Question

The following diagram represents the prograde sequence of metamorphic zones that develop during Buchan-type of metamorphism (<4 kbar) of typical pelites. Identify the zones labelled P, Q, and R. 

• A. P – Biotite zone; Q – Garnet zone; R – Kyanite zone
• B. P – Garnet zone; Q – Kyanite zone; R – Staurolite zone
• C. P – Biotite zone; Q – Cordierite zone; R – Andalusite zone
• D. P – Andalusite zone; Q – Biotite zone; R – Cordierite zone

Hint:

Buchan sequence (low-P, high-T): Chlorite → Biotite → Cordierite → Andalusite → Sillimanite.

Answer 1

The Correct Option is C

Step 1: Understanding Buchan-type metamorphism.
Buchan-type metamorphism occurs under low-pressure (<4 kbar) and high-temperature conditions. It typically develops in pelitic rocks during regional metamorphism associated with contact aureoles.
Step 2: Zone sequence.
The progressive mineral zones for Buchan metamorphism are:
Chlorite → Biotite → Cordierite → Andalusite → Sillimanite.
Step 3: Interpretation.
From the diagram:
- P: Next to chlorite zone → Biotite zone.
- Q: Intermediate → Cordierite zone.
- R: High-temperature → Andalusite zone.
Step 4: Conclusion.
Hence, the correct identification is P – Biotite zone; Q – Cordierite zone; R – Andalusite zone.