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A-Level Geology (2nd Year) > Metamorphic Petrology > Flashcards

Metamorphic Petrology Flashcards

1
Q

What is another word for metamorphic textures?

A

Metamorphic fabrics

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2
Q

What are the characteristics of slaty cleavage?

A

Splits into thin sheets along cleavage planes. Fine grained.

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3
Q

What are some examples of rocks with slaty cleavage?

A

Slate and phyllite

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4
Q

How is slaty cleavage formed?

A

By low grade metamorphism. Flat, platy minerals like clays and micas align at 90° to the direction of pressure and they’re usually parallel to the axial planes in folds

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5
Q

What does slaty cleavage represent?

A

Low grade metamorphism

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6
Q

How is schistosity formed?

A

Regional metamorphism. Alignment of flat platy minerals. 90° from direction of metamorphic pressure.

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7
Q

What is schistosity?

A

Thin parallel beds and shiny appearance where surface of micas visible (micaceous sheen)

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8
Q

What does schistosity indicate?

A

Higher grade metamorphism

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9
Q

What is the difference between slaty cleavage and schistosity?

A

Slaty cleavage: low grade. Fine grained.
Schistosity: higher grade. Coarser grained

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10
Q

What does gneissose banding look like?

A

Clearly distinguishable light and dark bands.

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11
Q

What makes up the light bands of gneissose banding?

A

Quartz and feldspar.
Usually granoblastic and equidimensional

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12
Q

What are the dark bands made of in gneissose banding?

A

Biotite and Mafic material. Foliated

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13
Q

Are bands in gneissose banding always straight?

A

No. They may be contorted or folded at 90° to maximum pressure

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14
Q

What are porphyroblasts?

A

Large metamorphic crystals within a finer matrix

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15
Q

What is the difference between porphyroblasts and phenocrysts?

A

Porphyroblasts are metamorphic.
Phenocrysts are igneous and formed by two stage cooling

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16
Q

How are porphyroblasts formed?

A

Both contact and regional metamorphism

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17
Q

How can porphyroblasts show the order of metamorphic events?

A

If they are pre, syn or post-kinematic

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18
Q

What are some examples of porphyroblasts?

A

Cubic pyrite in slate.
Garnet in schists

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19
Q

What is meant by granoblastic?

A

Unfoliated fabric formed by contact metamorphism. Randomly oriented equidimensional crystals

20
Q

What is granoblsstic sometimes called?

A

Sugary texture

21
Q

What rocks can be seen with a granoblastic texture?

A

Hornfels, marble, metaquartzite

22
Q

What is crenulation cleavage?

A

Several foliations in different directions

23
Q

How is crenulation cleavage formed?

A

Regional metamorphism. Two or more stress directions

24
Q

How does crenulation cleavage make the rock look?

A

Gives it a wavy appearance

25
Q

What rocks can have crenulation cleavage?

A

Phyllite, schist, and some gneisses

26
Q

How do metamorphic minerals form?

A

As mountain buildings happen, metamorphic minerals form due to high temps. High temps increase rate ions diffuse (as a solid) between minerals.

27
Q

How does water effect the formation of new metamorphic minerals?

A

Mineral formation takes millions of years but water speeds up this reaction as it allows ions to diffuse more rapidly

28
Q

How may we identify parent rock from a newly formed metamorphic mineral?

A

They may grow over sedimentary structures/features

29
Q

What happens if you rapidly apply stress to a rock?

A

Mineral grains form brittle fractures and are cursed and ground down mechanically at fault zones (cataclasis). Forms range of fabrics.

30
Q

What happens when stress is applied slowly to a rock?

A

Results in low strain rates. Rock doesn’t fracture but gradually deform. Later stages of deformation may produce crenulation cleavage which can be used to determine stages of metamorphism

31
Q

What is a pre-kinematic mineral?

A

Mineral was formed before deformation. Mineral will be affected by deformation

32
Q

What is a syn-kinematic mineral?

A

Form at the same time as deformation.

33
Q

What is a post-kinematic mineral?

A

Formed after the deformation

34
Q

What are index minerals (metamorphic)?

A

Specific minerals that help to identify the metamorphic grade

35
Q

Who discovered index minerals?

A

George barrow

36
Q

What are the index minerals discovered by Barrow?

A

Chlorine (shale + phyllite)
Biotite (phyllite + schists)
Garnet (schists)
Kyanite (schists)
Sillimanite (gneiss)
Chlorine found furthest from mountain centre

37
Q

How are Al2SiO5 Polymorphs formed?

A

Regional metamorphism

38
Q

Where is Kyanite in Al2SiO5 polymorphs?

A

Top left in diagram.
Found in gneisses. Found in high grade metamorphism. High pressure, low temperature

39
Q

Where is Sillimanite in Al2SiO5 polymorphs?

A

Right on the diagram.
High grade regional metamorphism. High temps and high pressure. Usually gneisses

40
Q

What temperatures and pressures is sillimanite formed in Al2SiO5 polymorphs?

A

Maximum temp of 700°C. Maximum pressure 700MPa. Pressure exists at depths about 25km. Geothermal gradient of 28°C/km

41
Q

Where is andalusite in Al2SiO5 polymorphs?

A

Bottom of the diagram.
Low pressure and high temps. Contact metamorphism

42
Q

What is it called when all the points meet in an Al2SiO5 polymorph diagram?

A

Triple point

43
Q

What types of metamorphism are contact, regional and dynamic?

A

Prograde metamorphic processes

44
Q

What happens during prograde metamorphism?

A

Fluids such as water and CO2 are driven off. These fluids are necessary to form the hydrous minerals that are stable at earth’s surface

45
Q

What is retrograde metamorphism?

A

Involves mineral hydration, carbonation and oxidation as metamorphic rocks are returned to the surface. Chemical reactions take place more slowly as temp decreases

A-Level Geology (2nd Year) (7 decks)

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