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FoG > Petrology > Flashcards

Petrology Flashcards

1
Q

What is the AFM diagram used for

A

to show the relative proportions of magnesium, iron, and alkaline oxides

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2
Q

What are the 2 trends on the AFM diagram

A

Tholeiitic
Calc-alkaline

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3
Q

Describe the path of the tholeiitic trend and where it is seen on Earth

A

initially moves upwards towards Fe as Mg end member crystallised first, then moves down to alkali
Seen at rift and plume volcanism

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4
Q

Describe the Calc-alkaline trend and where it is seen in magmas on earth

A

follows a straight path of increasing alkali
seen at subduction zone volcanism, depleted in Fe

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5
Q

Why are subduction zone magmas depleted in Fe

A

oxidised conditions due to melting involving water, magnetite readily crystallises out of the melt

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6
Q

What does the TAS diagram look like and what is it used for

A

graph of alkali content (TA) again silica (S), with
rift volcanism,
ocean island volcanism,
subduction volcanism
in order of decreasing gradient

Used to classify rocks based on major element chemistry

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7
Q

Where are flood basalts formed and what is special about their chemistry

A

A combination of rifting and hotspot volcanism causes extensive eutectic melting, resulting in high proportion of quartz

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8
Q

What is the granite problem

A

it is very difficult to produce large volumes of granitic magma from fractional crystallisation of basaltic melts

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9
Q

What is the solution to the granite problem

A

Add crustal material to basaltic melt

  • plume moves up through continental, or at subduction zone
  • rises though C.C. causing melting of silica rich material
  • incorporated into melt
  • becomes more viscous therefore gets stuck
    = not flood rhyolites

another = P+T from mountain building melts crustal material

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10
Q

What are the 4 types of granite

A

S-type
I-type
A-type
M-type

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11
Q

How are S-type granites formed

A

Melting of sedimentary rocks in cores of mountain belts

clockwise P-T curve (pressure first)

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12
Q

How are I-type granites formed

A

melting igneous material
- subduction zones -> basaltic melt -> incorporate as move up

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13
Q

How are A-type granites formed

A

form in plume melting settings
anorogenic, anhydrous

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14
Q

How are M-type granites formed

A

extensive fractional crystallisation of basaltic melts - usually v.small volumes

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15
Q
A
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16
Q

why do granite plutons rise to the surface as regularly spaced heads and what does this tell us

A

due to high viscosity and low density. Distance between plutons depends on density contrast between granite and crust, so can be used to calculate viscosity

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17
Q

what are the 2 categories of metamorphism when P-T increases and decreases

A

Prograde metamorphism (increase)
Retrograde metamorphism (decrease)

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18
Q

Why can we see high P-T assemblages at the surface

A

retrograde metamorphism restricted due to
- chem reactions faster at high T
- fluids used up making hydrous minerals in prograde
- fluids unable to catalise reactions

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19
Q

What are the 4 types of metamorphism

A

Thermal
Dynamic
Dynamothermal
Chemical

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20
Q

Name 5 metamorphic environments

A

Contact (intrusion)
Regional (orogenesis)
Fault
Chemical (hydrothermal)
Shock (impact events)

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21
Q

Equation and conditions for contact metamorphism

A
  • at shallow depths, large temperature gradient between magma and crust

Q = -k . dT/dz
(k=thermal conductivity)

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22
Q

What is Fourier’s Law of heat conduction

A

time for heat to dissipate:
dT/dt = k . ∂2T/∂x2
(k = thermal diffusivity)

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23
Q

Equation for thermal diffusivity

A

thermal diffusivity (kappa) = conductivity / (density . shc)

24
Q

equation for characteristic time it takes for an intrusion to cool

A

t = L^2 / π^2 . kappa

(kappa = diffusivity)

