Download Igneous Rock Classification

64
Igneous Rock Classification
Igneous rocks: very diverse in chemistry and texture, yet
they have very gradational boundaries (Table 3-7). We must
pick a rational basis for classifying them. The classification
system used, will depend on how much we know about the
rock being examined.
Basis for Classification
1) Field and hand specimen examination: texture, colour etc.
2) Chemical Data: rock chemistry.
3) Petrographic examination: mineral identification
Examine these classification systems in more detail.
1) Field and hand specimen examination
The most primitive classifications are based on rock
characteristics such as:
a) Extrusive or Intrusive (grain size)
Extrusive Volcanic rocks are formed near the earth’s surface.
They are fine grained to glassy except for coarser grained
pheoncrysts (which formed at depth before eruption). Eg
volcanic flows or ashes.
65
Igneous Rock Classification cont
Intrusive Hypabyssal rocks are formed at shallow depths
(less than 1 km). They are fine grained, may contain
phenocrysts. Eg tabular dykes or sills. (Often lumped with
volcanics because of similarity).
Intrusive Plutonic rocks form at depth greater than 1 km.
They are medium to coarse grained. Eg granite diorite etc.
(also often used for regional metamorphic rocks formed at
depth such as granite gneiss).
b) Colour index
(% of dark minerals)
c) Other features visible to the naked eye. Eg phenocrysts,
vesicles, flow banding, cumulate textures etc.
2) Chemical Classification
As technology improves, the use of chemical classification
has become more common, easier and cheaper. Eg 30 years
ago 10 major elements cost about $100. Now you get the
same analysis, REE and some minor elements for
$10.Geologists use an informal classification of major
elements and minor elements:
66
Igneous Rock Classification cont
a) Major Elements: make up the bulk of the rock. Eg Si, Al,
Fe2+, Fe3+, Mn. Mg, Ca, K, Na, P, Ti, H2O.
b) Minor elements: present in ppm quantities. Eg Cr, Ni, Zr,
Rb, Sr, REE’s.
Chemistry is most useful when dealing with altered and very
fine grained rocks. In general, if you test a suite of rocks, the
boundary between rock types becomes less arbitrary.
Chemistry of igneous rocks is reported in % oxides (Table 37). Note the ranges for most rocks.
SiO2 35-75% (basalts 45-50%, granites 70%, Ultramafic 30-40%
Al2O3 5-20%
TiO2 0-5%
CO2 0-5%
MgO 1-40%
Na2 0.5-5%
MnO 0-0.5%
CaO 1-20%
K2O 0-5%
P2O5 0-0.5%
Fetot 1-15%
H2O 0.2-5%
Now we can apply one of a number of classifications:
A] Classification based on Silica Percentage
67
Igneous Rock Classification cont
This can be combined with Table 3-3 with leucocratic being
applied to felsic rocks, mesocratic being applied to
intermediate rocks and melanocratic being applied to mafic
and ultramafic rocks.
Problems arise with this classification system because you
are comparing a chemical system (SiO2 %) with a system
based on % of dark minerals. You sometimes run into
problems: nepheline syenite is considered a felsic rock yet it
does not contain >66% SiO2.
B] Silica Saturation
As SiO2 is so abundant, a classification can also be based on
the presence or absence of various mineral phases which
reflect the SiO2 content in relation to the other chemical
components.
Typical saturated minerals that can occur with free quartz
include feldspar, Al & Ti poor pyroxene, amphibole, mica,
almandine garnet. Typical undersaturated minerals that are
not stable in the presence of free SiO2 include leucite,
nephelene, sodalite, olivine, melanite garnet, corundum, Al
& Ti rich clinopyroxene.
68
Igneous Rock Classification cont
Classification
Oversaturated rocks - have quartz and tridimite in
abundance
Saturated rocks - have no free quartz and no undersaturated
minerals
Undersaturated rocks - have no quartz and have
undersaturated minerals.
This system is therefore based primarily on relationships of
silica content to the rest of the rock.
C] Alumina Saturation
Based on Al2O3 similar to the SiO2 classification system
Peraluminous: molecular proportion of Al2O3 exceeds the
sum of CaO, Na2O and K2O. For plagioclase + alkali
feldspar, this ratio is about 1:1. Any Al2O3 that is left over
goes in to forming corundum. These rocks tend to be mica
rich.
