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Geology 443 Metamorphic-University of Calgary

Metamorphic Petrology

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Geology (GLGY 443-UCAL) Final - Metamorphic

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You may download this exam as a PDF file here.

Credits: Based on the excellent class notes provided by, Igneous: Dr. Rajeev Nair and Metamorphic: Dr. David Pattison during Winter 2014.
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Study Questions (Dr. Dave’s questions)

Additional contributors: Hue Jass, Shawn Sun, Nadia Shaikh, Michelle Baumgartner, Garang Riak, Dylan Riley, Jenna Sie and Gurpreet Multani. Special mention Kamil Sameer for annoying entertainment.

1. Discuss how metamorphic rocks can viewed as objects of natural history on one hand, and as chemical systems on the other hand. Give examples.

The textures and minerals formed during metamorphism can be used to determine past chemical and mechanical processes. For example, fabric (foliation, lineation, etc) of rocks often provide information on Geological history. These textures are formed as a result of changes in pressure, temperature (metamorphism) and fluid composition (metasomatism).

The mineral assemblages can be linked to different geological and chemical conditions within a rock body. Therefore, we can use the minerals present in a rock sample to determine the chemical systems in various stages of the metamorphism. For example, metamorphic facies derived from mineralogical data is used for determining if the metamorphism primarily occurred due to contact metamorphism or regional metamorphism.

2. What skills are needed to be a good metamorphic petrologist?

Fundamentally any petrologist should be able to identify minerals using hand samples, thin section or use of both. Additionally, a good metamorphic petrologist would be familiar with the mineral assemblages related to each facies in different types of metamorphism.

During experiments, always keep the observations separated from inferences/interpretations. One of the most common mistakes in petrology is that students make interpretations while taking observation hence altering “what you see” in samples.

Do not always ask help from TAs and Profs. Instead, try to screw the information and see where it goes. 🙂

Other concepts which a metamorphic petrologist should be familiar with would be crystal chemistry, phase equilibria, physical chemistry (thermodynamics and kinetics), microscopy, textural analysis, structural geology, igneous and sedimentary petrology, geochronology, regional geology, geodynamics (tectonics).

3. Discuss the range of scale in metamorphic petrology, and how study of metamorphic processes at one scale influences the understanding of metamorphic processes at other scales. Give examples.

Metamorphic petrology can be studied in micro to macro scales using thin sections. Often chemical processes associated with metamorphism derived from observations and interpretations made under the microscope. For example, the infamous corona sequence is determined using micro scale observations. Another example would be positive identification of prophyroblasts under thin sections or identifying mineral assemblages.

In hand samples, mm to cm scale, we can also make observations on metamorphic textures and mineral assemblages. This is a very important process because when collecting samples in the field, we do not have access to thin sections.

After collecting data in the field using mm to cm scale observations and positive identification of assemblages, petrologists can map metamorphic fields and/or other changes in mineralogy, chemistry, pressure-temperature, etc. This is a large scale process and could be as small as few square meters to few square kilometers.

Lastly, a metamorphic petrology data from a wide area such as Western Canada Sedimentary Basin can be used to interpret the history of area as large as Rocky Mountains (few square kilometers) to history of areas as large as entire Russia (thousands of square kilometers). These large scale studies are used for understanding the geological processes of mountain belts, large zones of subduction, etc.

4. Discuss how pressure influences texture development in metamorphic rocks. Give examples.
Foliation, flattening and deformation are examples of primarily pressure influenced metamorphic textures.

Depending on how the forces are applied to a rock body, the rock will undergo different types of metamorphism. There is a difference between lithostatic (hydrostatic) stress and deviatoric stress. Under lithostatic stress, there will be no lineation of minerals or foliation within the rock.

Under the deviatoric stress, there will be lineation, foliation or both. If the stress is different in all directions, then the rock will have both lineation of minerals and foliation. If the stress is greater in sigma-1 direction and smaller but equal in sigma-2 and sigma-3 directions, then there will be foliation but no lineation. If the sigma-3 is smaller and the other two are larger and equal then, the rock will have lineation but no foliation.

