Metamorphic Petrology And Processes Flashcards
what is metamorphism?
the process of mineralogical and structural adjustment by which sedimentary and igneous rocks are modified in the earths crust.
why does metamorphism occur?
plate tectonics drive continuous change
(orogenesis, subduction, rifting, magmatism)
what are mineralogical changes controlled by?
thermodynamics
what is the pressure of a rock controlled by?
weight of overlying rock (lithostatic pressure)
tectonic forces
what is lithostatic pressure?
weight of overlying rock
=density x gravity x height
what pressure does 1km of overlying rock exert?
0.294 kbar
what is the standard density of rocks, and thus their pressure?
2700 to 3300kgm-3
0.264-0.323kbar km-1
what is the typical pressure of crust that is 30-40km?
9-12 kbar
where is pressure the highest in the crust?
subduction zones - 30-40kbar
what is the temperature range at metamorphic pressures?
200-1000 degrees C
What is the geothermal gradient?
temperature increases with depth
where does heat for metamorphism come from?
mantle conduction and advection
radiogenic isotopes
how is heat distributed within the crust?
conduction - flow of heat
convection - fluid circulation
advection - intrusion of igneous bodies
what are changing pressures and temperatures driven by?
plate tectonics (orogeny or magmatism)
why do minerals change at different temperatures and pressures?
thermodynamic stability
laws of thermodynamics dictate that matter will try to form the lowest energy configuration.
what are the types of metamorphism?
regional or contact metamorphism
hydrothermal, impact, fault related
why is the temperature-depth relationship not linear?
conductivity and radioactivity
how does regional metamorphism work?
orogenesis causes thickening and deformation
thickened crust causes burial and heating
long time scale
how does contact metamorphism work?
intrusion of magma into the crust causing a local thermal anomalyw
what is a contact aureole?
metamorphic halo that forms around an intrusion
temperature driven
short time scale
can occur on regional scales (regional aureoles)
why does contact metamorphism occur?
result of a high geothermal gradient produced locally around intruding magma
how does hydrothermal metamorphism occur?
may accompany regional and contact metamorphism
occurs near ocean ridges
temperature and pressure gradients drive fluid flow - metasomatism
how does impact metamorphism occur?
drop a big ass rock on the earth
enormous transient pressure and temperature changes
very short lived
temperature from friction
how does fault related metamorphism occur?
brittle or ductile deformation
friction provides additional heat
where does metamorphism occur?
restricted to tectonically or magmatically active parts of the crust
why do we see metamorphic rocks?
exhumed by isostasy (subsidence + erosion)
extensional collapse of mountains (rapid uplight and outward dipping faults
what controls what metamorphic rocks look like?
composition
pressure and temperature attained
what deformation has occured
coarser grains = hotter
what happens when a rock is metamorphosed?
new minerals replace old ones
texture changes
what happens to siltstones at different metamorphic grades?
low temperature = fine grained micas = slate
high temperature = coarse micas (dark blobs)
higher temperatures = banded appearance and is coarsely grained
what is slate?
very fine grained, mica rich rock
individual micas cannot be easilt seen
what is a schist?
fine to coarse grained rock
continuous layers of mica or amphibole
breaks along foliation
what is a gneiss?
coarse grained rock
individual mica grains are not in contact/discontinuous lines
what is a mylonite?
very fine grained rock formed via intense deformation in shear zones
what is migmatite/migmatite gneiss?
coarse grained rock composed of distinct light and dark layers
formed from partial melting
what is a hornfel?
fine to medium grained rock that lacks obvious deformation features (no foliation or lineation)
contact metamorphism
what is a granofel?
medium to coarse grained rock that lacks obvious deformation features (no foliation or lineation
high temp contact metamorphism or low strain regional meta
what is a pelite?
siltstones and sandstones
metapelite
what is a psammite?
arenite and sandstones
what are the protoliths of metamorphosed sedimentary rocks?
metapelite
metapsammite
metagreywacke
what are the protoliths of metamorphosed igneous rocks>
metabasic/metabasalt
metagranite
what are metamorphic assemblages controlled by?
bulk composition (protolith)
temperature
pressure
what determines what minerals, proportions and compositions occur in a given bulk rock composition at P&T?
equilibrium thermodynamics
how do we make a phase diagram?
qualitive and experiments constraints
what are qualitive measures?
simple comparisons of changes in assemblages
what is a phase diagram>
pictorial representations of mineral relationships
what are the main types of metamorphic phase diagrams?
