Question

Which one of the following mineral reactions will result in the formation of a diagnostic granulite facies assemblage during regional metamorphism of a pelite?

• A. muscovite + staurolite + chlorite = biotite + kyanite + quartz + H₂O
• B. muscovite + quartz = K-feldspar + sillimanite + melt
• C. biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + melt
• D. staurolite + quartz = garnet + sillimanite + H₂O

Hint:

Granulite facies pelites are marked by the melt forming reaction that grows cordierite, garnet and K-feldspar together.

Answer 1

The Correct Option is C

Step 1: Rank the four reactions by grade.
Metamorphic reactions in pelites follow a rough temperature ladder: chlorite and staurolite zone reactions sit at the low end, sillimanite and melt bearing reactions sit at the high end. Placing the four options on this ladder helps pick the granulite facies one.

Step 2: Place option (A) on the ladder.
Growing biotite, kyanite and quartz from muscovite, staurolite and chlorite, with plain water released, belongs low on the ladder, around the staurolite zone. Too cool for granulite.

Step 3: Place option (D) on the ladder.
Staurolite breaking down to garnet and sillimanite with water is a step higher, in the upper amphibolite zone, but it is still a fluid releasing reaction, not a melt forming one.

Step 4: Compare options (B) and (C).
Both make melt, so both sit near the top of the ladder. Option (B), muscovite breaking down with quartz, happens first as temperature rises and simply marks the onset of melting. Option (C), biotite breaking down with sillimanite and quartz, happens at a distinctly higher temperature and builds cordierite, garnet and K-feldspar together, the mineral trio geologists use as the fingerprint of a granulite facies pelite.

Step 5: Pick the top rung.
Since granulite facies is the highest grade before the rock is dominated by melt, the biotite dehydration melting reaction sits highest on the ladder and gives the diagnostic assemblage.
\[ \boxed{\text{biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + melt}} \]