σ1 > σ2 = σ3 results in foliation but no lineation
σ1 = σ2 > σ3 results in lineation but no foliation
σ1 > σ2 > σ3 results in lineation and foliation

Examples: schist (foliated) vs hornfels (unforliated)

5. Discuss how temperature influences texture development in metamorphic rocks. Give examples.

In short;
-Increase grain size with temperature
-Porphyroblast formation
-metamorphic grading formation
-Pseudomorphism
-Coronas formation
-brittle vs ductile deformation
-Partial melting–migmatites

Grading, development of large prophyroblasts (pressure also a factor) and pseudomorphs are examples of temperature driven metamorphic textures. Corona texture also caused by increased in temperature which results in successive reaction rims around an unstable mineral.

Grading is associated with contact metamorphism. It is refer to as reverse grading because finer grains are metamorphosed to produce larger grains than the larger grains at the bottom which undergoes slower rate of metamorphism (?).

6. Discuss the interplay between melting and deformation in high grade magmatic rocks.

Migmatites are formed as a result of partial melting of pre-existing minerals during metamorphism. As a result of partial melting in high grade metamorphism, segregation of minerals occur causing felsic (light) bands and mafic (dark) bands in the rock. The dark components are known as melanosomes while the light components are known as leucosomes.

7. Discuss the metamorphic aspects of ‘granite’ magma generation.

The granite magma is produced as a result of partial melting. The melt that produced is highly felsic with mostly quartz and feldspars. As a result of pressure and chemical difference between the melt and the rest of the rock, this melt gets segregated out of the host rock. As a consequence of this, the granitic magma is produced. Later this magma gets cool down to form granite. Hence the process that produced the metamorphic rock also produced the granitic rock.

8. Discuss the granite-granulite connection.

The dehydration melting process that formed the granulite also forms granite with mafic minerals by producing granitic melt. The partially melted phase during granulite formation gets squeezed and segregated creating granite dykes. This can be considered as the interface between metamorphic and igneous processes. Note that granite and granulite forms at the same time due to the same process.

9. What is the significance of prophyroblasts in metamorphic petrology? Give examples.

Porphyroblasts are large mineral growths within metamorphic rocks. They formed in existing rocks as a result of change in pressure and temperature during metamorphism. They can be used to determine sequence of events. For example, the timing of the foliation can be determined based on the microfabric within the prophyroblast and its surrounding. We use pre-, syn- and post-tectonic to define the sequence of events. In addition in certain cases like successive development of grains can be used to describe chemical systems (remember that garnet with two distinct microfabrics? Inside one fabric and outside another! So freaking cool.)

They can also be used to determine the mineralogy and chemical composition of rocks before the alterations caused by metamorphic processes.

10. Discuss how microstructures in metamorphic rocks can be used to decipher the interplay between deformation and thermal processes. Give examples.

Deformations will result in textures like lineation and foliation. Thermal processes often result in formation of new minerals such as coronas, prophyroblasts, pseudomorphs and grading.

We can differentiate rocks that underwent contact metamorphism (thermal processes) from regional metamorphism (deformation) by analyzing the mineral assemblages and the corresponding textures. In some unique situations you may find evidence of both deformation and thermal processes in a single sample.

11. How can a natural rock consisting of at least 10 elemental components be reduced to three or fewer components amendable to graphical analysis of mineral assemblages? What are some of the pitfalls? Give examples.

Chemographic diagrams are often used to analyze chemistry of metamorphic rocks. In order to reduce complicated diagrams, conventionally we use three (3) components to categorize bulk compositions (in some cases even less than 3). We can do this by taking only the main control minerals in the assemblage, which varies with different rock types and pressure temperature conditions.

In order to reduce the number of components for analysis, (1) ignore the trace elements due their minor role in shaping overall composition. (2) Combine components using Goldschmidt’s rules (eg. Fe and Mn; Fe and Mg). (3) Limit the types of rocks to be shown on a single diagram. Only deal with sub-set of rock types for in which the chemographic diagram can satisfy. (4) Finally, use projections to make 2D diagrams out of 3D ones. Additionally we can use fixed pressure and temperature values, and leave out the common minerals such as quartz, which appears under all conditions out of the diagram itself.

12. How protolith important in determining the characteristic of metamorphic rocks? Give examples.

Protoliths are types of rocks before they underwent metamorphism. They are important because they provide the mineralogical composition for the metamorphic process. Protolith provide the bulk mineral composition. Depending on the initial mineralogy of the protolith, the final products of the metamorphism varies (direct chemical relationship between the protolith and the metamorphosed rock).