petrogenetic grids or P-T projections
compatibility diagrams
pseudosections - cut through of a total phase diagram
what is a total phase diagram
a diagram that has all the pressure, temperature and composition information
what is varience?
degrees of freedom
what are the twins of pressure, temperature and composition?
pressure = volume
temperature = entropy
composition = chemical potential
what are the basic rules determining what are phase diagram looks like?
phase rule
schreinemakers rule
multivariant boundary rule
what is the phase rule?
relates the number of phases to the number of chemical components being considered and the vairance
what is schreinemakers rule?
controls the topography (shapes) of diagrams
no mineral assemblages can exist >180 arc
what is the multivariant rule?
controls topography of diagrams
variance across a boundary between two assemblages can only change by 1
variance change through a point is either 0 or 2
what is a component?
basic chemical building blocks by which we define mineral and rock composition
what is a system?
chemical system being studied
sum of components
what is variance equal to?
Variance = Components - number of Phases + 2
how do we annotate variance?
V= 0 = invariant
1=univariant
2 =divariant
3=trivariant
4=quadrivariant
5=quinivariant
6=hexivariant
7=septivariant
8=octivariant
what happens at an invariant point?
reaction will stop
how do you form a phase diagram?
plot experimental lines
annotate each side of the lines with the phase
remove lines between the same phases
check logically
What is a petrogenetic grid?
Bundles of univariant reactions and invariant points
What are the limitations of a petrogenetic grid?
Only shows stable reactions
Specific to a given chemical system
What is KFMASH?
K2O - FeO - MgO - Al2O3 - SiO2 - H2O
What does a compatibility diagram look like?
Usually triangular plots showing mineral compositions and stable assemblages
Fixed p and t
What are the criteria for a compatibility diagram?
Usually triangular plots showing mineral compositions and stable assemblages
Fixed p and t
What does a compatibility diagram show?
What minerals are stable for different pressure and temperature conditions
Where does biotite plot on a compatibility diagram?
Below the triangle
What does crossing tie lines mean?
There is a reaction possible
What does a pseudo section show?
All mineral equilibria relationships of a given rock
Stability of mineral assemblages in a composition
What do lighter shades on a compatibility diagram typically represent?
More minerals and less variance
What is a terminal reaction?
When a phase gets stuck in the middle of a diagram and is surrounded by 3 triangles
1:3 reaction
Ie ctd -> g + chl + st
What does ctd +als give us?
St + chl
What does ctd react to?
G+ chl + st
What des g + chl react to?
St + bi
What does st + chl react to?
Bi + als
What does bi + st react to?
G + als
what is the most common type of metamorphism?
regional, characterised by metamorphism and deformation
what are the conditions of regional metamorphism?
intermediate pressure and temperature
what are the facies associated with regional metamorphism?
greenschist
amphibolite
granulite
what are the stages of metamorphism?
prograde metamorphism
peak metamorphism
retrograde metamorphism
what happens during prograde metamorphism?
burial and heating
what happens during peak metamorphism?
when a rock reaches its peak temperature
what happens during retrograde metamorphism?
exhumation and cooling
why does peak temperature not occur at peak pressure?
erosion and exhumation
(closer to the surface)
what is a P-T path?
the evolution of a rock over time
when would you use a P-T-t path?
when there are timing constraints from geochronology
what information does a P-T path give us?
nature of orogenesis
- relative rates of burial or heating
- relate back to tectonic setting
in what way can a P-T path vary through an orogen?
variation
- vertically
- horizontally
- timing of peak metamorphism
why do rocks not reverse react when they are retrograded?
need water to do reverse reaction that is not available
what is the progression of minerals with increasing metamorphism?
chlorite ->biotite -> garnet -> staurolite -> kyanite -> sillimanite
what do we see with increased metamorphic grade?
increasing grain size
what is an isograd?
the point at which a new mineral is observed with increasing temperature
what do we see in the chlorite zone?
pelitic rocks of chlorite, muscovite, quartz
slates=phyllites
some albite and graphite
what do we see in the biotite zone?
pelitic rocks of biotite, chlorite, muscovite, albite and quartz#
increase in biotite towards garnet zone
what do we see in the garnet zone?
pelitic rocks of garnet, biotite, muscovite, albite and quartz.
sometime chlorite present
phyllites and schist
garnet grains are Fe rich
what are Fe rich garnets called?
almandine
what do we see in the staurolite zone?
pelitic rocks of garnet, staurolite, biotite, muscovite, plagioclase and quartz
schists
what do we see in the kyanite zone?
staurolite - kyanite schist is dominant
garnet/biotite, muscovite, plagioclase, quartz
schists
what do we see in the sillimanite zone?
sillimanite replaces kyanite due to stability
schists
lower sillimanite zone has muscovite
upper sillimanite zone has k-feldspar
what is a field gradient?
lateral grade changes
give an example of a barrovian sequence?
