Lab 5 Metamorphic Rocks-4
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Oregon State University, Corvallis *
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Course
201
Subject
Geology
Date
Dec 6, 2023
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9
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GEO 201 Physical Geology Page 1 of 9 Lab 6: Metamorphic Rocks GEO 201 Physical Geology Name: _____Grace Heassler______ Lab 6
–
Metamorphic Rocks (25 points) Lab Date/Time
__Thursday 11am_____ Learning Outcomes •
Describe grain size of metamorphic rocks (exercises 1 and 3) •
Identify and describe type of foliation, if present (exercises 1 and 3) •
Identify common minerals in metamorphic rocks (exercises 1 and 3) •
Name metamorphic rocks using observations of grain size, foliation, and minerals present (exercises 1 and 3) •
Identify protoliths of different types of metamorphic rocks (exercise 1) •
Identify metamorphic grades produced by different pressure and temperature conditions (questions 1-3) •
Identify index minerals indicative of different metamorphic facies (exercise 3) •
Identify metamorphic facies produced by different pressure and temperature conditions or different types of geothermal gradients (questions 4, 7) •
Interpret tectonic setting producing different geothermal gradients and different metamorphic grades and facies (questions 5-6) •
Infer history of tectonic collision and orogenesis from observed distribution of metamorphic grades and facies (questions 8-9)
GEO 201 Physical Geology Page 2 of 9 Lab 6: Metamorphic Rocks Exercise 1 –
Identifying Metamorphic Rocks (12 points –
2 points per sample) Procedure: Describe each metamorphic rock sample labeled 1-6 by making careful observations of its characteristics in the following table. 1.
Begin by describing the size of the mineral grains
(microscopic, small but visible, or large and easy to see). 2.
Next, describe the foliation
of the sample, if present. Types of foliation include: •
Slatey cleavage: rock will split into thin planar layers due to alignment of platy minerals) •
Schistosity: wavy texture imparted as platy minerals are crowded by growing quartz and feldspar minerals. Schistosity may be subtle or conspicuous, depending on the size of the platy mica minerals •
Gneissic banding: platy and needle-shaped mineral grains, which tend to be dark, are crowded into bands by large and growing block-shaped crystals of light colored quartz and feldspars. This segregates the minerals into mafic and felsic bands. •
If no foliation is present, describe the rock as non-foliated. 3.
Identify any specific minerals
that you think are present. Common metamorphic minerals include quartz, muscovite, biotite, and feldspar, which we have studied in lab before. Some metamorphic rocks may be composed entirely of calcite, in which case the rock is soft (hardness 3). Others are mostly of quartz, and show physical properties associated with quartz such as high hardness (7), concave faces and sharp edges (conchoidal fracture). Minerals unique to metamorphic rocks include garnets (blocky shape, usually red in color) and chlorite (a platy mica mineral that is green in color). 4.
Once you have examined each rock, name each rock
based on the characteristics you observed. Refer to your lecture notes for help. 5.
Finally, identify the rock’s likely protolith
(s) (parent rock)
based on the minerals present in the rock, which are controlled by the conditions of metamorphism and the available minerals in the parent rock. Refer to the figure below for help.
GEO 201 Physical Geology Page 3 of 9 Lab 6: Metamorphic Rocks Description and Naming of Metamorphic Rock Samples (12 points -two points per sample) Sample Size of Mineral Grains Foliation Minerals Present Rock Name Protolith 1 Microscopic Slate cleavage Quartz, chlorite, muscovite slate shale 2 Large/ very easy to see N/A calcite marble limestone 3 Large/ very easy to see Compositional and segregation banding Quartz, amphiboles, feldspars gneiss granite 4 Small/ hard to see schistosity Quartz, kyanite, muscovite phyllite shale 5 Large/ very easy to see N/A quartz quartzite sandstone 6 Large/ very easy to see Compositional schistosity Quartz, garnet, biotite, sillimanite schist shale
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GEO 201 Physical Geology Page 4 of 9 Lab 6: Metamorphic Rocks Exercise 2 - Grades and Facies of Basalt Metamorphism (8 points) Metamorphism of basalt produces a series of metamorphic rocks that form under different sets of pressure and temperature conditions (
grades
), each with a distinctive mineral assemblage (
facies)
,
that is stable under a given set of pressure and temperature conditions. In the case of basalt metamorphism, the resulting rocks are given the same name as their facies. Metamorphism of basalt under different geothermal gradients will produce a different series of rocks. The rock samples labeled A-E were all produced by metamorphism of basalt. Their bulk chemistry is similar, yet they have distinct assemblages of mineral types (facies) produced under different pressure and temperature conditions. Describe each metamorphic rock sample labeled A-E by making careful observations of its characteristics in the following table. 1.
Begin by describing the size of the mineral grains
(microscopic, small but visible, or large and easy to see). 2.
Next, describe the foliation
of the sample, if present. In basalt metamorphism, the presence and/or type of foliation depends on the types of minerals present (facies), rather than the grade (intensity of metamorphism). •
If platy minerals are present, the rock will show schistosity. •
If a mix of mafic (amphibole/pyroxene) and felsic (quartz, feldspar) minerals are present, the rock may show banding similar to gneiss. •
If only blocky minerals are present, no foliation will be present. 3.
