminous”.

i.e; Al2O3 < (Na2O + K2O + CaO) but Al2O3 > (Na2O + K2O)

Sub-aluminous

If the conc. of alumina is equal to the concentration of the sum of Na2O and K2O, then it is

known as “Sub-alumimous”.

i.e; Al2O3 = (Na2O + K2O)

Per-alkaline

If the conc. of alumina is less than the concentration of the sum of Na2O and K2O, then it is

known as “Per-alkaline.”

i.e; Al2O3 < (Na2O + K2O)

http://www.brocku.ca/earthsciences/people/gfinn/petrology/al2o3sat.htm


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Bowen’s Reaction Series

The crystallization of magma (the process by which hot, liquid magma cools and solidifies to

become a rock) is one of the most important concepts in igneous geology and is described by

Bowen’s Reaction Series. This process is called Magmatic Differentiation –the process by

which liquid magma can crystallize (solidify) to form volcanic rock of different compositions.

Bowen’s Reaction Series was performed common in a la extrusive igneous rocks: Basalt. Basalt

is a dark volcanic rock rich in iron, magnesium, calcium and silicates. It is thought to be

representative of the magma that exits deep within the Earth’s crust and is the reason Bowen chose to

use it in his laboratory experiments.

The Bowen’s Reaction Series diagram above shows the relative, but not exact, sequence of

crystallization: Olivine and Calcic Plagioclase crystallize first (at approximately the same time)

and, as temperatures cool, other minerals form until the minerals comprising the Lower Series

ultimately crystallize, which then completes the reaction series.

The detailed sequence description for Bowens Reaction Series:

Liquid magma, with the composition of basalt, is allowed to cool slowly. The first

minerals to crystallize (solidify) from the cooling melt are Olivine and Calcic

Plagioclase.

As temperatures continue to cool, the Discontinuous Series (on the left) progresses:

Olivine reacts with the melt to form Pyroxene, Pyroxene reacts with the melt to form

Amphibole, and Amphibole reacts with the melt to form Biotite.

At the same time, Plagioclase (the Continuous Series on the right) crystallizes and reacts

with the remaining melt to form other Plagioclase minerals which are increasing rich in


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sodium. In other words, as temperatures cool, Plagioclase crystallizes but its composition

becomes more sodic with falling temperatures.

The process of crystallization of minerals in the Discontinuous and Continuous Series

ends when all of the iron-magnesium minerals (Discontinuous Series) and Plagioclase

(Continuous Series) are formed.

After all the Discontinuous Series and Continuous Series minerals crystallize,

temperatures continue to cool and minerals in the Lower Series begin form from the

remaining magma melt. Potassium Feldspar, Muscovite, and Quartz crystallize in that

order, and comprise the Lower Series. As these minerals form, they do NOT react with

the remaining melt, they simply cool to become solid.

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 Variation or Bi-variant (x-y) Diagrams

These diagrams visually present the variation in 2 chemical parameters. It is the oldest method

and is known as the variation diagram or Harker diagram which dates from 1909, and plots

oxides of elements against SiO2.

Explanation

Harker Diagrams gives a concept about Bowen Reaction Series.

Oxides (K2O, Na2O, CaO, MgO, and Al2O3) plotted against Silica (SiO2) form linear arrays. A

set of such plots is called a Harker diagram.

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.

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.

The explanation of these plots is as follows:


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FeO with silica:

This oxide has inverse relationship with silica. It means that with increasing FeO, silica decrease

and when conc. of FeO decreases, silica increases as shown in fig.

MgO with silica:

This oxide has inverse relationship with silica. It means that with increasing MgO, silica

decrease and when conc. of MgO decreases, silica increases as shown in fig.

CaO with silica:

This oxide has inverse relationship with silica. It means that with increasing MgO, silica

decrease and when conc. of MgO decreases, silica increases as shown in fig.

K2O with silica:

This oxide has direct relationship with silica. It means that with increasing K2O, silica increases

and when conc. of K2O decreases, silica also decreases as shown in fig.

Na2O with silica:

This oxide has direct relationship with silica. It means that with increasing Na2O, silica increases

and when conc. of Na2O decreases, silica also decreases as shown in fig.

Al2O3 with silica:

Al2O3 does not exhibit a strong variation.



Triangular or Ternary Variation Diagrams

These diagrams visually present the variation in 3 chemical parameters. These diagrams show

only the ratios of various oxides or elements, rather than their actual values

Two types of triangular variation diagrams are commonly used:

1. AFM Diagrams - Mainly for Mafic Rocks

A = Na2O + K2O

F = FeO (+Fe2O3)

M = MgO

Plotted as either molecular or weight percent values.

2. Na2O - K2O - CaO Diagrams- 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 readily determined.

Spider diagrams

Spider diagrams 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 spider diagram

• 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 element (log scale!)



• Link the dots

• Look at the “anomalies”!

It is also known as Transition metal diagrams

Magmatic series

It reflect first order differences b/w rock groups.

TAS diagram séparâtes alkali and sub-alkali series

Sub-alkali series are further separated on the basis of their Fe-Mg contens

AFM diagram: can further subdivide the sub-alkaline magma series into a tholeitic and a

calc-alkaline series

There are 3 types of Magmatic Series :

1. Tholeitic series

Fe-rich, alkali poor.

Metaluminous

Px/Hb/Bt-bearing basalts, andésites, 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

températures).

