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?
Which one of the following mineral reactions will result in the formation of a diagnostic granulite facies assemblage during regional metamorphism of a pelite?
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Granulite facies pelites are marked by the melt forming reaction that grows cordierite, garnet and K-feldspar together.
Updated On: Aug 14, 2026
- muscovite + staurolite + chlorite = biotite + kyanite + quartz + H2O
- muscovite + quartz = K-feldspar + sillimanite + melt
- biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + melt
- staurolite + quartz = garnet + sillimanite + H2O
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The Correct Option is C
Solution and Explanation
Step 1: Understand what marks granulite facies.
Granulite facies is the highest grade of regional metamorphism reached before melting takes over completely. Its pelitic rocks are marked by anhydrous, high temperature mineral assemblages formed through fluid absent (dehydration) melting, where a hydrous mineral breaks down and releases its water straight into a silicate melt instead of a free fluid.
Step 2: Check option (A).
Muscovite + staurolite + chlorite = biotite + kyanite + quartz + H2O releases a free water fluid and builds staurolite grade minerals. This is a garnet to staurolite zone reaction of upper greenschist to lower amphibolite facies, well below granulite grade.
Step 3: Check option (B).
Muscovite + quartz = K-feldspar + sillimanite + melt is the muscovite dehydration melting reaction. It happens near \(650\) to \(700\) degrees C and marks the start of crustal anatexis, at the amphibolite to granulite transition, but it is not yet the diagnostic high grade granulite assemblage.
Step 4: Check option (C).
Biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + melt is the biotite dehydration melting reaction, active at higher temperature (around \(800\) degrees C). It produces the classic cordierite plus garnet plus K-feldspar assemblage together with melt, which is exactly the textbook diagnostic mineral set of pelitic granulites.
Step 5: Check option (D).
Staurolite + quartz = garnet + sillimanite + H2O is a mid amphibolite facies dehydration reaction that destroys staurolite and grows garnet and sillimanite, still well below granulite conditions.
Step 6: Final conclusion.
Only the biotite dehydration melting reaction builds the cordierite, garnet and K-feldspar assemblage that defines granulite facies pelites.
\[ \boxed{\text{biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + melt}} \]
Granulite facies is the highest grade of regional metamorphism reached before melting takes over completely. Its pelitic rocks are marked by anhydrous, high temperature mineral assemblages formed through fluid absent (dehydration) melting, where a hydrous mineral breaks down and releases its water straight into a silicate melt instead of a free fluid.
Step 2: Check option (A).
Muscovite + staurolite + chlorite = biotite + kyanite + quartz + H2O releases a free water fluid and builds staurolite grade minerals. This is a garnet to staurolite zone reaction of upper greenschist to lower amphibolite facies, well below granulite grade.
Step 3: Check option (B).
Muscovite + quartz = K-feldspar + sillimanite + melt is the muscovite dehydration melting reaction. It happens near \(650\) to \(700\) degrees C and marks the start of crustal anatexis, at the amphibolite to granulite transition, but it is not yet the diagnostic high grade granulite assemblage.
Step 4: Check option (C).
Biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + melt is the biotite dehydration melting reaction, active at higher temperature (around \(800\) degrees C). It produces the classic cordierite plus garnet plus K-feldspar assemblage together with melt, which is exactly the textbook diagnostic mineral set of pelitic granulites.
Step 5: Check option (D).
Staurolite + quartz = garnet + sillimanite + H2O is a mid amphibolite facies dehydration reaction that destroys staurolite and grows garnet and sillimanite, still well below granulite conditions.
Step 6: Final conclusion.
Only the biotite dehydration melting reaction builds the cordierite, garnet and K-feldspar assemblage that defines granulite facies pelites.
\[ \boxed{\text{biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + melt}} \]
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