Examples are, mafic protoliths such as gabbros, basalts, and andesite will provide metamorphic rocks high in Fe, Mg and Ca. Alos, protolith of a slate is a shale or mudstone. Except in metasormatism, only what is originally in the rock will be metamorphosed.

Another example would be metapelites (sedimentary origin) contain a lot of Al, for this reason they will likely promote the growth of metamorphic Al-rich minerals such as garnet, kyanite, siliminite, and andalusite.

13. Discuss how metamorphic mineral assemblages represent interplay between bulk composition and grade.

Both bulk composition and grade control the mineral assemblages and therefore isograds. In case of peletic rock composition, the higher aluminum pelite bulk composition shows the most mineral assemblage changes (and therefore isograds) and so it is a sensitive indicator of changes in metamorphic grade. The bulk composition limits the types of minerals that form during metamorphism. In some cases restrictions in the bulk composition will prevent certain minerals from forming that are characteristic of a P-T condition.

14. What are the chemographic diagrams and how do they work? Give examples.

Chemographic diagrams are graphical representation of chemical components in a given system. They are used to categorize and analyze mineral assemblages. These diagrams are composed of tie-lines in which each tie-line connects phases that coexist in equilibrium. Please note that in metamorphic petrology, the elemental proportions are recorded in molar quantities (as opposed to wt%).

These diagrams are usually derived based on pressure-temperature (P-T) conditions. The change in P-T conditions will change the mineral assemblages. To graphically show these changes, we either add or remove the tie-lines on the chemographic diagram. The corners of each section (restricted by tie-lines and corners of the diagram) indicate the mineral assemblage for the bulk composition within that section in a given P-T condition.

Examples are ACF, AKF, AFM

15. What is the difference between phase diagram projections and phase diagram sections? Give examples.

Phase diagram projections involve more than three minerals in 3D figure in order to represent all the important mineral assemblage changes. For example, Fe-Mg-Al minerals occur on one side (A-F side) of the diagram and they are difficult to show in AKF. By creating a tetrahedral with magnesium we can show these changes. However, we also project one end of the tetrahedral from a phase that is always present in order to make a 2D version of the diagram (AFM goes to AKFM).

Phase diagram sections are a slice through P-T-x. They are used to represent specific bulk compositions. Unlike the phase diagram projection model, you cannot represent all possible mineral assemblages for the entire bulk composition in phase diagram sections. We also leave out the excess components from projection diagrams (?).

16. Why metapelites thought to be such good indicators of metamorphic grade? Give examples.

Metapelites are derived from clay rich protoliths such as shales and mudstones. These clay rich protoliths are highly sensitive to variations in pressure and temperature, undergoing extensive changes in mineralogy during progressive metamorphism. Hence metapelites would have a better resolution of mineralogic and chemical changes of metamorphism.

The best example is the Barrow’s 1912 Scotland publication.

17. What is meant by Barrovian and Buchan metamorphism? Give examples.

Barrovian metamorphic series is developed based on observations of regional metamorphism while the Buchan metamorphic series is developed based on contact metamorphism. Buchan can also be described as high temperature series as opposed to Barrovian being high pressure series.

Scotland is an example of Barrovian (regional) while New England is an example of Buchan (contact) series.

18. What is the relationship between isograds and metamorphic reactions? Give examples.

Isograds are lines on maps that separates different metamorphic grades from one another. Hence each line (isograd) separates one mineral assemblage from another. Therefore crossing an isograd will result in change in mineral assemblage. This change in mineral assemblage is a metamorphic reaction. When going from low grade metamorphic rocks to high grade metamorphic rocks, the key minerals such as Chlorite in low grade will disappear while key minerals in high grade such as Biotite will appear as the bulk composition move across the Chlorite-Biotite isograd.

19. What is the difference between continuous and discontinuous reaction, and how does this relate to the Gibbs Phase Rule? Give examples.

Continuous reactions take place in a wide range of P-T conditions in which the proportion of each end members changes as reaction progress. For example, the biotite-chlorite reaction area.

Discontinuous reactions will result in abrupt change in chemical composition at a specific point (known as invariant point) as the reaction progress. Discontinuous reactions occurred at the intersection between two or more continuous reactions.

With respect the Gibbs Phase Rule; there are zero degrees of freedom at the point in which the discontinuous reaction will undergo an abrupt change. In continuous reactions, the degree of freedom does not reach zero at any point.