3. Alkaline series

Alkali rich, Fe-rich

Metaluminous to Peralkaline

Evolution towards trachytes (Moderately 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 température.

Characteristic

Series Convergent Divergent Oceanic Continental

Alkaline yes yes yes

Tholeiitic yes yes yes yes

Calc-alkaline yes

Plate Margin Within Plate



Trace elements

Trace elements as a tool to determine paleotectonic environment.

• Useful for rocks in mobile belts that are no longer recognizably in their original setting.

• Trace elements can be discriminators of igneous environment.

• Approach is empirical on modern occurrences.

• Concentrate on elements that are immobile during low/medium grade metamorphism.

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 alkalis, 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 analysis 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.

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:

1. Measure the surface area of mineral grains of the same mineral, relative to the total

surface area of the thin section.

2. Measure the intercepts of each mineral along a series of lines.

3. 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

dependent on the mean grain size of the sample.

Advantages

1. One can compare rocks from different areas if you only have a thin section

2. No chemical analysis is required, using a petrographic microscope.

3. Gives the maximum and minimum grain sizes.

Disadvantages

1. Meaningless if the sample has a preferred orientation of one or more minerals.

2. Porphyritic rocks are difficult to count.

3. 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, and Barth.

Each of these proposals has its own specific advantages and/or disadvantages.



CIPW Norm

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 ferromagnesian minerals are assumed to be free of Al2O3.

The Fe/Mg ratio for all ferromagnesian 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.

Barth mesonorm

It is used commonly when examining granitic rocks.

OPHIOLITES

Ophio is Greek word for "snake", lite means "stone" from the Greek lithos.

The name is given because ophiolites have similarity in colour and texture with snakes,

some greenish colour.

Definition

An abductive part of the oceanic crust is known as “ophiolites”. OR

An Ophiolite is a section of the Earth's oceanic crust and the underlying upper mantle that has

been uplifted or emplaced to be exposed within continental crustal rocks.

Formation of Ophiolites

It is formed at the convergent plate boundaries during the subduction of oceanic plate beneath

continental plate. Due to compression, fragments of oceanic crust and upper mantle uplifted and

emplaced on continental margins, known as “ophiolite”.

http://en.wikipedia.org/wiki/Oceanic_crust

http://en.wikipedia.org/wiki/Earth%27s_mantle

http://en.wikipedia.org/wiki/Continental_crust



Lithology of Ophiolites

Ophiolites consist of five distinct layers:

1. The first layer is the youngest and is primarily sediment that was accumulated on the

seafloor.

2. The second layer is pillow basalt. Pillow basalt is characterized by large pillow or cloud

shaped blobs.

3. The next layer consists of sheeted dikes. Sheeted dikes form by rising magma within the

earth's crust.

4. Sheeted dikes are underlain by gabbro, which is compositionally similar to basalt, but

very coarse grained due to the slow cooling process.

5. The bottommost layer is Peridotite, which is believed to be mantle rock composition.

Ophiolites in Pakistan

• Indian plate collision with Eurasian plate and afghan plate.

• East-west trending ophiolites due to I.P collision with E.P.

• North-south trending ophiolites due to I.P collision with A.P.

Eurasian block ophiolites

Dargai

Mingora – bajaware

Chilas etc.

Afghan block ophiolites

Waziristan



Zhob

Muslim Bagh

Bela etc.

Conclusions

Ophiolites are slabs of ancient oceanic crust abducted/preserved onto the continental

crust/earth surface.

They are located in collisional boundaries.

Their compostion is sediments, lavas, sheeted dikes, gabbros, and ultramafic rocks.

Have similarity with oceanic crust.

Granitoids

“Granitoids” (sensu lato): loosely applied to a wide range of felsic plutonic rocks, similar to

granite which mineralogically are composed predominantly of feldspar and quartz

Examples

Granite

Quartz Monzonite

Quartz Diorite and

Syenite etc.

Explanation

Associated volcanics are common and have same origin, but are typically eroded away.

Most granitoids of significant volume occur in areas where the continental crust has been

thickened by orogeny, either continental arc subduction or collision of sialic masses.

Many granites, however, may post-date the thickening event by tens of millions of years.

Because the crust is solid in its normal state, some thermal disturbance is required to form

granitoids

Most workers are of the opinion that the majority of granitoids are derived by crustal

anatexis, but that the mantle may also be involved. The mantle contribution may range

from that of a source of heat for crustal anatexis, or it may be the source of material as

well.

Granitoids Classification

There are usually four types:

1. I-type

2. S-type



3. M-type

4. A-type

I-type Granitoids

These types of granitoids derived from the melting of mafic mantle-derived igneous

source, probably sub-crustal under plate. e.g; Andean Granites

It may be Metaluminous and peraluminous.

Its common oxide is magnetite.

It is hornblende-rich

S-type Granitoids

It is derived from partial melting of peraluminous sedimentary rocks imprinted by

weathering at surface of earth.

It is always peraluminous.

Its common oxide is ilmenite.

It is biotite-rich and may contain muscovite, andalusite etc.

M-type Granitoids

It is derived from mantle source.

It includes both immature arc plutons found in ophiolites oceanic crust.

A-type Granitoids

It is derived from anorogenic.

It is commonly intruded into non-orogenic setting.

Generally higher in SiO2, alkalies, Fe/Mg, halogens F, Cl, Zr than I-type.

IUGS Classification of Igneous Rocks

IUGS Classification of igneous rocks is as follows:



Ultramafic Rocks











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Igneous petrology.....Masoom