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: Turn the table into two simple ratios.
For each rock, work out how much of the feldspar is alkali feldspar rather than plagioclase, and note the quartz percentage separately, since the QAPF diagram is really just these two numbers plotted against each other.

Step 2: Work rock X.
X has $65$ parts alkali feldspar against $20$ parts plagioclase, out of $85$ total feldspar. That is about $76\%$ alkali feldspar in the feldspar mix, a strongly alkali feldspar dominated rock. Its quartz sits at $15\%$, a modest amount, enough to earn the "quartz" prefix but not enough to call it a true granitoid. That combination names it quartz syenite.

Step 3: Work rock Y.
Y has $15$ parts alkali feldspar against $60$ parts plagioclase, out of $75$ total feldspar, so only about $20\%$ alkali feldspar, meaning plagioclase clearly dominates. Its quartz sits higher at $25\%$, which is squarely in the granite family range. A plagioclase dominated feldspar mix with granite range quartz is the textbook granodiorite.

Step 4: Compare against the choices.
Only the pairing "quartz syenite for X, granodiorite for Y" matches both ratios worked out above; the other three choices mismatch either the quartz amount or the feldspar ratio for at least one rock.
\[ \boxed{\text{X - Quartz syenite; Y - Granodiorite}} \]