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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
Lithostatic pressure =
Due to overlying rock weight Therefore increases with depth Can be isostatic or non isostatic (=mineral alignment)
26
Tectonic overpressure =
Developed during orogenic metamorphism/deformation DOES NOT AFFECT EXISTENCE OF METAMORPHIC MINERALS BUT IMPORTANT FOR DEVELOPMENT OF TEXTURES Limited by rock strength
27
Fluid pressure =
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
Types of stress
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
Post-kinematic porphyroblast growths
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
Pre kinematic porphyroblast growth
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
Syn kinematic porphyroblast growth
Rotational porphyroblasts where internal schistosity is continuous with external schistosity Suggests deformation did not outlast porphyroblast growth
32
Mafic rock metamorphism - 3 types
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
Low series
Contact hornfels
34
Medium series
Greenschist >> amphibolite >> granulite Most common for regional metamorphism
36
Greenschist
Correlates with chlorite and biotite pelitic zones ``` Biotite CHLORITE Plagioclase GREEN AMPHIBOLE Epidote Quartz ```
37
Amphibolite
AMPHIBOLE | PLAGIOCLASE
38
Granulite N.B...
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
Ultra mafic garnet-pyroxenite
PYROXENES GARNET PLAGIOCLASE
40
What are UHTS?
Medium pressure granulite Davies where peak temperatures reach 900-1100’C at 0.7-1.3GPa
41
Where do UHTS occur?
Continental back arc settings due to crustal thinning and mantle rise
42
What causes UHP?
Continued underthrusting of “cold” oceanic crust causes low T high P metamorphism within the slab and accretionary wedge
43
High series
Blueschist >> eclogite SUBDUCTION ZONES
44
Blueschist
Ancient subduction zone indicator | SODIC BLUE AMPHIBOLE (GLAUCOPHANE) which is only present at high pressure
45
Eclogite
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
METAMORPHIC GRADE =
Temperature and Pressure AT TMAX PMAX occurs before TMAX (picture a loop with pressure on y axis and temp on x axis)
47
How do we measure the geotherm in ancient mountain belts?
We can’t, we can only plot P-T histories of individual rocks Study an array to create a METAMORPHIC GEOTHERM
48
Diagenesis of quartz
Stable Precipitated as cement - overgrowth on reworked quartz Dust line showing original boundary of grain
49
Feldspar
To clay minerals, especially kaolinite
50
Calcite
Common cement in sandstones, often poikiolitic
51
Dolomite
CaMg(CO3)2 | Characteristic rhombic crystals
52
Hematite
Thin coating
53
Limestone diagenesis
1) compaction 2) cementation 3) dissolution 4) replacement/recrystallisation 5) dolomitization
54
Carbonate diagenetic environments
MARINE Little circulation Sea floor becomes cemented to form hard ground METEORIC Fresh water affects sediments BURIAL Compaction and cementation
55
Lamination vs beds
Laminations = few mm thick Beds = greater than 1cm thick
56
How to determine metamorphic P-T paths
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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