Review Questions for Chapter 25:
Metamorphic Facies and Metamorphosed Mafic Rocks
1. Pentii Eskola originally defined metamorphic facies on the basis of predictable mineral
assemblages that develop in metamorphosed (mafic) rocks and occur worldwide. How does a
modern approach to metamorphic facies differ?
2. Compare the concepts of facies and facies series.
3. What are the three common types of facies series, and what sequence of facies occurs along each
series?
4. To what tectonic situations might you attribute each facies series?
5. What mineral or minerals impart the green color to greenschists?
6. What is the composition range of plagioclase in the greenschist facies?
Where does Ca from the original igneous plagioclase (and clinopyroxene) go?
7. What are the two important mineral changes that accompany the transition from greenschist facies
to amphibolite facies in mafic rocks?
Which change occurs first at higher pressure?
Which change occurs first at lower pressure?
Show how this variation is possible on a sketched P-T phase diagram.
8. Sketch an ACF diagram for mafic rocks in the amphibolite facies. What two coexisting minerals
serve to define amphibolite facies mafics? Why does garnet occur in some amphibolite facies mafic
rocks and diopside in others?
9. What do minerals of the granulite facies typically have in common?
10. What mineral is diagnostic of the blueschist facies in mafic rocks?
11. What mineral’s absence is diagnostic of the eclogite facies in mafic rocks? Why is it missing and
what replaces it?
12. What minerals are characteristic of UHP metamorphism?
In what context are they typically found/preserved (on the microscopic scale)? Why?
13. What principal tectonic implications do the 20+ UHP localities have for crustal behavior?
14. What is UHT metamorphism, and what mineralogical evidence is there for it?
15. Describe the tectonic–petrologic evolution of a rock along a typical clockwise P-T-t path.
16. Describe the tectonic–petrologic evolution of a rock along a typical counterclockwise P-T-t path.
17. How does the sequence of pressures and temperatures along a typical metamorphic field gradient
differ from the sequence along a typical clockwise P-T-t path? Explain.