Question

The given table shows modal volume percentage of quartz, alkali feldspar and plagioclase in coarse-grained rocks, X and Y. Which of the following rock names of X and Y are CORRECT according to the IUGS classification?

Mineral	X (volume %)	Y (volume %)
Quartz	15	25
Plagioclase	20	60
Alkali feldspar	65	15

• A. X - Alkali feldspar granite; Y - Tonalite
• B. X - Granodiorite; Y - Granite
• C. X - Alkali feldspar granite; Y - Quartz monzonite
• D. X - Quartz syenite; Y - Granodiorite

Hint:

Normalise Q, A and P to 100 and use the quartz percentage with the A/(A+P) ratio to read the QAPF diagram.

Answer 1

The Correct Option is D

Step 1: Recall the IUGS QAPF method.
For coarse grained igneous rocks, the IUGS system plots the normalised percentages of quartz (Q), alkali feldspar (A) and plagioclase (P), taking Q + A + P as \(100\%\). Two numbers place a rock in the diagram: the quartz percentage Q, and the ratio A over (A + P), which tells how alkali feldspar rich or plagioclase rich the feldspar fraction is.

Step 2: Normalise rock X.
Rock X has quartz \(15\), plagioclase \(20\) and alkali feldspar \(65\), which already sum to \(100\). So Q = \(15\%\), and:
\[ \frac{A}{A+P} = \frac{65}{65+20} = \frac{65}{85} = 0.76 = 76\% \]

Step 3: Place rock X in the diagram.
With Q between \(5\) and \(20\%\), rock X falls in the quartz bearing fields. With A/(A+P) between \(65\) and \(90\%\), it falls in the quartz syenite column. So rock X is a quartz syenite.

Step 4: Normalise and place rock Y.
Rock Y has quartz \(25\), plagioclase \(60\) and alkali feldspar \(15\), again summing to \(100\). So Q = \(25\%\), and:
\[ \frac{A}{A+P} = \frac{15}{15+60} = \frac{15}{75} = 0.20 = 20\% \]
With Q between \(20\) and \(60\%\), rock Y sits in the true granitoid row. With A/(A+P) between \(10\) and \(35\%\), that row gives granodiorite. So rock Y is a granodiorite.

Step 5: Final conclusion.
X is a quartz syenite and Y is a granodiorite.
\[ \boxed{\text{X - Quartz syenite; Y - Granodiorite}} \]