20. What is the Gibbs Phase Rule? How is it used in metamorphic petrology? Give examples.

The Gibbs Phase Rule is F (degree of freedom) = C (components) – Φ (phases) + 2 (fixed P-T). The rule is used to determine the possible pathways in which reactions could occur. Examples can be found on petrogrenetic grids and phase diagrams.

At invariant point, this indicate that a discontinuous reaction has occurred which represent based on the gibbs phase rule, the degree of freedom is zero. The reaction will stay at that invariant point unless a reactant gets completely used up.

21. How does the pressure relate to depth? Give examples.

Pressure is caused by a forced applied to an area; P = F/A. The force is directly related to depth (due to gravity) and density of the material. Hence the relationship among these variables can be written as P = ρgh. Therefore increased in depth, h will result in increased in pressure P. Deeper the rock is buried, higher the pressure which it experience.

22. What are the metamorphic facies? Give examples.

Metamorphic facies are defined by distinct mineral assemblages. These mineral assemblages are controlled by the pressure-temperature conditions.

Examples are Blueschist facies, Amphibolite facies, Eclogite, etc.

23. What are the rationales for the metamorphic facies concept? Give examples.

Each facies is a set of stable metamorphic minerals that formed under similar pressure-temperature (P-T) conditions. Therefore there is a predictable relationship between the bulk composition and the occurrence of such minerals. If a mineral assemblage of a sample is found to be identical or very similar to that of known assemblages from a facies, it is thought to be from the same facies as the one with the information. Metamorphic facies are defined by mafic rocks (metabasites). The metamorphic facies of a rock is defined by the highest grade mineral.

(By Mitchell B.)As any rock can be metamorphosed, and even re-metamorphosedand it is hard to keep track of all of the possible metamorphic rocks that could originate from all of the possible protoliths. Many different protoliths might ultimately be metamorphosed to the same metamorphic rock. To avoid this problem we discuss groups of rocks as metamorphic facies. Metamorphic facies are a set of associated metamorphic rocks formed over a specific range of temperatures and pressures that can be distinguished from other metamorphic rocks by characteristic mineral assemblages. Metamorphic facies are often associated with restricted range of tectonic settings. They are named after specific index minerals or fabrics associated with one or more protolith types. An example of a protolith and associated metamorphic assemblage and faice are: 1) basalt metamorphosed to a metabasite in the greenschist facie 2) Siliciclastic to a blueschist facie.

24. Discuss amphiboles in metamorphism. Give examples.

Many amphibole minerals can be used to define metamorphic facies.

Example: metabasites—actinolite (greenschist facies), but actinolite does not appear when we go to amplibolite facies, Hbl shows up instead.

Example: Metacarbontaes—tremolite

Amphiboles are hydrous thus provide water in some dehydration metamorphic reactions.

Examples ???

25. Discuss the mineralogical changes in metapelites and metabasites going from greenschist to the amphibolites facies.

Metabasites: Albite + Epidote + Actinolite + Chlorite ==> Plagioclase + Hornblende + Garnet + Cummingtonite
Metepleites: Chlorite + Biotite ==> Biotite + Garnet + Kyanite +Staurolite

Please refer to the metapelite and metabasite zone/facies verses the mineral chart in your notes.

26. Discuss the mineralogical changes in metabasites and meta-siliciclastic rocks going from the greenschist to the blueschist facies.

Metabasites: Chlorite + Garnet + Epidote +Albite ==> Chlorite + Garnet + Epidote + Plagioclase (???? NOT SURE ABOUT THIS)
Meta-silicates: Plagioclase + Water ==> Jadite + Lawsonite + Quartz

27. Discuss the mineralogical and textural changes in metapelites and metabasites going from amphibolite to the granulite facies.

Metapelites: Muscovite + Chlorite + Staurolite + Kyanite ==> K-Feldspar + Silimanite + Water (??? NOT SURE ABOUT THIS)
Metabasites: Hornblende + Plagioclase + Garnet ==> Orthopyroxene + Clinopyroxene + Plagioclase + Garnet

28. What is the relation between changes in Gibbs free energy, enthalpy, entropy and volume of a reaction and the equilibrium constant of the reaction? Give examples.

