Ch 02 igneous classification

Classification of Igneous RocksClassification of Igneous Rocks
“personally, I doubt that an exact petrological
classification of igneous rocks can ever be attained.
We may arrive at some sort of approximation to an
orderly arrangement for the purposes of
petrographic description and petrological discussion,
which might by courtesy be called a
classification…………”
H S Washington, 1922

“A rock may be given one name on
the ground of field occurrence and
from hand lens examination, only to
require another when it is studied in
thin section, and perhaps a third when
it is chemically analyzed. . . .
Different schemes have different
objects in view”

Classification based on FabricClassification based on Fabric
Four principal types of fabric occur in magmatic rocks: phaneritic, aphanitic,
glassy, and volcaniclastic.
The first two refer to the dominant crystal grain size, which ranges over
several orders of magnitude, from 10-3
to 10 m.
Phaneritic applies to rocks that have mineral grains sufficiently large to be
identifiable by eye (minute accessory minerals excepted).
This texture is typical of rocks crystallized from slowly cooled intrusions of
magma.
Aphanitic rocks have mineral grains too small to be identifiable by eye and
require a microscope or some other laboratory device for accurate
identification.

Classification based on fabric:
Aphanitic- crystals too small to see by
eye
Phaneritic- can see the constituent
minerals
Fine grained- < 1 mm diameter
Medium grained- 1-3 mm diameter
Coarse grained- 3-50 mm diameter
Very coarse grained- > 50 mm
diameter
Porphyritic- bimodal grain size
distribution
Glassy- no crystals formed

Aphanitic texture is most common in rapidly solidified extruded magma but can
also be found in marginal parts of magma intrusions emplaced in the cool shallow
crust.
Some magmatic rocks contain essentially two grain-size populations and few of
intermediate size; such texture is said to be porphyritic.
The larger grains are phenocrysts, and the smaller constitute the groundmass, or
matrix.
Porphyritic aphanitic rocks are far more common than porphyritic phaneritic
rocks.
Glassy, or vitric, rocks contain variable proportions of glass, in contrast to
holocrystalline rocks made entirely of crystals.
A vitrophyre is a porphyritic rock that contains scattered phenocrysts in a glassy
matrix.

The fabric of volcaniclastic rocks is produced by any
fragmenting process that creates broken pieces of volcanic rock
and/or mineral grains.
Classification of volcaniclasts parallels that of sedimentary
clasts according to their particle size, as follows:

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

Classification based on Field RelationsClassification based on Field Relations
The location where magma was emplaced provides a
basis for rock classification.
Some petrologists recognize three categories for rocks
solidified from magmas emplaced onto the surface of
the Earth (volcanic or extrusive), into the shallow
crust (intrusive hypabyssal), and into the deep crust
(intrusive plutonic).
The first and the last categories are readily
distinguished on the basis of their field relations but
less directly on the basis of their grain size, degree of
crystallinity (proportion of crystals to glass), and
mineralogical composition.

Magmas emplaced onto the surface of the Earth as coherent lava
flows or as fragmental deposits form extrusive, or volcanic rocks.
These rocks are typically aphanitic and glassy. Many are
porphyritic.
Some have fragmental (volcaniclastic) fabric.
Intrusive, or plutonic, rocks form where magma was intruded into
preexisting rock beneath the surface of the Earth as intrusions, or
plutons.
Plutonic rocks are typically phaneritic. Monomineralic rocks
composed only of plagioclase, or olivine, or pyroxene are well
known but rare.

Characteristics of intermediate-depth hypabyssal rocks are not clearly
distinct from those of volcanic and plutonic rocks.
Many occur in shallow crustal dikes, sills, and plugs that represent
feeding conduits for surface extrusions of magma.
But dikes and sills are also intruded deep in the crust. Hypabyssal rocks
can have fabric similar to that of plutonic and volcanic rocks.
Because of these ambiguities, many petrologists tend to categorize
magmatic rocks in the field simply as plutonic or volcanic.

Classification based on MineralogicalClassification based on Mineralogical
and Modal compositionsand Modal compositions
These different compositional aspects are related but may lead to
confusion at times.
Quartz, plagioclase, alkali feldspar, muscovite, biotite, hornblende,
pyroxene and olivine.
Felsic: Feldspar and Silica, Mafic: Magnesium and Ferric iron.
Felsic is also used for rocks composed of feldspathoids.
Smoky quartz and darker plagioclase are felsic.
Apatite, zircon, titanite, epidote, chlorite are accessory.
Ultramafic rocks are especially rich in Mg and Fe and generally have
little or no feldspar, eg is peridotite.
Silicic rocks contain large concentrations of silica, rhyolite.

Color is not a valid basis of rock classification as well as indicator of
composition.
Eg. When when plagioclase becomes more calcic than about An50, it
is commonly dark grey or even black. Smoky quartz is also quite
dark.
Most geologists would resist to call these mafics because roots of
felsic and mafic lie in terms of their chemical composition.
A rock composed of 90% of dark feldspar would thus be
considered as felsic.
Instead of color, the term Color Index has been widely used
which means volume percentage for dark minerals.

