GEO1115_2021_Activity10.
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Geology
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Dec 6, 2023
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THEME 10 – METAMORPHIC ROCKS [ 40/45 : A ]
A
CTIVITY
10.2S – T
ECHNICAL
QUESTIONS
( /45)
Reference material:
-
Lecture of associated theme (Brightspace)
-
Glossary (
https://mysite.science.uottawa.ca/dumas/
)
-
Reference document “Section of the lab manual” in Brightspace
Activity:
1.
Refer to the figure below to answer the following questions about metamorphic facies and grade. (
/14)
(see questions next page…)
If a rock contains the following minerals: quartz, muscovite, biotite, garnet…
a)
To which metamorphic facies and metamorphic grade does this assemblage of minerals correspond? ( /2)
Amphibolite facies, Intermediate grade
b)
What is the name of the rock formed by this assemblage of minerals? ( /1)
Schist
c)
According to this table, what would be the protolith of this rock (give a rock name), and what is the rock type of this protolith (igneous, sedimentary, or metamorphic)? ( /2)
Amphibole, Igneous
d)
Estimate the conditions of pressure (value and units) and temperature (value and units) of formation of this rock. Answer by providing ranges of values. ( /2)
Pressure: 1.5 bar to 7 bar
Temperature: 500°C to 650°C
e)
To what depth (in kilometers) does the pressure interval given in the previous question ("d)") correspond? Here again, provide a depth interval (in km). ( /1)
5.55km to 25.9km
f)
Referring to the course notes, indicate to what type of metamorphism the term "Barrovian" corresponds according to the nomenclature used in the course notes. ( /1)
Dynamo-thermal metamorphism
g)
Would the rock produced in this context exhibit foliation? Justify your answer. ( /2)
Yes because this kind of metamorphism utilizes differential pressure which results in foliation.
h)
If we replaced muscovite in the assemblage of minerals listed in "a)" with sillimanite, what would be the implications on the name
of the rock formed and the associated metamorphic grade
and facies
? ( /3)
If you replaced the muscovite with sillimanite it would likely instead make gneiss, which corresponds with granulite facies and is of high metamorphic grade.
2.
Refer to the figure below to answer the following questions about index minerals and metamorphic grade. ( /8)
a)
Evaluate the metamorphic grade and temperature of formation of a rock containing the following minerals: quartz, feldspar, garnet, and sillimanite. ( /2)
With this combination it would fall under high grade with a temperature of about 700-800°C.
b)
Evaluate the metamorphic grade and temperature of formation of a rock containing instead the following minerals: quartz, feldspar, garnet, and chlorite. ( /2)
c)
With this combination it would fall under intermediate grade with a temperature of about 500°C.
d)
Which rock (the one described in "a)" or the one described in "b)") represents a better geothermometer (i.e. the one that most accurately indicates the conditions under which a rock was formed)? Justify your answer. ( /2)
The rock in “b)” is a better geothermometer since there is a much smaller interval in which this combination of minerals all metamorphize therefore gives a more precise estimate for the conditions under which it would have formed.
e)
Is quartz a good index mineral? Justify your answer. ( /2)
No since it metamorphizes at both low and high grade conditions.
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3.
Refer to the figure below to answer the following questions. ( /8)
a)
Based on the tectonic context illustrated in the figure above, what two main types of metamorphism would you possibly expect to find in the space occupied by the granulite facies? Justify your answer. ( /4)
Contact metamorphism due to the presence of the heat from the magma which will cause this kind of metamorphism. Dynamo-thermal metamorphism due to this taking place on a convergent boundary of tectonic plates which is where this type of metamorphism is commonly found.
b)
What metamorphic grade corresponds to the granulite facies
? According to this figure, what are the factors contributing to achieving this metamorphic grade? In other words, why do we find this metamorphic grade at this precise location on this figure? ( /3)
Granulite facies corresponds to a high metamorphic grade. In this figure specifically the high metamorphic grade corresponds to the extremely high temperatures from the close proximity to
the magma.
c)
What will happen to the rocks if the P and T conditions continue to increase beyond the granulite facies conditions? ( /1)
They will exceed the conditions the rocks can handle and remain in solid form and so will instead
melt eventually becoming igneous rocks when cooling off and hardening again.
4.
Refer to the figure below to answer the following questions. ( /9)
a)
What type of metamorphism is illustrated by the letter “B” in this diagram? ( /1)
Burial metamorphism
b)
Would you expect the metamorphic rocks produced in the context shown in the diagram above to be foliated? Justify your answer. ( /2)
No because burial metamorphism is low and uniform pressure which does not cause foliation.
c)
Assuming that the protolith is a sedimentary rock, would it be possible to preserve the characteristics of this rock (e.g. stratification) following this type of metamorphism? Justify your answer. ( /2)
Yes as it occurs at low temperature and pressure therefore is able to preserve some original characteristics of the rock
d)
If the protolith was a quartz sandstone, what would be the metamorphic rock produced at the letter “B” in the diagram above? Also, indicate an observable change that would allow you to say
that this rock has undergone metamorphism (i.e. it is no longer a sedimentary rock). ( /2)
Quartzite. When you break quartzite it will break through the grains, as opposed to breaking a quartz sandstone which will break around the grains instead.
e)
Draw a simple version of the P-T diagram and mark on your diagram the area representing this type of metamorphism. ( /2)
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5.
Can a metamorphic rock have a foliation even if it does not contain any flat mineral (e.g. mica)? Justify your answer. ( /2)
No because if it doesn’t contain flat minerals the minerals will not align and layer in order to create foliation.
6.
Briefly explain the effect of the presence of fluids (water) during metamorphic reactions and indicate
the different origins of these fluids. Then, indicate whether these fluids play an important role in the final composition of the resulting metamorphic rock: if so, explain how/why these fluids greatly affect the composition; if not, indicate what controls the composition of the resulting metamorphic rock more. ( /4)
Fluids in metamorphic reaction can come from either magma or the dehydration or degassing of rocks, for example a subduction zone. These fluids cause metamorphic reactions to take place faster and at lower temperatures. However they do not affect the final composition of a metamorphic rock,
this is determined solely by the protolith.