GEOL 320-Metamorphism
NAME ________________________
Rocks and Minerals
Lab 8: Isograds, Index Minerals, Facies Series, P-T-t paths, and other
Fun Topics
DUE DATES
At the beginning of lab next week.
OBJECTIVES
 To define how protolith and facies impact mineral assemblages
 To review and analyze the following concepts: index mineral, isograd, facies, metamorphic
facies series/metamorphic field gradient
 To determine how metamorphism impacts texture
 To explain P-T-t paths
 To define equilibrium and metastable state
ASSIGNMENT (ANSWER QUESTIONS DIRECTLY ON LAB.
ATTACH SEPARATE SHEETS.)
IF NECESSARY
1. [12 points] List four of the main protolith types for metamorphic rocks and contrast the minerals one would find
if each of these protoliths was metamorphosed to the greenschist facies.
PROTOLITH 1:
PROTOLITH 2:
PROTOLITH 3:
PROTOLITH 4:
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GEOL 320-Metamorphism
2. For the four main protoliths defined in your text, answer the following questions assuming that the relevant
metamorphic facies is greenschist. It may help to refer to the mineral formulae in your text, found on pgs. 362-363.
a. [3 points] Explain why actinolite is more likely to form from a mafic protolith whereas tremolite is
more likely to form from an ultramafic protolith.
b. [2 points] Why is muscovite found in metamorphosed mudrocks (pelites)?
c. [2 points] Why is calcite only found in metamorphosed calcareous rocks?
d. [5 points] What evidence is there that water-rich fluids are typically present either during formation of
these protoliths and/or during greenschist metamorphism?
3. This question is designed to allow you to contrast the conditions of amphibolite vs. eclogite facies
metamorphism. Answer the questions below.
a. [4 points] Describe the differences in P-T conditions for these two facies. Be specific—give P and T
ranges for both.
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GEOL 320-Metamorphism
b. [4 points] For the mafic protolith, list the typical minerals and their mineral formula found in these two
facies:
c. [4 points] With increasing metamorphic grade, the minerals typically dehydrate. What is the evidence
for this?
4. [8 points] There is a sequence of facies that includes hornfels and sanidinite (see Figure 17-4). Briefly describe
how metamorphism occurs in these facies, and contrast the textures of typical rocks in these facies vs. those found
in the sequence slate gneiss that metamorphose over the P-T range from greenschist to granulite.
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GEOL 320-Metamorphism
5. Refer to Figure 17-2 in your text (pg. 342) and associated reading and class notes. Based on these, answer the
following questions.
a. [4 points] Define index mineral and isograd.
b. [4 points] Figure 17-2 illustrates the Barrovian Zones. Describe what these zones are and why they are
important.
c. [6 points] Figure 1 is a very simple representation of a metamorphic map. Based on the information
given, draw in appropriate isograds for the index minerals garnet, staurolite, kyanite, sillimanite. One rock
specimen represents each metamorphic zone. Describe how P and T change in this terrane and where one
would expect to find the highest grade metamorphic rocks and why. Hand in figure 1 with your lab.
Based on your examination of the map and hand specimens, identify the map symbol corresponding to
each index mineral: _____garnet, _____staurolite, _____kyanite, _____sillimanite.
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GEOL 320-Metamorphism
6. Based on our discussions in class and your reading, answer the following questions about facies series (the
equivalent “modern” term is metamorphic field gradient).
a. [3 points] Define facies series/metamorphic field gradient.
b. [4 points] List two examples of facies series and describe the differences between the two. Be specific
about the differences.
c. [3 points] Explain the importance of the concept of facies series/metamorphic field gradient.
7. An important consideration in understanding metamorphism involves the path that a particular rock package takes
during its full metamorphic history. Because time is involved, this path is commonly referred to as a P-T-t
(Pressure, Temperature, time) path. Refer to pages 348-350 and Figure 17-10 in your text.
a. [3 points] Identify the three phases in a typical regional metamorphic P-T-t path
b. [9 points] For each of these phases, and describe the P-T changes that occur and explain why the P-T
path looks the way it does.
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GEOL 320-Metamorphism
9. [8 points] One of the important processes that occurs during metamorphism is recrystallization. Based on your
reading, describe what happens to crystals when they are exposed to higher temperatures during metamorphism. In
your answer, include a discussion of WHY this change occurs and HOW the change occurs. Be specific.
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GEOL 320-Metamorphism
10. We have recently discussed the concepts of metamorphic assemblages and reactions. Define the following
terms:
a. [3 points] equilibrium
b. [3 points] metastable state
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GEOL 320-Metamorphism
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