If one attempts to devise a practical classification of igneous rocks, many
problems arise that can be averted if one separates the volcanic from the
plutonic rocks.
These 2 groups of rocks generally cool, and congeal, at different rates; and this
cooling tends to occur in different physical and often chemical environments.
Plutonic rocks are generally medium- to coarse-grained whereas many
volcanic rocks are partially, or wholly, glassy and/or fragmental, and the
phenocrysts/megacrysts they contain are often not a clear guide to the overall
composition of the rock.
It thus seems rational to use modal criteria to classify plutonic rocks that
contain easily, recognizable minerals, and chemical criteria to classify
volcanic rocks.

In 1973, International Union of Geological Sciences subcommission
on the systematics of igneous rocks (IUGS) introduced a new modal
classification of the plutonic rocks.
The classification uses
1. Q = Quartz or high temperature polymorphs
2. A = Alkali feldspars (orthoclase, Albite (An00-05)
3. P = Plagioclase (An05-100)
4. F = Feldspathoids and
5. M= Mafic and related minerals (micas, amphiboles, pyroxenes,
olivines), opaque minerals (Fe-Ti oxides), accessory minerals and
primary carbonates.

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

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

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

Classification ofClassification of
Igneous RocksIgneous Rocks
Figure 2.3. A classification and nomenclature
of volcanic rocks. After IUGS.

Does an Igneous rock analysis representDoes an Igneous rock analysis represent
magma composition?magma composition?
In the quest to discover the origins of magmas and the conditions
under which they form, extensive use is made of major element,
trace element and isotopic analyses of volcanic rocks.
The key assumption in doing so is that such analyses accurately
represent the chemical compositions of the magmas from which
the volcanic rocks crystallized.
How widely can this assumption be justified, and what factors
limit its application?

1. Degassing and volatile release1. Degassing and volatile release
Magma confined at depth in the Earth contains gases dissolved in the
melt.
As load pressure is relieved during ascent toward the surface, this gas
content will progressively come out of solution to form separate
bubbles of gas (often apparent as vesicles in erupted lavas), which may
escape from the magma into the atmosphere.
Such ‘degassing’ is common in all magmas held in shallow magma
chambers or erupted on the surface.
It follows that the volatile content measured in a fresh volcanic rock
sample will generally be less than the true content originally dissolved
in the melt at depth.

1. Degassing and volatile release1. Degassing and volatile release
How can we determine the true pre - eruption ‘ magmatic ’ volatile
contents of erupted volcanic rocks?
One approach is to analyse the volatile content of minute glass
inclusions (generally referred to loosely – even though no longer
molten – as ‘ melt inclusions ’ because they represent trapped melt)
within individual phenocrysts.
The tensile strength of the surrounding crystal effectively ‘armours’
the melt inclusion against rupture and gas escape as the host magma
ascends towards the surface; micro - analysis of the ‘ trapped ’ volatile
content of these glass inclusions provides the best available measure of
that of the undegassed melt.

2. Hydrothermal alteration and low2. Hydrothermal alteration and low
grade metamorphismgrade metamorphism
Anhydrous minerals formed at melt temperatures such as olivine and plagioclase,
if exposed to hydrous fluids at lower temperatures during cooling, are prone to
react and recrystallize into hydrous secondary minerals such as smectite,
serpentine, chlorite and epidote.
The analysis of a volcanic rock that has undergone such alteration or low-grade
metamorphic reactions will therefore show elevated contents of H2O and other
volatile species, introduced by these post-magmatic reactions, that bear no relation
to the original volatile content of the magma.
It is much harder to correct for any changes in the contents of relatively soluble
non-volatile elements such as Na2O, K2O and CaO that may also have
accompanied these reactions.
For this reason geochemical work needs to be based on unaltered samples that
show negligible amounts of such post - magmatic minerals under the microscope.

3. Crystal accumulation3. Crystal accumulation
In deep-seated magma chambers where the cooling rate is slow,
crystals may sink or float in the melt according to their density and
crystal size, and may then form deposits in which one mineral (or more
than one) is selectively concentrated at particular horizons.
Alternatively, one type of crystal may nucleate more efficiently on the
chamber floor and walls than other minerals and thereby become
selectively concentrated there.
The possibility of such selective accumulation processes, operating on
various scales, means that the composition of a plutonic rock hand–
specimen will not accurately record the melt composition from which
it crystallized.

3. Crystal accumulation3. Crystal accumulation
Moreover, accumulations of early crystals will generally have higher
Mg/Fe (in the case of ferromagnesian minerals) or higher Ca/Na (in
plagioclase) than the melt from which they separated.
Though crystal accumulation processes exhibit their most dramatic
effects in large layered intrusions, they are also known to occur in
minor intrusions and even in thick lava flows.
Whereas in volcanic rocks the minerals that form can be seen as
dictated by magma chemistry, in plutonic rocks where crystal sorting
may have occurred the converse applies: the whole-rock chemical
composition is in part a consequence of the minerals present and the
proportions in which they happen to be combined.