69
Igneous Rock Classification cont
Metaluminous: molecular proportion of Al2O3 exceeds the
sum of Na2O and K2O, but is less that the sum of Na2O, K2O
and CaO. These rocks tend to be rich in anorthite and usually
also contain hornblende, epidote, biotite and pyroxene.
Subaluminous: molecular proportion of Al2O3 is
approximately equal to the sum of Na2O and K2O. These
rocks tend to form alkali feldspar and a little Ca plagioclase
and usually contain olivine and pyroxenes.
Peralkaline : molecular proportion of Al2O3 is less than the
sum of Na2O and K2O. There is insufficient alumina to use
all the Na2O and K2O by making feldspar. The free alkalis
become incorporated into alkali rich ferromagnesium
minerals such as aegerine or reibeckite.
D] Alkali-Lime Index
This system tells us about the alkalinity of the rocks.
70
Igneous Rock Classification cont
Figure 3-6 plots CaO vs SiO2 and Na2O+K2O vs SiO2. Since
CaO usually decreases as Na2O+K2O increases with respect
to SiO2, therefore the curves cross. The SiO2 content, at the
point at which the curves cross, indicates the alkalinity of the
rock suite.
E] Common Chemical X-Y and Ternary Plots
Typically, for X-Y plots you plot oxides against a common
or stable or highly variable component. Which components
to plot depends on experience and what you wish to know.
Tholeiitic basalts - ophiolites, ocean
floor, greenstone belts.
Alkali basalts - crustal melts, Hawaii
or
or Figure 6-16 - normalized REE
71
Igneous Rock Classification cont
Common Ternary Plots - 3 component systems
a) A(B)FM Diagram (J.B.Thompson 1957)
A=Al2O3
B=K2O
F=FeO
M=MgO
b) ACF Diagram (Eskola early 1900’s)
A=Al2O3+Fe2O3-(Na2O+K2O)
C=CaO
F=MgO+FeO+MnO
c) AKF Diagram (Eskola, early 1900’s)
A=Al2O3-(CaO+Na2O+K2O)
K=K2O
F=FeO+MgO+MnO
72
Igneous Rock Classification cont
These plots are used to easily and clearly distinguish
different rock types. They are particularly useful for fine
grained or altered rocks where identification can be difficult.
Some of these ternary plots (Figures 6-20 and 6-16) are
specific for a particular rock type: Ti-Zr-Y(Sr) Diagram for
basalts. Field A+B are low K tholeitic, field B are ocean
floor basalts, field B+C are calc-alkali basalts and field D are
oceanic island or continental basalts.
These are all relatively immobile trace elements. These
diagrams are useful if the original environment is
scrambled. Eg: ocean floor basalts thrust onto the continent;
basalts within the plate (oceanic or continental) VS plate
margin (ocean ridge to ocean floor).
73
Igneous Rock Classification cont
3) Classification based on Petrographic Examination
Thin sections of rocks are relatively easy to make and
identification of rocks based on the mineralogy observed is
possible.
Rules:
a) Make sure the thin section is representative of the rock.
b) Identify the major components of mineralogy and
estimate their relative proportions.
c) Use proportions to classify the rock according to a
scheme. Any scheme is somewhat arbitrary. See handout and
Streckeisen.
Criteria which are important:
1) Proportion of mafic to felsic components
2) Composition of the plagioclase
3) Proportion of alkali feldspar to plagioclase
4) Presence or absence of quartz
5) Presence or absence of feldspathoid minerals
6) Grain size or texture (extrusive or intrusive)
74
Igneous Rock Classification cont
Discussion - In general
a) These methods are time consuming but relatively straight
forward for coarse grained rocks.
c) Volcanic rocks are harder to identify mineralogy. Grains
are small and difficult to identify petrographically.
c) Glassy rocks - often impossible to identify mineralogy
petrographically.
d) Altered rocks - Bad news, the system can break down.
Some problems related to some classification schemes:
i) No subdivisions of granites or rhyolites. All just felsic rich
acidic rocks.
ii) No subdivisions of basalts and andesites. Need further
rules.
iii) Lack of description for mafic rocks in general.
75
Igneous Rock Classification
Streckeisen Classification System
•
•
•
•
In 1967 Albert Streckeisen, with the cooperation of
many geologists in many countries, came up with a
generally accepted rock classification system.
The International Union of Geological Sciences
(IUGS) modified and expanded his work to form
what is an internationally accepted igneous rock
classification system.
In order to use this system, you must be able to
determine the percentage of five minerals (or
mineral groups): quartz, plagioclase, alkali
feldspars, ferromagnesian minerals and
feldspathoids (such as nepheline or leucite).