Identify any specific minerals
that you think are present and are characteristic of that facies (index minerals). Common minerals produced by basalt metamorphism include chlorite (platy, green), glaucophane (platy, blue to steel-gray), garnet (blocky, red), pyroxenes (blocky, green to black), amphibole (needle-like, black), epidote (blocky to rod-like, dark green), quartz, and plagioclase feldspar. 4.
Once you have examined each rock, name the rock
based on the characteristics you observed. Refer to your lecture notes for help.
GEO 201 Physical Geology Page 5 of 9 Lab 6: Metamorphic Rocks Description and Naming of Rock Samples Produced by Basalt Metamorphism (5 points –
one points per sample) Sample Foliation Size of Mineral Grains Index Minerals Rock Name A None Visible/ easy to see garnet eclogite B Indistinct Visible/ easy to see Plagioclase, amphibole amphibolite C Schistosity Visible/ very hard to see glaucophane blueschist D Banding Visible/ easy to see Plagioclase, garnet, pyroxene granulite E Schistosity Visible/ very hard to see chlorite greenschist
GEO 201 Physical Geology Page 6 of 9 Lab 6: Metamorphic Rocks A high geothermal gradient describes a rapid increase in temperature with depth, and is found in tectonic settings where magma is intruding to shallow depths and low pressures. Conversely, a low geothermal gradient describes a more rapid increase in pressure than temperature, and is found in tectonic settings where rocks are being compressed and squeezed without increasing in depth. An intermediate geothermal gradient describes concomitant increases in both temperature and pressure, and is found in regions of thickening continental crust associated with mountain building. Use the graph of metamorphic facies (below) to answer the following questions. Figure 2. Metamorphic facies 4. Which of the three geothermal gradients (high, normal, or low; see figure in Exercise 2) will produce the following series of rocks by metamorphism of a basalt protolith? (1 point) a) Blueschist, Eclogite- low b) Greenschist, Amphibolite, and Granulite- normal 5. Describe the tectonic setting (be as specific as you can) that would be expected to produce a low geothermal gradient (high pressures but low temperatures) and hence blueschist and eclogite facies. Hint: These conditions would occur where rocks are being squeezed without being dragged down to greater depth (If they did, they would experience higher temperatures, too). (1 point) Subduction zones would produce low geothermal gradient, because of the low temperatures but high pressure. 6.
In what tectonic setting would you expect hornfels to form? Explain why. (1 point) Hornfels are likely to form in volcanic arcs which have conditions of shallow depth, high temps, and low pressure.
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GEO 201 Physical Geology Page 7 of 9 Lab 6: Metamorphic Rocks Exercise 3 - Regional Metamorphic Grades (5 points) An orogeny
is a period of intense crustal deformation (folding, faulting, and metamorphism), usually caused by continental collisions. The tectonic history of New England was punctuated by three such events: the Taconic Orogeny (450 Ma –
million years ago-), followed by the Acadian Orogeny (410 Ma), and finally the Alleghenian Orogeny (300 Ma). During each orogeny, rocks were pushed together and deeper into the Earth, where they encountered higher temperatures and pressures producing metamorphism. Over time, uplift and erosion has exposed rocks that were once several kilometers deep at the surface during their metamorphism. 7. The map on the last page shows the distribution of different metamorphic facies now exposed at the surface in New England and provides clues to the conditions these rocks experienced during metamorphism.(2 points) a) Which of the facies shown on the map corresponds to the highest metamorphic grade (highest pressures and temperatures of metamorphism)? granulite b) According to Figure 2, approximately what temperature and pressure did the rocks from this facies experience as they were being metamorphosed? 0-1200 MP pressure 600-900 degrees celcius c) According to Figure 2, how deep were these rocks during metamorphism? 0-30 kilometers
GEO 201 Physical Geology Page 8 of 9 Lab 6: Metamorphic Rocks 8.
The landmasses that collided with North America during these orogenies came in from the southeast. Therefore, the oldest area of metamorphism in this region is to the north and west, and younger episodes of metamorphism affected rocks to the south and east. Each orogeny resulted in a distinct belt of metamorphism with multiple facies; the oldest to the northwest and the youngest to the southeast. Much like a series of cars rear-ending each other in a chain reaction collision. (2 points) a)
The oldest orogeny (the Taconic orogeny) primarily affected rocks in what state(s)? What was the highest metamorphic facies produced in this orogeny? New York, Massachusetts, Connecticut, Vermont. In this orogeny the highest facies produced is amphibolite b)
The next orogeny (the Acadian orogeny primarily affected rocks in what state(s)? What was the highest metamorphic facies of this orogeny? New Hampshire, Massachusetts, Maine granulite c)
The youngest orogeny (the Alleghenian Orogeny) primarily affected rocks in what state(s)? What was the highest metamorphic facies of this orogeny? Rhode Island and Massachusetts amphibolite 9.
Which of these orogenies involved a collision with the largest land mass? Why? Hint: which orogeny produced the highest pressures over the largest area, as shown by the distribution of metamorphic facies?(1 point) Acadian orogeny as it has high pressure and the largest area of granulite.
GEO 201 Physical Geology Page 9 of 9 Lab 6: Metamorphic Rocks
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