4. Xenocrysts and Xenoliths4. Xenocrysts and Xenoliths
Many igneous rocks contain foreign material in the form of xenoliths,
torn from conduit walls during magma ascent, or present in a
disaggregated state as individual xenocrysts.
A bulk chemical analysis of the host rock will not faithfully represent
the composition of the host magma if such exotic matter has not been
carefully picked out during sample preparation.
Even when obvious foreign bodies have been removed, the analysis
may be distorted by chemical exchange between magma and xenoliths,
especially in the case of slowly cooled plutonic host rocks.

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

Classification based on whole rockClassification based on whole rock
chemical compositionchemical composition
In order to discuss problems relating to the nature, origin and evolution
of magmas, to compare igneous rock series and to interpret chemical
analysis themselves, it is desirable to have chemical classification of
igneous rocks.
Obviously, these tell nothing of rock textures and only hint at
mineral content.
Thus they ignore the cooling and crystallization history, for magmas of
generally similar (not identical) chemical composition may yield rocks
of many different textures and markedly different mineral content like
obsidian, rhyolite and granite.

Because the mode is commonly difficult to determine for the
volcanics, since the matrix of many volcanics is composed of
minerals of extremely fine grain size and may even consist of a
considerable proportion of glassy or amorphous material.
The most reliable way to avoid the matrix problem is to
analyze the volcanic rock chemically and use a classification
scheme based on the analytical results.
The IUGS recommended a classification of volcanics based on
a simple diagram comparing total alkalis with silica, also
called as TAS diagram.

The use of silica in the classification of igneous rocks is particularly
appropriate because it is the dominant oxide in the common magmatic rocks of
Earth, and the silica content of the melt exerts considerable control over the
physical nature and structure of the melt.
It is also important to take Na2O and K2O into account as these oxides,
together with silica, essentially determine the silica saturation of the common
magmatic rocks.
This is because the SiO2, Na2O and K2O contents of a rock usually determine
the quantity and type of felsic minerals that form.
Alumina, CaO and MgO, are generally the next most important oxides in
determining the gross chemical and modal character of the common magmatic
rocks.

A chemical classification of
volcanics based on total
alkalis vs. silica. After Le
Maitre (2002) .

Classification based on whole rock chemicalClassification based on whole rock chemical
composition- Silica saturation (Quasi-chemical)composition- Silica saturation (Quasi-chemical)
Shand in 1950, divided igneous rocks into undersaturated, saturated
and oversaturated rocks.
Consider a simple hypothetical magma consisting only of O, Si, Al,
and Na.
If there is an excess of molar SiO2 relative to that needed to make
albite from Na2O, that is, SiO2/Na2O > 6, then the magma can
crystallize quartz in addition to albite.
(In a natural magma, the albite would be in solid solution in
plagioclase and/or alkali feldspar.)
This magma and the corresponding rock are silica-oversaturated.

chemical composition- Silica saturationchemical composition- Silica saturation
If the magma contains SiO2 and Na2O in the exact ratio of 6, then
these two constituents can only combine into albite; the magma and
rock are silica-saturated.
If the molar ratio SiO2/Na2O < 6 but 2 in the magma, then there is
insufficient SiO2 to combine with all of the Na2O into albite and some
nepheline is created instead; the magma and rock are silica-
undersaturated.
If the molar ratio SiO2/Na2O < 2 in the magma, then there is
insufficient SiO2 to combine with the Na2O to create any albite at all
and only nepheline can be produced; the magma and rock still qualify
as silica-undersaturated.

chemical composition- Alumina saturationchemical composition- Alumina saturation
Al2O3 is the second most abundant constituent in most magmatic
rocks and provides another means of classification, especially for
felsic rocks, such as granitic ones.
The concept of alumina saturation is based on whether or not there is
an excess or lack of Al to make up the feldspars.
The alumina saturation index is defined as the molecular ratio
Al2O3/(K2O + Na2O + CaO).
In alumina-oversaturated, or peraluminous rocks, Al2O3 >
CaO+Na2O+K2O.
In peraluminous rocks, we expect to find an Al2O3 rich mineral to be
present as modal mineral, Muscovite, Corundum, Topaz, Andalusite

In alumina-undersaturated, or metaluminous rocks,
CaO+Na2O+K2O > Al2O3 > Na2O+K2O.
These are more common types of igneous rocks. They are
characterized by lack of an Al2O3-rich mineral.
In peralkaline rocks, they are oversaturated with alkalis, and thus
undersaturated with respect to Al2O3. Al2O3 < Na2O+K2O+CaO.
They have Na-rich minerals like aegirine, riebeckite in the mode.

Here, classification divides the rocks into alkaline and
subalkaline. It is based solely on alkali vs silica diagram.
Very alkaline rocks, are also silica undersaturated.

Ch 02 igneous classification

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