Lab_6_Metamorphic_EXERCISE
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University of Washington *
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BEARTH
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Geology
Date
Apr 3, 2024
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Lab 6: Metamorphic Rocks and Processes Lab Section: _______ Group Members: ________________________________________________________ One answer sheet per group should be turned into your T.A. A.
Metamorphic Rock Composition 1.
Identify eight of the common metamorphic rock-forming minerals
(specimens M-1 through M-8). You identified many of these minerals in previous labs. Refer to your mineral identification charts (Tables A-1, A-2, and A-3) and the diagnostic properties of metamorphic rock-forming minerals (Table 6-1 in laboratory exercise). Specimen # Mineral Name Specimen # Mineral Name M-1 M-5 M-2 M-6 M-3 M-7 M-4 M-8 2.
Rock A (located at the front of the class) is a sedimentary rock that you have seen previously identified, called shale (this specimen is an oil shale). What is the mineralogical composition of Rock A? 3.
Look at rock R-5, a metamorphosed shale, in your tray. A larger hand-sized specimen is set up at the front of the class.
Use Figure 6-3 (refer to laboratory exercise), which shows the changes in mineral assemblage in a shale due to metamorphism, to assist in answering the following questions. (a)
Name two or more minerals in this metamorphic rock
. (b)
Use the minerals you have identified and Figure 6-3 to determine the likely range of pressures and temperatures at which this rock formed. List the range
of temperatures and the range
of pressures below (in °C and kilobars).
(c)
Do these minerals form under conditions of low-, medium-, or high-grade metamorphism? (d)
If the pressure during metamorphism dropped below 1 kilobar, what minerals would you expect to find in this metamorphic rock?
(Use Figure 6-3)
4.
Which would be more useful to infer the given information about a metamorphic rock: its mineral composition
or its chemical (elemental) composition
? Explain your answer
. (a)
the protolith of the rock before metamorphism? (b)
the pressure and temperature conditions experienced during metamorphism? B.
Metamorphic Rock Texture
5.
Identify the metamorphic textures of specimens R-2, R-4, and R-5
as “slaty cleavage”, “schistosity”, or “gneissic banding”. Interpret their grade of metamorphism
(“low”, “medium”, or “high”) based on the texture (Refer to Figure 6-5 in laboratory exercise). Specimen # Texture Metamorphic Grade R-2 R-4 R-5 6.
Specimens R-2, R-4, and R-5 have the same protolith (the same parent rock, Rock A). Arrange the specimens from lowest to highest metamorphic grade. What changes do you observe?
7.
S
ketch any evidence of high pressures you see present in R-4. Indicate what you think is the orientation of greatest of stress with arrows. 8.
Look closely at R-7, found in your tray. a)
Is it mainly comprised of lithic (rock) fragments or enlarged crystals?
b)
The protolith for R-7 is the sedimentary Rock B, located at the front of classroom. What is the protolith for R-7 named (name Rock B)? c)
Describe the changes that occur with metamorphism of Rock B to form R7.
C.
Identifying Metamorphic Rocks
A metamorphic rock may be classified by comparing its textural and mineralogical properties with the Metamorphic Rock ID Chart (Table 6-6 in laboratory exercise, page 10). Be aware that metamorphic rock types are texturally and mineralogically gradational. Therefore, the subdivisions between rock types are somewhat arbitrary. Use your Metamorphic Rock ID Chart (Table 6-6 in your laboratory exercise, page 10) to name the seven rock specimens in your tray.
1.
Identify the minerals present in each of the rock specimens in your tray (R-1 to R-7).
Write your answer on the Metamorphic Rock ID Chart (page 9) under “composition.” Along with the diagnostic minerals (Table 6-1), the composition may also include "clay minerals" and "rock fragments". 2.
Describe the texture of specimens R-1 through R-7 on your Metamorphic Rock ID Chart.
First determine if they are foliated or nonfoliated and write “F” or “NF”
in the appropriate column. Then describe their texture in the column labeled “texture”. i.
If they are nonfoliated, you can describe the texture as “crystalline”. ii.
If foliated, describe the texture as consisting of “slaty cleavage”, “schistosity”, or “gneissic banding”. Specimen # Composition Foliated (F) or Nonfoliated (NF) Texture Rock Name R-1 R-2 R-3 R-4 R-5 R-6 R-7 *Please note that we do not include a sample of phyllite
in the metamorphic rock collection for this lab. It is an intermediate grade of metamorphism between slate and schist. The microscopic minerals, such as, muscovite, chlorite and graphite give it a silky to golden sheen called the “phyllitic luster”
shown in Figure 6-5 in the Metamorphic Rocks and Processes information binder.
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9.
Use the metamorphic minerals present in the following rock specimens to determine a possible protolith
. Specimen # Protolith type R-1 R-3 R-6 10.
The protoliths of R-1, R-3, and R-6 are sedimentary in origin. While sedimentary rock texture may or may not be preserved through metamorphism, it is possible to infer parts of a rock’s pre-metamorphic history. List at least one depositional environment that each rock could have originally come from.
R-1: R-3: R-6:
D. Metamorphic Rocks and Plate Tectonics Most metamorphic rocks were probably formed in orogenic (mountain) belts at convergent plate margins. A subducting oceanic plate sets in motion a series of processes, each of which is related to one or more kinds of metamorphism. (1)
First, the subducting oceanic plate creates a trench that collects sediments eroding from the continent. These sediments get dragged down with the subducting plate and become exposed to increasing pressures. (A in Figure 6-6.) (2)
Second, the cold ocean plate and the "wet" sediments it carries begin to heat as they subduct. At a depth of about 120 km the “wet” sediments are hot enough to melt, releasing magma that rises into the overlying continental crust. (B in Figure 6-6.) (3)
Third, the heat from the rising magmas and the compression of the two plates coming together cause the crust to buckle upward and form a mountain range. Figure 6-6.
Zones of metamorphism associated with a convergent plate margin. The pressure and temperature conditions that define each of these zones are the result of the processes described above. Pressure increases with depth below the surface, and so does temperature. However, the subducting oceanic plate is cold,
and affects the temperature of the surrounding crust
. Magma bodies heat the crust locally
.
11.
Look at Rock C on the side table of the lab room. This rock is a "blueschist
”
.
Which region on Figure 6-6 is Rock C from? Explain why.
12.
Look at Rock D on the side table of the lab room. This rock is a "hornfels". Which region on Figure 6-6 is Rock D from? Explain why.
13.
Examine the rock labelled with a star at the front of the room. Describe the composition, texture, name, and protolith of the rock.
After identifying the rock, approximate the temperature and pressure conditions under which the rock formed. Discuss the geologic history of this rock, and include things such as the setting in which the original rock might have formed and the tectonic history which led to metamorphism of the rock.
Composition: Texture: Name: Protolith: Geologic History:
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