NE America Massechutus /New Hampshire
Scottish Dalradian
what is HPLT?
High pressure low temperature metamorphim
what is HPLT typified by?
blueschist to eclogite transition
what causes HPLT metamorphism?
subduction processes
fast burial and exhumation too fast for thermal equilibrium
what is glaucophane?
BLUESCHIST
a blue amphibole mineral found in blueschist metamorphic rocks
what is omphacite?
ECLOGITE
a green pyroxene found in very high pressure metamorphic rocks
what happens during the glaucophane -> omphacite transition?
water is lost from the system and density increases
release of water lowers the melting point of the mantle
What is UHP?
Ultra high pressure metamorphism
seen in coesite and diamonds
what is special about coesite?
exists outside normal stability fields for normal P and T
mostly observed in impact craters, Alps
what are the hot and cold zones of HPLT?
Hot = arc
Cold = subducting plate
what are the characteristics of coesite?
exists at high pressure
always partially degenerated to quartz - pallisade
present in garnet as intrusions with radial fractures present as quartz expands
why dont we find coesite in older, archean rocks?
warmer conditions affect the thermal equilibrium so it doesnt form
change in tectonics
erosion
where do we find coesite?
Dora Maria (Alps)
Norway
Dabie-Sulu belt (china)
himalayas
are diamonds present in UHP metamorphic rocks?
yes, as microdiamonds in crustal rocks
5-100 u m
where are diamonds typically found?
at mantle conditions
exist as microdiamonds in crustal rocks
how can diamond be identified in thin section?
creates radial scratches in other minerals like a scary sea urchin
what is the upper date boundary for blueschists and eclogite?
800Ma blueschist boundary
eclogite becomes increasingly rarer the further back in the geological record you go
what is the origin of UHP rocks?
continent=continent collision less buoyant crust goes down
greater depths can be achieved if the crust is attached to oceanic crust further down. This section breaks up, reducing the pull force and causing buoyancy to force the crust back to the surface
what is HTLP?
High temperature low pressure metamorphism
similar minerals to contact metamorphism but is regional and deformed
give examples of where HTLP rocks are found
Banff Buchan Zones
Australia
Antarctica
South America
continental gondwana
what are characteristics of HTLP rocks?
andalusite-sillimanite transitions
k feldspar before melting
cordierite assemblages
fe-spinel assemblages
why are HTLP rocks characteristic of gondwana?
different tectonic history/style
what are the buchan zones?
from low to high grade:
biotite zone
cordierute zone
andalusite zone
sillimanite zone
what is broken hill?
a HTLP area larger than buchan zones
has an andalusite zone, sillimanite-muscovite zone, sillimanite-kfeldspar zone, and two pyroxene zone
what is the most extreme example of HTLP?
Mt Stafford, central australia
800 degrees at 10-15km depth
lots of Fe spinel
melting
what causes HTLP zones?
thinning of the lithosphere or loss of mantle
magmatic heat (thermal anomaly)
high internal heat production from radiogenic elements
What does fluids within a rock allow?
Stability
Element diffusion through the rock
What minerals are fluids tied up in?
Mica, chlorite, amphibole
Breakdown of these hydrous minerals release OH as H2O
Are rocks open or closed systems?
Open
What does it mean that rocks are open systems?>
Composition changes during metamorphism
Material may enter or leave the rock
What happens to fluid during metamorphism?
Fluid is highly mobile so most leaves the rock
But as rocks heat up more fluid is produced
What are the characteristics of fluids?
Can treat as another phase in a rock that can move
Mixture-solution
Can host ionic species and complexes
high pressure and high temperature = increased fluid
What are potential fluids?
H20
CO2
CH4 - low grade
What are potential ionic species dissolved by fluids?
Na, K, Ca, Cl, Au, Pb, Zn
What are the characteristics of deep basinal brines?
Very saline
Mostly diagenetic
What are the characteristics of metamorphic fluids?
Low salinity- salty fluid removed
Dominated by H2O-CO2 mixes
Produced by the breakdown of hydrous minerals (H2O)
Produced by the breakdown of carbonate minerals (CO2)
What do the H2O-CO2 mixes create?
