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.
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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:
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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 3-7). Note the ranges for most rocks.
SiO235-75% (basalts 45-50%, granites 70%, Ultramafic 30-40%
Al2O3 5-20%TiO20-5%CO20-5%
MgO 1-40%Na20.5-5%MnO 0-0.5%
CaO 1-20%K2O 0-5%P2O50-0.5%
Fetot1-15%H2O 0.2-5%
Now we can apply one of a number of classifications:
A] Classification based on Silica Percentage
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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 SiO2is so abundant, a classification can also be based on the presence or absence of various mineral phases which reflect the SiO2content 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 SiO2include leucite, nephelene, sodalite, olivine, melanite garnet, corundum, Al & Ti rich clinopyroxene.
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Igneous Rock Classification cont
Classification
Oversaturatedrocks -have quartz and tridimite in abundance
Saturatedrocks -have no free quartz and no undersaturated minerals
Undersaturatedrocks -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 Al2O3similar to the SiO2classification system
Peraluminous: molecular proportion of Al2O3exceeds the sum of CaO, Na2O and K2O. For plagioclase + alkali feldspar, this ratio is about 1:1. Any Al2O3that is left over goes in to forming corundum. These rocks tend to be mica rich.
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Igneous Rock Classification cont
Metaluminous: molecular proportion of Al2O3exceeds 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 Al2O3is 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 Al2O3is 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.
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Igneous Rock Classification cont
Figure 3-6 plots CaO vs SiO2and Na2O+K2O vs SiO2. Since CaO usually decreases as Na2O+K2O increases with respect to SiO2, therefore the curves cross. The SiO2content, at the point at which the curves cross, indicates the alkalinity of therock 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
oror Figure 6-16 -normalized REE
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Igneous Rock Classification cont
Common Ternary Plots -3 component systemsa) A(B)FM Diagram(J.B.Thompson 1957)A=Al2O3B=K2OF=FeOM=MgOb) ACF Diagram(Eskola early 1900’s)A=Al2O3+Fe2O3-(Na2O+K2O)C=CaOF=MgO+FeO+MnOc) AKF Diagram(Eskola, early 1900’s)A=Al2O3-(CaO+Na2O+K2O)K=K2OF=FeO+MgO+MnO
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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).
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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)
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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.
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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.
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Igneous Rock Classification
Streckeisen Classification System
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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% SiO2is andesite while one with <52% SiO2is 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.
78Ultramafic Rocks contPyroxenite: 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.
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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.
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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 SiO272.67 ÷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 1Al2O36SiO2
4) Use remaining Al2O3making Corundum
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5) Allocate remaining FeO, MgO to hypersthene. Molecular proportion of FeO+MgO = molecular proportion of SiO2.
6) Allocate the remaining SiO2to 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/Fe2O3allocation can cause problems. It is assigned to magnetite but what about other iron minerals and iron in silicate structures?
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