Discussion
Two pyroxene granulites in which garnet is not present indicate :
Answer: A
Garnet-free two-pyroxene granulites indicate low pressure granulite metamorphism.
Granulites of mafic composition consist of orthopyroxene, clinopyroxene and plagioclase. As pressure increases, garnet appears by reaction of pyroxene and plagioclase, giving garnet-clinopyroxene granulites in the high-pressure part of the facies. The absence of garnet therefore points to the low-pressure part of the granulite facies.
High-pressure granulites are recognised precisely by garnet. Amphibolite facies lacks orthopyroxene, so a two-pyroxene rock is not amphibolite facies, and contact metamorphism at high temperature produces hornfelses (pyroxene-hornfels facies) rather than regional granulites.
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