Metamorphic Rocks Flashcards
Metamorphism
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
Key controls on metamorphism (and explain)
Temperature Pressure Fluids Original rock composition Time
Evidence for the existence of metamorphic fluids
Fluid inclusions
- required for hydrous or carbonate phases
- reactions involving volatiles occur at temperatures and pressure different to anhydrous systems
Types of regional metamorphism
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
Size and shape of the aureole depends on…
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
Foliation vs lineation
Foliation = any planar fabric element (“flattened”)
Lineation = any linear fabric elements (“stretched”)
Slaty cleavage =
Any type of foliation in which platy phyllosilicates are to fine to see individually unaided
Schistosity =
Preferred mineral grain/grain aggregate orientation due to metamorphic processes
Coarse enough to see unaided
Gneissose fabric
Segregated into layers due to metamorphic processes
BANDING
Slate =
Compact
Fine grained
Well developed cleavage
Freshly cleaved surfaces are dull
Phyllite
Very fine phyllosilicates
Silky sheen to foliation surface
Schist =
Metamorphic rock exhibiting schistosity
Slates and phyllites are also schists
Gneiss =
Metamorphic rock displaying gneissose structure
Non foliated rocks
And their definitions
GRANOFELS
Any isotropic rock
HORNFELS
Typically fine grained and compact, occurs in contact aureoles
Tough and splinter when broken
Porphyroblastic =
Metamorphic rock with one or more minerals that grew much larger than the others
Individual crystals = porphyroblasts
Pelitic =
Clay rich
Barrovian zones
Chlorite
Biotite
Garnet
Staurolite
Kyanite
Sillimanite
Chlorite zone
Slates and phyllites
CHLORITE
Muscovite
Quartz
Plagioclase
Biotite zone
Phyllites and schists
Biotite Chlorite Muscovite Quartz Plagioclase
400-450 DEGREES
Garnet zone
Schists
Alamandine garnet (red) Biotite Chlorite Muscovite Quartz Plagioclase
~550 DEGREES
Staurolite zone
Schists
STAUROLITE Biotite Muscovite Quartz Plagioclase Garnet ~chlorite
550-600 DEGREES
Kyanite zone
Schists
Kyanite Biotite Muscovite Quartz Plagioclase Garnet Staurolite
530-630 DEGREES
Sillimanite zone
Schists/gneisses
Sillimanite Biotite Muscovite Quartz Plagioclase Garnet ~staurolite
530-630 DEGREES
Isograd =
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
Lithostatic pressure =
Due to overlying rock weight
Therefore increases with depth
Can be isostatic or non isostatic (=mineral alignment)
Tectonic overpressure =
Developed during orogenic metamorphism/deformation
DOES NOT AFFECT EXISTENCE OF METAMORPHIC MINERALS BUT IMPORTANT FOR DEVELOPMENT OF TEXTURES
Limited by rock strength
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
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
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
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
Syn kinematic porphyroblast growth
Rotational porphyroblasts where internal schistosity is continuous with external schistosity
Suggests deformation did not outlast porphyroblast growth
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
Low series
Contact hornfels
Medium series
Greenschist»_space; amphibolite»_space; granulite
Most common for regional metamorphism
Greenschist
Correlates with chlorite and biotite pelitic zones
Biotite CHLORITE Plagioclase GREEN AMPHIBOLE Epidote Quartz
Amphibolite
AMPHIBOLE
PLAGIOCLASE
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
Ultra mafic garnet-pyroxenite
PYROXENES
GARNET
PLAGIOCLASE
What are UHTS?
Medium pressure granulite Davies where peak temperatures reach 900-1100’C at 0.7-1.3GPa
Where do UHTS occur?
Continental back arc settings due to crustal thinning and mantle rise
What causes UHP?
Continued underthrusting of “cold” oceanic crust causes low T high P metamorphism within the slab and accretionary wedge
High series
Blueschist»_space; eclogite
SUBDUCTION ZONES
Blueschist
Ancient subduction zone indicator
SODIC BLUE AMPHIBOLE (GLAUCOPHANE) which is only present at high pressure
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
METAMORPHIC GRADE =
Temperature and Pressure AT TMAX
PMAX occurs before TMAX (picture a loop with pressure on y axis and temp on x axis)
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
Diagenesis of quartz
Stable
Precipitated as cement - overgrowth on reworked quartz
Dust line showing original boundary of grain
Feldspar
To clay minerals, especially kaolinite
Calcite
Common cement in sandstones, often poikiolitic
Dolomite
CaMg(CO3)2
Characteristic rhombic crystals
Hematite
Thin coating
Limestone diagenesis
1) compaction
2) cementation
3) dissolution
4) replacement/recrystallisation
5) dolomitization
Carbonate diagenetic environments
MARINE
Little circulation
Sea floor becomes cemented to form hard ground
METEORIC
Fresh water affects sediments
BURIAL
Compaction and cementation
Lamination vs beds
Laminations = few mm thick
Beds = greater than 1cm thick
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
