Classification of plutonic rocks
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Plutonic rocks are classified primarily based on their mineral content (or mode). According to the International Union of Geological Sciences (IUGS), all rocks with a phaneritic, holocrystalline, and coarse-grained texture (> 3 mm) must be classified as plutonic rocks. The classification is centered on the QAPF diagram, developed by Streckeisen (1973).
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To use this diagram, it is necessary to establish the modal content (% in volume) of five group of minerals within a plutonic rock:
Q = quartz and its polymorphs
A = alkali feldspar. This parameter consists of all K-feldspars and Na-feldspars, including albite plagioclase with 0 to 5% of anorthite.
P = plagioclase (5 to 100% of anorthite) and scapolite.
F = feldspathoids or foids.
M = mafic or related minerals. This includes micas, pyroxenes, amphiboles, oxides, hydroxides, and all other phases (accessory phases, primary carbonates, epidote, garnet, melilite, etc.).
The sum of Q + A + P + F + M must always close at 100%. Moreover, Q and F cannot be present together in the same rock (quartz forms in silica oversaturated rocks, foids in silica undersaturated ones).
A = alkali feldspar. This parameter consists of all K-feldspars and Na-feldspars, including albite plagioclase with 0 to 5% of anorthite.
P = plagioclase (5 to 100% of anorthite) and scapolite.
F = feldspathoids or foids.
M = mafic or related minerals. This includes micas, pyroxenes, amphiboles, oxides, hydroxides, and all other phases (accessory phases, primary carbonates, epidote, garnet, melilite, etc.).
The sum of Q + A + P + F + M must always close at 100%. Moreover, Q and F cannot be present together in the same rock (quartz forms in silica oversaturated rocks, foids in silica undersaturated ones).
In order to classify the rock, the first thing to do is establishing the color index M, which comprises all minerals that generally have a dark color. The QAPF classification can be used only if M is less than 90%, otherwise the rock is ultramafic e there is another classification for it.
If the rock can be classified with the QAPF diagram, it is necessary to recalculate to 100% the sum of Q + A + P + F. For instance, a rock containing Q = 10%, A = 30%, P = 20% e M = 40%, can be recalculated as follows:
Q = 100 * 10 / 60 = 16.7%
A = 100 * 30 / 60 = 50%
P = 100 * 20 / 60 = 33.3%
where 60 is 100 – M
A general formula can be written as follows
X = 100 * x / (100 – M), where X it the mineral normalized to 100% and x the content of that same mineral estimated in the rock.
At this point, we need to define the plagioclase ratio, which is the percentage of plagioclase (P) with respect to alkali feldspar (A), as follows:
plagioclase ratio = 100 * P / (A + P)
For instance, for the rock above, this value is 100 * 33,3 / (50 + 33,3) = 39,9%. On the QAPF diagram, contrarily to other ternary diagrams, the plagioclase ratio draws lines that start from the A-P side and converge in the Q and F vertices, at the opposite sides of the diagram.
In practice, for every rock that can be classified with the QAPF diagram, we just need to know the percentage of quartz (Q) or feldspathoids (F), normalized to 100%, and the plagioclase ratio between plagioclase and alkali feldspar. In the example used so far, the rock has 16,7% recalculated quartz and plagioclase ratio aroun 39,9%. Therefore, it is a quartz monzonite.
What’s missing?
Note, if you have used this guide to learn how to classify plutonic rocks, that not all plutonic rocks can be classified with Streckeisen’s diagram (e.g. lamproites, kimberlites, and others).
Stay tuned for updates!
Bibliografia
Cox, K. G. (Ed.). (2013). The interpretation of igneous rocks. Springer Science & Business Media.
Le Bas, M. J., & Streckeisen, A. L. (1991). The IUGS systematics of igneous rocks. Journal of the Geological Society, 148(5), 825-833.
Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
Streckeisen, A. (1976). To each plutonic rock its proper name. Earth-science reviews, 12(1), 1-33.
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