Gibbs free energy: ΔG = Delta;H – TΔS+ ΔV+Rln(k)

29. What is geothermobarometry, and how is it used in metamorphic petrology? Give examples.

Geothermobarometry is the process in which the history of pressure and temperature (P-T) conditions of a rock is determined using mineralogy and chemistry of a sample. It relates mineral assemblages to different P-T conditions in which the rock sample may have formed.

Examples in the TUTORIAL 8; Garnet-biotite thermometer (important to contact metamorphism) and Garnet-Kyanite-quartz barometer (important to regional metamorphism).

30. What are the relative strength and weakness in estimating P-T conditions of metamorphic rocks using (1) thermodynamically-calculated phase diagrams and (2) geothermobarometry? Give examples.

Thermodynamically-calculated phase diagrams will provide a very wide range of P-T conditions for a given mineral assemblage. TO BE ADDED MORE DATA!

Geothermobarometery can provide more accurate P-T conditions for a specific assemblage. However we assume the values of H, S, and v used in calculates are accurate.

31. What makes a good geothermometer? What makes a good geobarometer? Give examples.

Good geothermometer: Sensitive to temperature changes but not pressure. Examples are Garnet – Biotite exchange reaction, Fe-Mg exchange reaction. (Note: Related to contact metamorphism)

Good geobarometer: Sensitive to pressure but not temperature. Examples are Garnet – Silimanite – Plagioclase – Quartz reaction. (Note: related to regional metamorphism)

Omitted 32 to 35 for obvious reasons, but check them out.

36. What is forward verses inverse modelling with respect to P-T estimation in metamorphic rocks? Give examples.

In forward modelling, we use theoretical models to predict how a system would respond to changes. In inverse modelling, we use known set of data (results) to determine its history. In other words, working backwards from a given results. Best example of inverse modelling is the geothermobarometry in which a known mineral assemblage of a rock is used to determine how such assemblages formed. Using petrogenetic grid and AFM diagrams to determine the mineral assemblages are forward modelling approaches.

37. What are some differences in the metamorphism of meta-carbonate rocks compared to other protoliths? Give examples.

Meta-carbonate rocks are the protolith of metamorphic rocks such as marbles and other calc silicate metamorphic rocks. They are mainly coming from a carbonate origin made up mainly limestones or dolostones. Comparing with other protolith such as metabasites or metapelites, the mineralogical and rock origins are different. For metapelites, it is mainly made up of aluminum silicate minerals and for metabasites, it’s mainly made up of mafic minerals due to a likely basaltic origin. Also, meta-carbonate rocks are also formed from reactions due to fluid infiltration from intrusions, meta-carbonates can possess a mixed fluids. Meta-carbonates primarily contain calcsilicates (ex. Talc, tremonlite).

38. Describe different types of fluid evolution in the metamorphism of carbonates. Give examples.

TBA

39. Distinguish between internal buffering, external buffering and fluid infiltration in the metamorphism of carbonates. Give examples.

All reactions cause changes in concentration of chemical components. If the fluid composition of a reaction remains constant, then the reaction must be externally buffered (externally controlled). Throughout the reaction fluids must have been added or subtracted to keep the system constant. In other words, the fluid composition remains the same and is controlled only by the external fluid introduction.This is not observed in nature, since you will have to assume that all internal reaction within a rock does not produce any H2O or CO2. (Move straight up the CO2 vs T diagram).

Internal buffering is caused by changes in the fluid concentrations within the rock itself, this occurs due to reaction with in the rock which results in the consumption/production of H2O/CO2. No external fluid sources are added, all changes are due to internal reactions, thus fluid composition is only controlled by internal reactions. We are moving along the reaction lines.

Fluid infiltration will often will result in introduction of fluids that belongs to completely different chemical composition that the primary reaction itself. This causes chemical disequilibrium. Once a fluid is introduced the reaction will proceed to eliminate the chemical potential difference to reach equilibrium. (Move horizontally on the CO2 vs T diagram)

40. Can granulite facies rocks and amphibolites facies rocks exit together at the same P-T conditions? Explain.

They can coexist at the same pressure-temperature (P-T) conditions if a CO2-rich fluid is infiltrated into an Amphibolite facies rocks. The CO2-rich fluid will act as a catalyst for the growth of granulite facies by providing CO2 and lowering the temperature needed to produce granulite. (IS THIS CORRECT?)

