Question

In the AFM diagram below, W, X, Y and Z sepresent four pelitic assemblages. Which combinations of assemblages contain the same mineral phases in equilibrium in the staurolite zone?

• A. (X-Y) & (W-Z)
• B. (X-W) & (Y-Z)
• C. (X-Z) & (Y-W)
• D. (W-X-Y) & (Z)

Answer 1

The Correct Option is B

To determine which combinations of assemblages (W, X, Y, and Z) contain the same mineral phases in equilibrium in the staurolite zone, we need to analyze the AFM diagram provided.

The AFM diagram is a ternary plot used to represent the phase relations between minerals in metamorphic rocks. It is particularly useful to study pelitic (clay-rich) assemblages in metamorphic petrology.

1. Identify the mineral phases present in each assemblage point from the diagram:
   • W and Z: These assemblages lie on the line where garnet is present with biotite and staurolite.
   • X and Y: These assemblages lie in the staurolite-chlorite field where garnet is absent.
2. Understand the equilibrium in the staurolite zone:
   • In the staurolite zone, the minerals commonly present can include garnet, staurolite, biotite, and chlorite, among others, depending on the precise PT conditions.
   • Based on the AFM diagram, X and W will share the same assemblage of mineral phases because X and W are in similar mineral phase fields. Similarly, Y and Z will share the same assemblage of mineral phases.
3. Conclusion and Answer Selection:
   • The combination of assemblages (X-W) and (Y-Z) shares the same minerals in equilibrium within the staurolite zone, based on their positions on the AFM diagram.
   • Thus, the correct answer is: (X-W) & (Y-Z).