Question

A pelitic rock consisting of cordierite + garnet + K-feldspar + sillimanite belongs to which one of the following metamorphic facies?

• A. Granulite
• B. Eclogite
• C. Greenschist
• D. Blueschist

Answer 1

The Correct Option is A

To determine the metamorphic facies of a pelitic rock consisting of cordierite, garnet, K-feldspar, and sillimanite, we need to understand the conditions under which these mineral assemblages are stable.

1. The granulite facies represents high-temperature and moderate to high-pressure metamorphic conditions. It is characterized by the presence of minerals such as orthopyroxene in mafic rocks, or, as in this case, the assemblage cordierite + garnet + K-feldspar + sillimanite in pelitic rocks.
2. Under granulite facies conditions, temperature typically exceeds approximately 700°C, and such conditions lead to dehydration reactions that produce these minerals in the rock.
3. Among the options provided:
   • Granulite: Suitable for the given mineral assemblage due to high-temperature conditions.
   • Eclogite: Generally associated with high-pressure, low-temperature conditions, typically containing omphacite and garnet in mafic rocks, not typical for the minerals provided.
   • Greenschist: Characterized by low-grade metamorphism, typically involving chlorite and actinolite, unsuitable for high-temperature minerals like sillimanite and K-feldspar.
   • Blueschist: Metamorphic facies defined by high-pressure, low-temperature conditions, with minerals like glaucophane, not typical for the given assemblage.

Therefore, based on mineral stability and metamorphic conditions, the correct answer is Granulite.