Question

In metabasitic rocks, plagioclase is not stable in

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

Hint:

In eclogite facies, the high pressure and temperature conditions lead to the breakdown of plagioclase into pyroxene and garnet.

Answer 1

The Correct Option is D

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: Plagioclase is unstable in granulite facies, where high-temperature conditions cause it to break down into other minerals like orthopyroxene.
(B) epidote amphibolite facies: In this facies, plagioclase is stable and often found along with amphibole.
(C) amphibolite facies: Plagioclase is stable in amphibolite facies, particularly in the presence of hornblende.
(D) eclogite facies: Correct — Plagioclase is unstable in eclogite facies, where it is typically replaced by minerals like pyroxene and garnet.
Step 3: Conclusion.
The correct answer is (D) eclogite facies, as plagioclase is not stable in eclogite facies, where high pressure and temperature conditions favor the formation of other minerals.