H2O»»CO2 = pelites, greywackes
H2O»CO2 = altered seafloor basalts
CO2>H2O = limestones, marls
What is evidence for fluids?
Trapped fluids - in quartz, minimal
Inclusions - easily overprinted, only useful for low grade
Inferred from our understanding - doesn’t tell us the composition of the fluid
Clays (high H2O) -> Schist (moderate H2O) -> Gneiss (low H2O)
Mineral equilibria - can predict fluid content and composition
why are fluids important in metamorphism
lots of minerals have water in their structure, therefore fluids allow those rocks to be stable
presence of a fluid changes the strength of a rock - deforms easier
allows diffusion between grade boundaries
what are the main groups of minerals that are hydrous and common?
mica
chlorite
amphibole
(their breakdown produces fluid)
what happens to SiO2 as a rock is heated?
SiO2 is lost, fills veins of fluids
why do we call fluids supercritical?
temperature wants it to boil
pressure wants it to be a solid
where is fluid stored during metamorphism
most of OH tied up in minerals, some in pores 1%
during metamorphism = OH in minerals decreases significantly, OH in porosity decreases a little
is metamorphism typically hydrous or anhydrous?
most reactions are hydrous on low pressure side (prograde) release water
wants to consume water on retrograde but its moved on cant reverse it
why is metamorphism asymmetric?
dehydration reactions and loss of fluid
where do we find retrogressed rocks?
shear zones & fault zones - ie where water can go lower in the crust ie can hydrate
very concentrated areas
what are the characteristics of retrogressed minerals?
low temperature
fine grained
distinctly replace particular minerals
what does biotite regress to?
chlorite
what does garnet regress to?
biotite or chlorite
what does andalusite regress to?
muscovite
what does orthopyroxene regress to?
biotite
what does clinopyroxene regress to?
amphibole
why is cordierite so easily regressed?
contains small pockets of water that are easily accessed
very low self esteem
what does cordierite regress to?
sillimanite and biotite (+chlorite)
what is the most common mixed fluid?
H2O-CO2
What is the hassle with mixed fluids?
unknown composition effect on rocks
externally controlled equilibria
need to consider the composition of the rock and the fluid
XCO2=CO2/(H2O+CO2)
What controls equilibria of a mixed fluid system?
lots of fluid influx = fluid composition controls equilibria
little fluid influx = rock composition controls equilibria
what is devolatilisation?
loss of a mixed fluid
what is meant by external buffering?
rock is subject to an infinite reservoir of fluid, minerals will change to be in equilibrium with the fluid
any H2O or CO2 taken up or released by the rock is too small to change the overall fluid composition
what is meant by internal buffering?
fluid is only present in rock porosity, changes in minerals control the composition of the fluid
any H2O or CO2 taken up or released by the rock changes the overall fluid composition
what happens with buffering on evolution?
external = connects with the reaction at certain points in a triangle sort of way
internal = stays on the reactions - trapped on reaction.
at what point do rocks start melting?
~650 degrees C
What happens to fluid in pore space as a rock begins to melt?
gets sucked up into the melt
rock becomes fluid absent, melt bearing
what are the majority of igneous rocks?
granitic (ie quartz & feldspar rich)
basaltic (Si poor, Fe Mg rich)
how do we make basalts and gabbros?
from the mantle
how do we make granites?
from melting the crust. always
what do we call a rock that shows evidence for melting?
migmatities - distinctive dark and light minerals
what are the characteristics of melt?
high viscosity
moves slowly
can get trapped in a rock
what is the solidius?
the point at which a rock with decreasing temperature becomes fully solid
what is the liquidus?
the point at which a rock with increasing energy becomes fully lquid
what happens to metapelites at ~650 degrees?
k feldspar replaces muscovite
garnet and cordierite replace biotite and sillimanite
orthopyroxene replaces biotite
what happens to metabasites at ~650 degrees?
biotite breaks down
sphene and epidote is lost
clinopyroxene and orthopyroxene occurs at some loss of amphibole
at high pressure garnet appears
what is a silicate melt?
complex hydrous silicate solution
what are the characteristics of a sillicate melt?
contains many other elements ie alkalis
is hydrophylic- needs water
exchanges minerals with solid
becomes less hydrous with increasing temperature
what is the solidus reaction?
silicate melt initially consuming H2O
What happen s in a silicate melt when H2O is not present?
forms from dehydration breakdown of hydrous minerals
incongruent melting reaction
what are the main minerals reacting with water to make a melt?
quartz
feldspar
muscovite
what is the classic wet solidus melting reaction?
solids + H2O = liquid (+little solid)
what is the classical incongruent melting reaction?
solids (in which one is hydrous) =liquid + less hydrous solids
what are examples of hydrous minerals?
muscovite
biotite
what are the types of granite?
granite (K feldspar rich)
granodiorite (plagioclase, lower silicate)
tonalite (plagioclase)
how much melt can incongruent melting provide?
