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Earth Materials > Metamorphic Rocks > Flashcards

Metamorphic Rocks Flashcards

1
Q

Metamorphism

A

A solid state process which causes changes in mineralogy and or texture, when there is a new physical or chemical environment and the mineral assemblage is no longer its most stable

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2
Q

Key controls on metamorphism (and explain)

A 
Temperature
Pressure
Fluids 
Original rock composition 
Time
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3
Q

Evidence for the existence of metamorphic fluids

A

Fluid inclusions

  • required for hydrous or carbonate phases
  • reactions involving volatiles occur at temperatures and pressure different to anhydrous systems
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4
Q

Types of regional metamorphism

A

Burial - in deep sedimentary basins

Orogenic - deviatoric stress
CAN GET A POLYMETAMORPHIC PATTERN

Seafloor - infiltration of seawater into hot oceanic crust as it upwells at MOR
WIDE T RANGE AND LOW P

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5
Q

Size and shape of the aureole depends on…

A

The pluton nature: it’s size, shape, orientation, temperature, composition

The country rock nature: it’s composition, permeability, depth and metamorphic grade prior to intrusion

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6
Q

Foliation vs lineation

A

Foliation = any planar fabric element (“flattened”)

Lineation = any linear fabric elements (“stretched”)

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7
Q

Slaty cleavage =

A

Any type of foliation in which platy phyllosilicates are to fine to see individually unaided

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8
Q

Schistosity =

A

Preferred mineral grain/grain aggregate orientation due to metamorphic processes

Coarse enough to see unaided

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9
Q

Gneissose fabric

A

Segregated into layers due to metamorphic processes

BANDING

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10
Q

Slate =

A

Compact
Fine grained
Well developed cleavage
Freshly cleaved surfaces are dull

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11
Q

Phyllite

A

Very fine phyllosilicates

Silky sheen to foliation surface

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12
Q

Schist =

A

Metamorphic rock exhibiting schistosity

Slates and phyllites are also schists

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13
Q

Gneiss =

A

Metamorphic rock displaying gneissose structure

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14
Q

Non foliated rocks

And their definitions

A

GRANOFELS
Any isotropic rock

HORNFELS
Typically fine grained and compact, occurs in contact aureoles
Tough and splinter when broken

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15
Q

Porphyroblastic =

A

Metamorphic rock with one or more minerals that grew much larger than the others

Individual crystals = porphyroblasts

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16
Q

Pelitic =

A

Clay rich

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17
Q

Barrovian zones

A

Chlorite

Biotite

Garnet

Staurolite

Kyanite

Sillimanite

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18
Q

Chlorite zone

A

Slates and phyllites

CHLORITE

Muscovite
Quartz
Plagioclase

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19
Q

Biotite zone

A

Phyllites and schists

Biotite
Chlorite
Muscovite
Quartz
Plagioclase

400-450 DEGREES

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20
Q

Garnet zone

A

Schists

Alamandine garnet (red)
Biotite
Chlorite
Muscovite
Quartz
Plagioclase

~550 DEGREES

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21
Q

Staurolite zone

A

Schists

STAUROLITE
Biotite
Muscovite
Quartz
Plagioclase
Garnet
~chlorite

550-600 DEGREES

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22
Q

Kyanite zone

A

Schists

Kyanite
Biotite
Muscovite
Quartz
Plagioclase
Garnet
Staurolite

530-630 DEGREES

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23
Q

Sillimanite zone

A

Schists/gneisses

Sillimanite
Biotite
Muscovite
Quartz
Plagioclase
Garnet
~staurolite

530-630 DEGREES

24
Q

Isograd =

A

Line that separates zones

Line in field of constant metamorphic grade

Intersection of isograd surface with earth’s surface

1st appearance of particular index mineral as you increase in grade NOT DISAPPEARANCE

25
Q

Lithostatic pressure =

A

Due to overlying rock weight

Therefore increases with depth

Can be isostatic or non isostatic (=mineral alignment)

26
Q

Tectonic overpressure =

A

Developed during orogenic metamorphism/deformation

DOES NOT AFFECT EXISTENCE OF METAMORPHIC MINERALS BUT IMPORTANT FOR DEVELOPMENT OF TEXTURES

Limited by rock strength

27
Q

Fluid pressure =

A

Fluids follow a hydrostatic pressure gradient in he upper crust

LOCALLY can be as high as lithostatic pressure causing hydraulic fracturing I.e. deformation

