Lab 4 Metamorphic
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School
University of Arkansas, Fort Smith *
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Course
2803
Subject
Geology
Date
Apr 3, 2024
Type
Pages
6
Uploaded by MateOryxMaster1065
METAMORPHIC ROCKS LAB
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Learning Outcomes After completing this lab exercise, you should be able to: 1. Identify about 12 common metamorphic rocks. 2. Identify minerals in hand samples of metamorphic rocks. 3. Estimate metamorphic grade on the basis of metamorphic minerals and textures. 4. Identify likely protoliths for common metamorphic rocks. Metamorphic Rocks Metamorphism
takes place when a pre-
existing rock (
protolith or parent rock
), is subjected to conditions of pressure, temperature, or chemical environment that are very different from those in which the protolith formed. Any type of rock –
igneous, sedimentary, or metamorphic –
may be metamorphosed. Metamorphism is largely a solid-state process
, occurring at temperatures higher than those at which sedimentary rocks form, but not so high that melting occurs (Fig. 1). Metamorphism is commonplace along convergent plate boundaries (Fig. 2). Metamorphism can significantly change the composition and texture of a protolith. When subjected to new conditions, some minerals become chemically unstable and react with each other in the solid state to form new minerals (Fig. 3). Even when minerals remain stable, they often recrystallize and grow larger. Figure 2. Three distinct metamorphic zones along convergent plate boundaries: A = High-T Low-P contact metamorphism. B = Regional metamorphism over a range of P and T. C = High-P Low-T metamorphism along subduction zones. Letters correspond to A, B, and C in Figure 1. A B C Figure 1. Metamorphism occurs over a wide range of temperatures and pressures that are intermediate between sedimentary and igneous conditions. The large arrows represent three commonly recognized metamorphic “regimes”
that correspond to different tectonic settings. A) Contact metamorphism occurs around the contacts of shallow plutons at high temperatures but relatively low pressures. B) Regional metamorphism occurs at deeper levels in the thickening crust along convergent plate boundaries. C) High-pressure metamorphism happens to relatively cool sediment and oceanic crust that gets carried down subduction zones. Compare to A, B, and C in Figure 2. A B C Figure 3. Garnet porphyroblasts in mica schist. Garnet, staurolite, and sillimanite commonly grow during medium-to-high grade metamorphism of sedimentary rocks.
METAMORPHIC ROCKS LAB
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Foliated and Non-foliated Metamorphic Rocks The texture of metamorphic rocks is strongly dependent on pressure conditions. Tectonically active environments such as convergent plate boundaries often generate differential pressures
, where the pressure exerted on the rock is higher in one direction than another. Under these conditions, new and recrystallized minerals become preferentially oriented (Fig. 4), giving the rock a metamorphic foliation
(i.e. layering). Three common types of metamorphic foliation are slaty cleavage
, schistosity
, and gneissic layering
(Fig.5). Non-foliated
metamorphic rocks form under lithostatic pressures
(i.e. identical in all directions), so lack preferential orientation of new and recrystallized minerals. A common example of metamorphism under lithostatic conditions is burial metamorphism
, where older sediments are compressed under the growing weight of younger sediments deposited above. Some non-foliated metamorphic rocks, including hornfels, quartzite, and marble, are produced by contact metamorphism
of mudstones, quartz sandstone, and limestone, respectively, around igneous plutons. Contact metamorphism, also known as thermal metamorphism
, is driven primarily by elevated temperatures rather than increased pressures. Metamorphic Grade and Index Minerals Metamorphic rocks are often classified as low-, medium-, or high-grade
, where grade refers to the temperature level. As metamorphism progresses from low to high-grade, the extent of recrystallization and new mineral growth increases. A good example is the progressive metamorphism of shale
into low-grade slate, then medium-grade schist, and finally to high-grade gneiss (Fig. 6). Low-grade rocks resemble their protoliths, medium-grade rocks are extensively recrystallized, and high-grade rocks may be entirely recrystallized with no original textures or fossils. The mineral composition of a metamorphic rock is a good indicator of metamorphic grade, because certain minerals are only stable over limited ranges of pressure and temperature (Fig. 7). These metamorphic
index minerals
include chlorite (low-grade), staurolite (medium-grade), kyanite (high pressure), and sillimanite (high temperature), and to a lesser extent biotite, garnet, amphibole, and pyroxene. Quartz and feldspar are not useful index minerals because they are stable over the entire range of metamorphic P and T. Figure 5. Three types of metamorphic foliation. Slaty cleavage and schistosity are due to the parallelism of mica crystals. Gneissic layering is due to the separation of mafic and felsic minerals into alternating bands. slate schist gneiss Figure 4. Metamorphic foliation is created when old minerals recrystallize and new minerals grow under conditions of differential pressure. Recrystallization of old minerals and growth of new minerals Foliated metamorphic rock Direction of greatest pressure
METAMORPHIC ROCKS LAB
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Parent Rock Metamorphic Grade Mafic igneous Felsic igneous Limestone Sandstone Shale Coal Approximate T (°C) 200 400 600 800 Approximate Temperature Ranges where Minerals are Stable Chlorite Biotite Garnet Staurolite Kyanite Sillimanite Amphibole Pyroxene Quartz Feldspar Muscovite Figure 7. Gradational metamorphism of various parent rocks and temperature ranges of metamorphic index minerals. Greenschist contains abundant chlorite. NOTE: Minerals do not apply to coal, which is composed of organic material and at high temperatures is converted to graphite (carbon). Figure 6. Progressive metamorphism of shale. At low grades, clay minerals in shale react to form microscopic biotite and muscovite flakes in slate. With increasing grade, the mica crystals grow larger and are often joined by garnet and staurolite to form mica schist. At high grade, mafic and felsic minerals grow larger still and begin to separate into alternating bands to form coarse-grained gneiss. At still higher temperatures approaching melting, the rock becomes a swirly mess called migmatite.
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METAMORPHIC ROCK CLASSIFICATION CHART
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Names, Textures, and Protoliths of Common Metamorphic Rocks
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Metamorphic Rocks Pre-Lab Answer these questions before the lab. 1.
What is another name for metamorphic parent rock? 2.
Compare the pressure and temperature range of metamorphism to those for the formation of sedimentary and igneous rocks. 3.
Describe the nature of metamorphism associated with convergent plate boundaries. 4.
In what 2 ways does metamorphism change the composition and texture of a protolith? 5.
What are three common types of metamorphic foliation? 6.
What causes metamorphic foliation? 7.
Describe the pressure conditions under which burial metamorphism takes place. 8.
Where does contact metamorphism occur and what drives it? 9.
Define metamorphic grade. 10.
Describe the changes that take place and the metamorphic rocks that form during the progressive metamorphism of shale. 11.
What are metamorphic index minerals? 12.
Why are quartz and feldspar poor index minerals? 13.
If given a hand sample of metamorphic rock, how can you estimate its metamorphic grade? 14.
What is the metamorphic grade of a rock that contains abundant chlorite and muscovite? 15.
What is the metamorphic grade of a rock that contains abundant garnet and staurolite? 16.
What is the dominant mineral in ma
rble, and what is marble’s parent rock?
Metamorphic Rocks Data Sheet
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No. Texture (foliation and grain size) Minerals (refer to Fig. 7) Other Observations (luster, smoothness, platiness, color, waviness, folding, etc.) Name Grade L, M, H Protolith 1 2 3 4 5 6 7 8 9 10 11 12
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