10-30%
why do we have areas of immense granite ie the cairngorms?
melt wants to rise
forms 1000’s km of plutons
lock and fold belt, e australia
what do high grade rocks look like?
larger crystals
pyroxenes, garnets etc
120 degree angle = granoblastic
what are migmities used for?
identifying melting
what controls the appearance of migmatites?
how much melt formed
where in the rock it formed
deformation
melt migration
whether melt separates from the solids or not
whether it escapes the source and by how much
what are the two divisions of migmatities?
metatexites - order
diatexites - chaos
what are metatextites?
migmatites in which earlier metamorphic and protolithic features have been largely preserved
discrete layers - stripey ferromagnesian and felsic layers
what are diatextites?
migmatites in which earlier metamorphic and protolithic features have been destroyed or disrupted
looks more like a magma than metamorphic rock.
melt and solid mixed together
rafts of metatextite presemt
what is a leucosome?
light coloured band
quartz and feldspar
what is a melanosome?
dark coloured band
Fe and Mg minerals
very dark layers are called Selvedge
what textures are seen within crystalised migmatites?
insy winsy baby granite
lack of deformation on cooling produces igneous textures i.e. K feldspars
what is a melt pseudomorph?
thin films of quartz or feldspar
forms leucosomes
one sitting in a pool of the other
perfect quartz = in a sea of feldspar
what are batholiths?
large scale melt features containing cubic kms of magma
mostly structurally controlled
why do crystals get larger with higher temperatures?
competing internal (Gibbs) and external (surface) energy
larger the crystal = decreased external energy compared to internal energy
how are prograde minerals preserved?
as inclusions in peak minerals ie chlorite in garnet
how are retrograde minerals presented?
overprint high grade minerals
i.e. chlorite replacing biotite or garnet
what does compositionally zoned minerals reflect?
reflecting change in equilibrium compositions
how can we determine prograde history that has been overprinted?
porphyroblasts
inclusions
prograde pseudomorph
if there are inclusions of glaucophane and lawsonite in eclogite facies garnet, what can we determine about the P-T path?
passed through the blueschist facies
what is a retrogression?
peak phase regresses to form an island of peak phase surrounded by fine grained retrograde minerals
ie garnet into garnet in a sea of chlorite
what are the common metamorphic textures?
simple - muscovite bearing quartzite
what are the characteristics of simple textures?
reflect basic relationships between coexisting minerals
controlled by metamorphic grade and intensity of deformation
what causes undulose extinction in quartz?
due to small defects in the crystal that has been deformed and is trying to grow n ew undeformed crystals
what are the types of foliations?
compositional layering
preferred orientation of platy minerals - micas
shape of deformed grains
grain size variation - fine vs coarse (deformation)
preferred orientation of platy minerals in a matrix without a preferred orientation - quartzite
preferred orientation of lenticular mineral aggregates
preferred orientation of fractures
mixture
what is a foliation evidence of?
deformation
how do you make ribbons of quartz?
shear and squashing it
stretches it out into lots of little grains, then recrystalises into a singular crystal
what is meant by granoblastic?
120 degree angles
large but similarly sized minerals
equant in shape
what determines the texture of a minerals?
the way different minerals try to minimise their energy, dependant on crystal structure
what are the six contact metamorphism textures?
granoblastic - 120 degree angles
decussate - straight rectangular
nodular - oval, rounded
porphyroblastic
poikiloblastic
skeletal/web
what is a pseudomorph?
where a new mineral mimics the shape of the older mineral - replacement texture
hornblende replaced by glaucophane by exchange of calcium for sodium - retrograde
garnet overgrowing biotite - prograde
why does garnet replace biotite along biotite cleavages?
where elements can diffuse more quicklu
what is symplectite?
intergrowth of spinel and cordierite
what is textural disequilibrium?
very high surface energy for the size of the mineral
reacting outwards, spikey, ragged crystal
what makes symplectite?
sillimanite + garnet being replaced
where do we see symplectite?
high temperature eclogite and granulite facies rocks
what is the relationship between foliations and porphyroblasts?
foliation wraps around porphyroblast = porph predates fol
porph overgorws the fol = porph postdates the fol