28
Q

Types of stress

A

TENSION
Strain = extension

COMPRESSION/PURE SHEAR
Strain = flattening/folding

SHEAR/SIMPLE SHEAR
Strain = slip along spaced cleavages/flow
N.B. Hard to distinguish between compression and shear

29
Q

Post-kinematic porphyroblast growths

A

Internal schistosity continuous with external schistosity

Porphyroblast formed after the deformation so it’s inclusions will have the same form as the surrounding fabric

30
Q

Pre kinematic porphyroblast growth

A

Internal schistosity is inherited from an earlier deformation

External schistosity is therefore compressed about the porphyroblast and a pressure shadow can develop

Might be a bit squished

31
Q

Syn kinematic porphyroblast growth

A

Rotational porphyroblasts where internal schistosity is continuous with external schistosity

Suggests deformation did not outlast porphyroblast growth

32
Q

Mafic rock metamorphism - 3 types

A

1) LOW PRESSURE/TEMP
contact metamorphism

2) MEDIUM
crustal thickening, like barrovian

3) HIGH PRESSURE and relatively low T for given P
subduction zone

33
Q

Low series

A

Contact hornfels

34
Q

Medium series

A

Greenschist&raquo_space; amphibolite&raquo_space; granulite

Most common for regional metamorphism

36
Q

Greenschist

A

Correlates with chlorite and biotite pelitic zones

Biotite
CHLORITE
Plagioclase
GREEN AMPHIBOLE
Epidote
Quartz
37
Q

Amphibolite

A

AMPHIBOLE

PLAGIOCLASE

38
Q

Granulite

N.B…

A

Amphibole (specifically hornblende) breaks down

PYROXENES
Garnet
Plagioclase

N.B.
Origin of granulite is controversial
However it is agreed that it is

UNUSUALLY HOT
Due to crustal thickening and excess heating

DRY
It doesn’t melt due to the lack of water
Any fluid inclusions within the rock are CO2 rich rather than H2O rich as in amphibolite

Due to:

1) partial melting leaving an anhydrous residue
2) fluxing of CO2 rich or saline fluids

39
Q

Ultra mafic garnet-pyroxenite

A

PYROXENES
GARNET
PLAGIOCLASE

40
Q

What are UHTS?

A

Medium pressure granulite Davies where peak temperatures reach 900-1100’C at 0.7-1.3GPa

41
Q

Where do UHTS occur?

A

Continental back arc settings due to crustal thinning and mantle rise

42
Q

What causes UHP?

A

Continued underthrusting of “cold” oceanic crust causes low T high P metamorphism within the slab and accretionary wedge

43
Q

High series

A

Blueschist&raquo_space; eclogite

SUBDUCTION ZONES

44
Q

Blueschist

A

Ancient subduction zone indicator

SODIC BLUE AMPHIBOLE (GLAUCOPHANE) which is only present at high pressure

45
Q

Eclogite

A

High density
(Subducted oceanic>surrounding mantle)

In eclogite facies conditions albite (plagioclase) breaks down at HIGH PRESSURE =

GREEN JADEITIC PYROXENE AND QUARTZ

NaAlSi3O8 = NaAlSi2O6 + SiO2

46
Q

METAMORPHIC GRADE =

A

Temperature and Pressure AT TMAX

PMAX occurs before TMAX (picture a loop with pressure on y axis and temp on x axis)

47
Q

How do we measure the geotherm in ancient mountain belts?

A

We can’t, we can only plot P-T histories of individual rocks

Study an array to create a METAMORPHIC GEOTHERM

48
Q

Diagenesis of quartz

A

Stable

Precipitated as cement - overgrowth on reworked quartz
Dust line showing original boundary of grain

49
Q

Feldspar

A

To clay minerals, especially kaolinite

50
Q

Calcite

A

Common cement in sandstones, often poikiolitic

51
Q

Dolomite

A

CaMg(CO3)2

Characteristic rhombic crystals

52
Q

Hematite

A

Thin coating

53
Q

Limestone diagenesis

A

1) compaction
2) cementation
3) dissolution
4) replacement/recrystallisation
5) dolomitization

54
Q

Carbonate diagenetic environments

A

MARINE
Little circulation
Sea floor becomes cemented to form hard ground

METEORIC
Fresh water affects sediments

BURIAL
Compaction and cementation

55
Q

Lamination vs beds

A

Laminations = few mm thick

Beds = greater than 1cm thick

56
Q

How to determine metamorphic P-T paths

A

1) observe partial overprints of one mineral assemblage upon another
2) geothermometers/geo barometers on core vs rim compositions of chemically zoned minerals

Earth Materials (3 decks)

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