Question

The shaded region (P) in the given ACF diagram represents the compositional range of mafic rocks that have undergone granulite facies metamorphism. Which of the following equilibrium mineral assemblages is/are identified in these mafic rocks? 

• A. plagioclase + clinopyroxene
• B. plagioclase + clinopyroxene + orthopyroxene
• C. plagioclase + orthopyroxene + garnet
• D. plagioclase + garnet + cordierite

Hint:

When analyzing ACF diagrams, focus on identifying the key minerals that form under specific facies conditions, such as clinopyroxene and orthopyroxene in granulite facies.

Answer 1

The Correct Option is B

Step 1: Understand the ACF Diagram.
The ACF diagram (Al2O3, CaO, FeO) is a triangular plot used in metamorphic petrology to understand the equilibrium mineral assemblages in mafic rocks. In granulite facies metamorphism, high temperature and pressure conditions lead to the stabilization of specific mineral phases.
Step 2: Interpreting the Shaded Region.
The shaded region (P) in the ACF diagram corresponds to a specific range of mafic rock compositions in the granulite facies. This includes minerals like plagioclase, clinopyroxene, and orthopyroxene. These minerals are commonly formed in high-temperature metamorphic environments, such as the granulite facies.
Step 3: Evaluating the Options.
- Option (A): Plagioclase + clinopyroxene is typical for lower temperature metamorphism and is not a complete assemblage for granulite facies.
- Option (B): Plagioclase + clinopyroxene + orthopyroxene is the correct assemblage for granulite facies mafic rocks.
- Option (C): Garnet is not typically part of the granulite facies assemblage for mafic rocks, making this option incorrect.
- Option (D): Garnet + cordierite is more typical for pelitic rocks under different metamorphic conditions.
Thus, the correct answer is (B) plagioclase + clinopyroxene + orthopyroxene.
Final Answer: \[ \boxed{\text{plagioclase + clinopyroxene + orthopyroxene}} \]