Question

The AFM diagram given below shows stability of minerals in the garnet zone. If P, Q, R and S represent the compositions of different pelitic rocks, which of the following is/are characterised by the equilibrium assemblage of muscovite + garnet + biotite + quartz?

• A. P
• B. Q
• C. R
• D. S

Answer 1

The Correct Option is A, C

The AFM diagram is used to understand the stability of minerals in different zones of metamorphic rocks, particularly pelitic rocks. In the given AFM diagram, we have to identify the compositions characterized by the equilibrium assemblage of muscovite, garnet, biotite, and quartz.

Understanding the Diagram: 

• The diagram is a ternary plot showing different mineral assemblages under specified conditions of pressure and temperature.
• The assemblage of muscovite, garnet, biotite, and quartz forms a typical characteristic in higher-grade metamorphic zones such as the garnet zone.

Based on the diagram:

• P and R fall within the garnet stability field which is expected to coexist with muscovite and biotite based on the assemblage given in the question.
• Q and S fall within the chlorite or other fields not aligned with the mentioned assemblage.

Conclusion:

Thus, the options characterized by the equilibrium assemblage of muscovite + garnet + biotite + quartz are P and R.