The Q (or F) , A and P mineral percentage is
recalculated to add to 100% and is plotted on the
triangular plot.
76
Igneous Rock Classification
Streckeisen Classification System
77
Igneous Rock Classification
Streckeisen Classification System
•
•
•
The plagioclase rich area of the diagram has some
additional requirements for rock distinction.
For plutonic rocks: anorthosite is a rock containing
>90% plagioclase, gabbro contains plagioclase
more calcic than An50 and usually contains >35%
mafic minerals (augite, hypersthene or olivine),
Diorite contains plagioclase more sodic than An50
and usually contains >35% mafic minerals
(hornblende or hypersthene ± augite).
For volcanic rocks: the distinction between basalt
and andesite is bases on the silica content. A rock
with >52% SiO2 is andesite while one with <52%
SiO2 is basalt.
Rocks that don’t fit the IUGS Classification
Ultramafic Rocks
Ultramafic rocks (containing more than 90% mafic
minerals) are classified by alternative methods. Some
of the most common types are defined as follows:
Peridotite: a rock containing 40-100% olivine, with the
remainder mainly pyroxene and/or hornblende.
Dunite: a rock containing 90-100% olivine with the
remainder mainly pyroxene.
78
Ultramafic Rocks cont
Pyroxenite: a rock composed mainly of pyroxene with the
remainder olivine and/or hornblende.
Hornblendite: a rock composed mainly of hornblende with
the remainder mainly pyroxene and/or olivine .
There are a few rocks that don’t fit the IUGS
classification system that are named on the basis of
texture, with mineral content being of secondary
consideration. Some of the more important of these are
defined as follows:
Pegmatite: a very coarse grained (>1 cm) rock with
interlocking grains. Usually granitic in composition.
Obsidian: a black volcanic glass with conchoidal
fracture, rhyolitic in composition.
Tuff: a compacted deposit of ash and dust containing up
to 50% sedimentary material.
79
Breccia: Similar to a tuff, but with large angular
fragments in a fine matrix.
There are also few well recognized igneous rocks
that are found in a highly altered state. The
alteration is related to their method of origin.
Some of the more important of these are defined
as follows:
Spilite: an altered, usually vesicular basalt
exhibiting pillow structures. Feldspars have been
altered to albite and is usually found with chlorite,
calcite, epidote, chalcedony or prehnite.
Serpentinite: a rock containing almost entirely
serpentine (from the alteration of olivine and
pyroxene).
Kimberlite: an altered porphyritic mica peridotite
containing olivine (altered to serpentine or a
carbonate mineral) and phlogopite (commonly
altered to chlorite). Some also contain diamonds.
80
Igneous Rock Classification cont
There are rock classification systems that attempt to
combine chemistry and mineralogy. In this case, you take
the chemistry data and transform it into theoretical
mineralogy. This is called the CIPW Normative
Classification (Cross, Iddings, Pirsson and Washington).
The norms are based on molecules of ideal composition.
Methodology
A] Convert % oxides into molecular proportions
wt% oxide ÷ formula wt = Molecular Proportion
Eg SiO2
72.67 ÷ 60.09 = 1.211
B] Allocate molecular proportions to minerals using the
following rules:
1) Apatite is one of the first minerals to precipitate. All P
is in apatite.
2) Allocate Fe2O3, FeO to magnetite. The limiting factor is
the total amount of Fe2O3. Molecular proportion of Fe2O3
= Molecular proportion of FeO.
3) Make pure Orthoclae, Albite and Anorthite.
Eg: Orthoclase 1K2O 1Al2O3 6SiO2
4) Use remaining Al2O3 making Corundum
81
Igneous Rock Classification cont
5) Allocate remaining FeO, MgO to hypersthene.
Molecular proportion of FeO+MgO = molecular
proportion of SiO2.
6) Allocate the remaining SiO2 to quartz.
C] Once the molecular fraction has been calculated for
each mineral, multiply through by the atomic weight of
that mineral. This will give you a proportion (5) of each
mineral species.
Limitations: Often Severe
1) Can only calculate anhydrous species, therefore biotite
and amphiboles are ignored.
2) normative mineralogy will not equal modal mineralogy
3) Theoretical end members are used which may not
match actual members present.
4) FeO/Fe2O3 allocation can cause problems. It is assigned
to magnetite but what about other iron minerals and iron in
silicate structures?