Question

In metabasitic rocks, plagioclase is not stable in

• A. granulite facies
• B. epidote amphibolite facies
• C. amphibolite facies
• D. eclogite facies

Hint:

Granulite facies represent high-temperature metamorphism, where minerals like plagioclase break down into other phases such as orthopyroxene.

Answer 1

The Correct Option is A

Step 1: Understanding the question.
The question is asking about the stability of plagioclase in different metamorphic facies in metabasitic rocks.
Step 2: Analyzing the options.
(A) granulite facies: Correct — Plagioclase is unstable in granulite facies, where high-temperature conditions lead to the breakdown of plagioclase into other minerals like orthopyroxene.
(B) epidote amphibolite facies: In this facies, plagioclase is stable and often present along with amphibole.
(C) amphibolite facies: Plagioclase is stable in amphibolite facies, especially in the presence of hornblende.
(D) eclogite facies: Plagioclase can exist in eclogite facies but may be unstable in certain conditions, typically replaced by pyroxene and garnet.
Step 3: Conclusion.
The correct answer is (A) granulite facies, as plagioclase is unstable under the high temperatures of granulite facies.