Review Questions and Problems for Chapter 23:
Structures and Textures of Metamorphic Rocks
1.
What is the difference between rock texture and rock structure?
2.
Imagine the mineral albite experiencing sufficiently high temperature that it breaks down to
jadeite (new) + quartz (already present). Write the reaction and describe what occurs at the
atomic scale, using the four steps introduced at the beginning of this chapter.
3.
How does cataclastic flow differ from intracrystalline plastic deformation?
4.
What causes strain hardening?
5.
What is recovery, and what processes aid it?
6.
What is recrystallization, and what processes aid it?
7.
Explain how deformation might enhance the prospects of recovery and recrystallization.
8.
Sketch a thin section view of porphyroblastic and poikiloblastic texture in a groundmass of
granoblastic polygonal quartz.
9.
What is the difference between a porphyroblastic, a porphyroclastic, and a blastoporphyritic
feldspar in a metamorphic rock?
10.
Sketch an S-C texture with sinistral shear and explain how it can be used to determine the sense
of shear.
11.
Sketch a -type mantled porphyroclast with sinistral shear and explain how it can be used to
determine the sense of shear.
What would be the sense of shear if the porphyroclast itself were rotated and not the result of
dragging the tails in the foliation? Explain.
12.
Find a sense-of-shear indicator in Figure 23.18 and t4race or sketch it. Does it indicate dextral or
sinistral shear? Explain.
13.
Sketch a P-T-t path (see Figure 16.19) for the burial, heating, uplift, cooling and exposure of a
polyphase orogenic metamorphic event with two thermal climaxes.
Can you imagine a plate tectonic process that might be able to produce this?
14.
Briefly describe the various types of foliations (including three primary/inherited types).
15.
In what ways can fabric elements become aligned in a metamorphic foliation?
16.
Briefly describe the various types of tectonic/metamorphic lineations.
17.
What determines whether a lineation or a foliation develops?
18.
How might an originally homogeneous rock develop gneissose layering?
19.
What is S0?
What is S1?
What is S2?
What is L1?
20.
If Si in a poikiloblast does not lead continuously into Se, what can you say about the duration of
deformation versus poikiloblast growth? Why?
21.
What is the relationship between deformation and crystallization in a continuous schistosity?
22.
How many deformational phases are indicated by a polygonal arc?
With respect to deformation, when did crystallization of the arc mineral occur? Explain.
23.
Describe four examples of clearly syn-kinematic porphyroblast growth. Explain why each is so.
24.
Redraw Figure 23.45, interpreting the spiral in garnet (Figure 23.44) as a second
deformational/metamorphic event.
25.
What is a major advantage of TIMS over conventional methods of isotopic analysis?
What is a major advantage of UV-LA-ICPMS and IMP over TIMS?
26.
What are the desirable properties of a mineral for textural dating? Cite an example for each
quality from the list of studies provided, if available.
27.
Use the average duration of growth and rate of growth to estimate the diameter of the garnets in the
study of Christensen et al (1989) in Section 23.7.1.
Problems.
1. Try to diagram the deformation/crystallization history of the six poikiloblasts in Figure 23.37.
What controversy exists as to whether one or two deformational events are involved?
2. Discuss the deformation/crystallization history of the garnet porphyroblast shown in a muscovite,
biotite, quartz schist. Chart the deformation–crystallization history. Support your conclusions with
evidence.
3. Describe the textures that you see in Figure 23.46, interpret the metamorphic crystallization–
deformation history, and create your own diagram that depicts your interpretation.
4. Suppose you began with a mole of uranium in average crustal proportions listed in Section 9.7.2.4
and half a mole of thorium. Use the decay constants in that section to calculate the proportions of
Pb isotopes you would expect in the same sample after 1 Ga. Why might this free you from having
to do isotopic analysis to determine a mineral age? What other assumptions would this require?