41. Explain what happens to plagioclase in metamorphic rocks as pressure increases. Give examples from different protoliths.

With the increased in pressure, plagioclase become more albite rich (Ca-rich to Na-rich) and eventually become unstable where it reacts to form a new mineral.

Ex 1. Granulite to Eclogite (high T):
Na-rich plag goes form a component of Omphacite (cpx). Omphacite is indicatory of eclogite. Ca-rich plag forms garnet and kyanite.

Ex 2. Grey wacke metamorphosed to blueschist:
Detrital plag (unaltered plag) forms Lawsonite and Jadite. The Lawsonite forms from anorthite component and jadite forms from the ablate component of plagioclase.

42. What is (are) the difference(s) between lawsonite blueschist facies and epidote blueschist facies?

Lawsonite Blueschist facies is low grade compared to the epidote blueschist facies. The latter have epidote while the other than lawsonite. (MUST ADD MORE TO THIS Q!)

43. What is (are) the difference(s) between amphibolite facies and epidote amphibolite facies?

Amphibolite facies is higher grade than the epidote amphibolite facies. The latter is characterized by the characterized by the epidote while higher grade amphibolite lacks epidotes.

44. Explain how the presence of aragonite in a rock reveals information both about P-T conditions of peak metamorphism and the P-T path followed by the rock? Give examples.

Aragonite is stable under high pressure conditions and is unstable at the surface. We would not expect to find aragonite in rocks unless rock has been passed across the Calcite-Aragonite boundary at a specific pressure and temperature. An example of an area with such occurrence is a subduction zone in which the rate of burial is almost same as the rate of exhumation.

45. What are the characteristics of eclogite facies metamorphism?

Eclogite facies formed at the extremely high pressure (also high temperature) environments. Therefore it is an important facies in understanding the Earth’s processes. The key minerals are Garnet and Omphacitic pyroxene. Developed in mantle conditions and the protolith is igneous rocks with basaltic composition.

46. How can equilibrium thermodynamics on the one hand, and reaction kinetics on the other, help us understand metamorphism? Give examples.

Equilibrium thermodynamics describes the state of equilibrium of a system using H, S and T. (NOT SURE ????)

Reaction kinetics is used to describe the rate (speed) of reactions. It provides information on how fast a system will reach its equilibrium (all systems reach equilibrium).

47. Use the reaction rate expression to calculate how much reaction takes place in a given amount of timer, or how long a reaction takes to proceed.

R = (A)e(-G/T)

48. What is the difference between metamorphism and metasomatism? Give examples.

Metamorphism is caused by solid state reactions that recrystallize minerals in the bulk composition. These reactions are caused as a respond to changes in Pressure and Temperature. Examples of metamorphism would be the formation of high grade metamorphic mineral assemblages.

Metasomatism is caused by chemical alteration of rocks due to hydrothermal alteration or fluid infiltration. Metasomatism changes the whole-rock composition. This is very important for the formation of metacarbonates. Example of metasomatism is the formation of skarns.

Skarns are formed when there is a contact zones between a granitic magma body and carbonate sedimentary rock, the igneous body releases fluids that cause alteration of the adjacent carbonate rocks. This results in high grade metamorphic rocks near the intrusion and lower grades away from the intrusion.

49. Give examples of how fluid infiltration can affect both metamorphic mineral assemblage development and rates of metamorphic processes.

Certain minerals form when they are reacted with a fluid phase. This occurs at certain pressures and temperatures (P-T) ranges. The fluid infiltration acts as a catalyst by lowering the P-T conditions for a given reaction, thus allowing the reaction to take place early on. Fluids increases the rates of reactions since it acts as a catalyst. Example: Dehydration fluids from solidifying igneous intrusions can cause the nearby rocks to produce higher grade mineral assemblages at much lower P-T conditions. (NEED A GOOD MINERAL BASED EXAMPLE HERE)

50. What are migmatites and how are they formed?

They are a mixture of metamorphic and igneous rocks. They formed in extremely high grade metamorphic environment and in partial melting phase. They are rich in mafic minerals such as quartz feldspars. They have bandings in which light coloured minerals assemblages are separated from darker units (melanosomes and leucosomes).

51. What are the two main types of melting reaction in prograde metamorphism, and under what conditions do they proceed? Give examples.