25
Q

what does the size of a contact aureole depend on

A
  • size
  • temperature
  • country rock wet or dry
26
Q

How does the wetness of the country rock impact a contact aureole

A

Impacts the method by which heat is lost and therefore the rate

wet = convection = more efficient = smaller aureole

dry = conduction = less efficient = wider aureole

27
Q

What are the conditions in regional metamorphism

A

orogenic events have P and T increase due to burial and radioactive decay

28
Q

equation for pressure a rock experiences by overlying rock column

A

P = rho . g . h

29
Q

estimates for crustal density and thickness

A

felsic: 2700, 35km
ultramafic: 3300, 7km

30
Q

how much does pressure increase with 10km crustal depth

A

0.3 GPa

31
Q

Regional metamorphism estimates for pressure in subduction zones and orogenesis

A

subduction: P>3 GPa
base of mountains: P=2 GPa

32
Q

Describe the process of orogenesis metamorphism

A
  • isotherms flat
  • isotherms pushed down as pressure increases
  • burial rate decreases, temp catches up, isotherms flatten (radioactive decay)
  • erosion of mountains = isotherms up
  • reaches equilibrium, thicker crust

(clockwise PTt curve)

33
Q

what are isograds and how are they identified

A

boundary between regions of different metamorphic grade, identified by new metamorphic phase appearing

34
Q

examples of index minerals used to identify P-T conditions in metamorphism

A
  • chlorite
  • biotite
  • garnet
  • andalusite
  • sillimanite
35
Q

what do the different PTt curves show

A

clockwise = rocks exhumed during metamorphic event
(high P then T)
(orogenesis)

anticlockwise = very deep and exhumed after event
(high T then P)
(intrusion)

36
Q

Example of how mineral inclusions can be used to identify PTt curves

A

decreasing P and increasing T = kyanite -> sillimanite

kyanite preserved in inclusions

37
Q

Example of reaction textures revealing PTt paths

A

Andalusite being replaced by sillimanite = increasing T conditions

38
Q

How is element zoning used to understand PTt paths

A

zoning = low T, or short time at high T

eg Mn rich core, Mn poor rim in Garnet

39
Q

what is metasomatism (chemical metamorphism)

A

change in mineral assemblage by percolating fluid
olivine -> serpentinite at subduction zone

40
Q

what is the reaction of olivine and water in subduction zones

A

olivine + water -> serpentine + H + methane + heat + magnetite

serpentine: percolates structure
magnetite: used in palaeomag

41
Q

what setting other than subduction zones does metasomatism occur

A

at slow spreading ridges, extension -> faulting -> enhance hydration

serpentine = very smooth and makes rock tectonically weak

42
Q

for mudstones, what is the sequence of 5 rocks with increasing grade

A
  • slate
  • phyllite
  • schist
  • gneiss
  • migmatite
43
Q

identifying characteristic of slate

A

slatey cleavage in defined orientation

thin layered cleavage, flakes away

44
Q

identifying characteristics of phyllite

A

shiny lusture due to presence of micas

45
Q

identifying characteristics of schist

A

growth of porphyroblasts

46
Q

identifying characteristics of gneiss

A

segregation of mafic and felsic components into bands

47
Q

identifying characteristics of migmatite

A

partial melting of segregated bands

48
Q

what are metamorphic facies

A

refer to specific regions of P-T space
defined by concept that rocks of similar composition metamorphosed under same P-T conditions should generate assemblage of similar minerals in similar proportions

49
Q

what is a metamorphic protolith

A

refers to the original unmetamorphosed rock

50
Q

how does Gibbs phase rule relate to mineral assemblages in metamorphic rocks

A

varying P+T means f=2
therefore c = p
high number of chemical components = large mineral assemblage formed (eg mafic) (opposite eg calcareous)

51
Q

What does each new zone in Barrow’s Zones represent

A

mineral with defining higher P-T conditions, therefore greater crustal burial depth

52
Q

What texture is seen in most metamorphic rocks (regional not contact)

A

foliation
- cleavage
- schistosity
- gneissosity
(with increasing grade)

53
Q

what metamorphic textures are seen in contact metamorphism

A

boundaries between minerals at 120º to minimise surface tension

54
Q

what is crenulation cleavage and what does it indicate

A

when original foliation is deformed by later compression at an angle to original foliation. Indicates multiple stages of deformation

55
Q

what do textures in porphyroblasts show about timing of deformation

A

comparing texture or porphyroblast and matrix:

same texture = p. growth post deformation
different texture = p. growth pre or during deformation

FoG (7 decks)

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