Question

Which of the following is/are true for crystallization of plagioclase phenocrysts from a basic magma forming a layered intrusion?

• A. Cumulus texture at the base
• B. Anorthite-rich early plagioclase at the base
• C. Albite-rich late plagioclase at the top
• D. Quench texture at the base

Hint:

In a layered intrusion, plagioclase crystallizes first as anorthite-rich and later as albite-rich, with the earlier layers being more calcium-rich.

Answer 1

The Correct Option is A, B, C

Step 1: Understanding crystallization in layered intrusions. 
In layered intrusions, plagioclase crystallizes from a basic magma as a function of temperature and pressure. Early crystallization forms anorthite-rich (calcium-rich) plagioclase at the base, while late-stage crystallization leads to albite-rich (sodium-rich) plagioclase at the top. 
 

Step 2: Analyzing the options. 
(A) Cumulus texture at the base: This is correct as cumulus texture refers to the texture formed by the settling of crystals.
(B) Anorthite-rich early plagioclase at the base: This is the correct answer as anorthite-rich plagioclase is the first to crystallize from a basic magma. 
(C) Albite-rich late plagioclase at the top: This is correct. 
(D) Quench texture at the base: This is incorrect because quench textures form rapidly from a cooling lava, not in the crystallization of plagioclase in a layered intrusion. 
 

Step 3: Conclusion. 
The correct answer is (A), (B) and (C).