Two types of melting:
1) reaction with water as a free fluid (below the wet solidus)
2) reaction with no water as a free fluid—Water enters melt phase (above the solidus)—Dehydration melting

As metamorphism increases the free fluids released and escapes rock (rock gets dehydrated). When metamorphism gets to extreme cases (ie. Partial melting occurs) we cross the wet solidus and any water released during reactions are dissolved in to the melt. Dehydration melting produces migmatites which contain granitic melts.

52. What is the significance of the wet melting reaction in prograde metamorphism? Give examples.

Since water is in the products side and is needed for the reaction to proceed, west melting reactions indicates the presence of fluids during metamorphism. Example would be amphibolites into granulites.

53. The muscovite+quartz-out dehydration reaction intersects the wet melting curve at arbout 3.7 kbar, 640 C. How does the muscobite+quartz-out reaction reaction differ above and below this interasction point?

Below this intersection point or solidus the reaction will generate water in a free phase form.

Ms +Qzt = Kfs + And or Sil + H20.

Above this solidus any free water goes into the melt phase and the rock is a migmatite. There is a loss of the free H2O phase as the muscovite is no longer stable and reacts and H20 dissolves into a melt phase. Ms + Qzt = Kfs + Sil or Ky + melt

54. How does what you have learned about dehydration melting in high grade metamorphism lead to an improved understanding of the chemistry and genesis of ‘granite’ magma in the continental crust?

Dehydration melting leads to two important processes. 1) it produces a granite (granite-like) melt and 2) dehydrated residue characteristic of the upper amphibolite or granulite facies. Residue’s mineral assemblage is Gartnet, Silimanite, Cordiorite and Orthopyroxene. The melting process is related to the crossing of the reaction lines on P-T diagrams.

55. How does the geothermal gradient vary on Earth?

Thinner lithosphere has a higher geothermal gradient than the thicker lithosphere (WHY ???). The much thicker asthenosphere has a lower geothermal gradient than lithosphere. There is a transition zone between lithosphere and asthenosphere gradients in which the gradient changes from small to large gradually.

Continental crust has a lower geothermal gradient than the oceanic crust.

56. What factors control Earth’s geothermal gradient?

– Thickness of the crust (thicker the crust, less conduction efficiency)
– Heat flux from mantle (closer the layer to the mantle, higher the gradient; heat requires to derive reactions)
– Heat production from radioactive decay of potassium (K), uranium (U) and Thorium (Th)
– Specifically for core and mantle: convection

57. Discuss differences in the geothermal gradient in the continental lithosphere and oceanic lithosphere, and what factors influence on those differences.

Continental lithosphere is older and it is thicker. It has less conduction efficiency leading to low geothermal gradient.

Oceanic lithosphere is much thinner than the continental one. Therefore it has better heat conduction leading to high geothermal gradient.

58. What is the advection and how does it control thermal patterns in the curst and mantle.

Advection is the process of heat transfer by physical movement of materials. The magma or rocks get uplifted with the heat which they have within them. The rock and magma movement (advection) is faster than the heat movement (conduction) on Earth. This is evident by the fact that Earth is not completely “hot”. Earth has patches of “hot” and “cold” areas.

59. “Metamorphic rocks represent perturbation of the normal geothermal gradient”. Justify this statement, giving examples.

Metamorphic gradients do not correlate with the greothermal gradient. Therefore they are perturbations of the normal (steady state) geothermal gradient. On a normal pressure-temperature (P-T) diagram, metamorphic sequences such as Greenschist, Buleshist facies do not align with the P-T field gradients. Hence the facieses represents different metamorphic paths and are perturbations from the normal geothermal gradient. Note that the geothermal gradient lines are steeper than that of metamorphic facies domains boundaries on P-T diagrams.

60. Draw cross-section of a subduction zone under a continent and show the distribution of isotherms and metamorphic facies.

Subduction Zone Cross-Section with Facies

Subduction Zone Cross-Section with Facies (click to enlarge)

Please note that this course did not cover Zeolite facies, therefore ignore it.

61. What are ‘paired metamorphic belts’ and what is their significance?

They are juxtaposed zones two distinct metamorphic environments at subductions zones. It was first observed at Japanese subduction zone. The down-going mantle lithosphere is subjected to low temperature, high pressure metamorphism while the overriding mantle lithosphere is subjected to high temperature, low pressure metamorphism. Hence, at the subduction margin, we will observe both sides next to each other. The paired belts provide evidence for the existence of subduction zone in a given region (very good indicator).

62. What is the style of the metamorphism in continent – continent collisional zones?

Orogeny, so things like mountain building because you are colliding between continental plates. This also generates barrovian or regional metamorphism. TO BE UPDATED…

63. What are the primary controls on the development of granulite facies and ultra-high temperature metamorphism?

Mineralogical controls. So between granulite facies and ultra-high temperature facies, they are separated by different mineralogies. For example, in granulite facies, garnet and cordierite is formed from biotite, silimanite and quartz due to dehydration melting? And for ultrahigh temperature facies, spinel starts form. Also, one other development control is the temperature affecting their development. Ultra-high temperature metamorphism occurs at much higher temperature than the granulite facies.

64. What is ultra-high pressure metamorphism and under what conditions does it occur?

They are high pressure facies typically represented as eclogites. They are formed below the mantle where pressure is greater than in crustal conditions. They contain diamonds. They are also made up of mainly strong garnet, which indicate that they were original mudstones. They were form due to subduction of continents and exhumation of UPH terrain which cause the lower of crust density compared to the mantle.

65. What is the difference between metamorphic field gradient and a P-T path? Use a sketch to illustrate your answer.

A metamorphic gradient is a trajectory that joins all peak metamorphic temperatures reached by ricks from different metamorphic zones. The P-T path is simply the P- T and conditions that the rock was subjugated to. The metamorphic gradient and the P-T path differ in that the maximum pressure does not always correspond to the maximum temperature, thus the gradient does not include any pressure readings and only focuses on the temperature.

66. Sketch some typical P-T paths in Barrovian-type metamorphic terrain and label P_max, T_max, P at T_max, metamorphic field gradient.

The metamorphic field gradient is the line which connects the maximum temperatures of each rock in an assemblage.

Metamorphic Field Gradient, P_max and T_max

Metamorphic Field Gradient, P_max and T_max

Additionally (kind of related to question), once the samples were plotted on a pressure-temperature diagram, they should be able to relate to each other by a straight line.

Note: Following image is modified from, Metamorphic P-T Phase Diagrams

Example of a hypothetical  field gradient.

Example of a hypothetical field gradient.


67. What are some models for formation and exhumation of blueschist facies rock, and what are some of the consequences of these different models fro P-T path?

The formation depth and the setting determines the pressure-temperature conditions. Therefore, in order to have the blueschist facies related mineral assemblage, the rock must form at certain depth (and also right potolith). What is important is the exhumation path because there are two models. One is that the temperatures decrease before pressure decreases. The other is the opposite. The mineral assemblages varies depending on which variable (P/T) decreases first. Example would be temperature drop before pressure result in preservation of aragonite.

68. How is metamorphism useful in understanding Earth processes? Give examples.

  • Metamorphic rocks are formed due to perturbation from the ‘normal’ Temperature gradient
  • Metamorphic rocks: natural objects that contain a record of Earth processes
      – Textures (ex.Folaitions) and changes in mineral assemblages show changes in stress regimes and P-T conditions. This ultimately reflects the geodymanic changes that the Earth was experiencing during that time.
      – Can tell us sequence of events
  • Chemical systems
      – Shows how chemical changes have occurred by the progressive change in mineral assemblages. The changes are highlighted in P-T diagrams that show characteristic assemblages for a range of P-T conditions.
      – Can mention how fluid composition changes can lead to different assemblages
  • Records Tectonic processes
      – Certain metamorphic rocks (ie. Ultrametamorphic rocks) can only occur in certain tectonic setting such as subduction zones. Cataclastic rocks would occur in areas where there is significant brittle deformation.
      – Eclogites can indicate the presence of subduction zones

Omitted 69 for obvious reasons, but check it out.

70. What area or areas of metamorphic petrology need(s) more reserch? Outline the area, explain what is missing from our understanding, and suggest some ways forward.

Open ended question…. Here are some ideas; understanding the history of Earth and processes that resulted in the current condition of this planet; understanding subduction zone processes, prediction of geological conditions for economic, academic, etc…

71. Discuss a topic in metamorphic petrology you found to be interesting, illustrating with examples.

This is an open ended question. Please pick a good topic and go for it. For example, textures and their relation to P-T conditions, the metamorphic facies and mineral assemblages, arragonite at the surface, etc.

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