How and where do metamorphic rocks form

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METAMORPHISM AND TECTONICS

• How and where do metamorphic rocks form?
• How and where are metamorphic rocks exposed?
• What does a typical P-T-t-d path look like? Why?
• How are P-T-t-d paths determined from rocks?
• How are P-T-t-d paths interpreted?

2

• How and where do metamorphic rocks form?

Metamorphism is a response to change in P, T, X,
s, e
DP crustal thickening and/or thinning,
subduction, rifting DT heat transport by/through
rocks, fluids (advection, conduction) DX mass
transport by fluids, diffusion Ds, e
deformation normally requires tectonic
processes e.g. mountain-building (orogeny)
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2. How and where are metamorphic rocks exposed?
Metamorphism cannot occur at the Earths
surface (that is called weathering!) Exposure of
(deeply) buried rocks is referred to as exhumation
NOT the same as erosion uplift
Why are these processes different? ..... a few
definitions
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2. How and where are metamorphic rocks exposed?
  erosion physical breakdown, chemical
solution, removal and transport of material
from a surface uplift increased elevation of a
surface or a rock (relative
to geoid or some other reference
horizon) exhumation removal of overlying
material from a surface or rock
(decreased burial depth) 
crustal thickening
A
crustal thinning
A
A
sea level
B
B
B
surface A rock B
A uplifted A subsided B buried
B exhumed
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2. How and where are metamorphic rocks exposed?
Metamorphism cannot occur at the Earths
surface (that is called weathering!) Exposure of
(deeply) buried rocks is referred to as exhumation

• Exhumation processes include
• syn-orogenic erosion (1-10 mm/y)
• post-orogenic erosion (ltlt1 mm/y)
• normal faulting (1-10 mm/y)
• combination of these

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METAMORPHISM AND TECTONICS
DP crustal thickening and/or thinning,
subduction, rifting exhumation DT heat
transport by/through rocks, fluids DX mass
transport by fluids, diffusion Ds, e
deformation all of these processes vary in
orogenic belts as a function of time
and space can link metamorphism and tectonics
via construction, interpretation, modelling
of P-T-t-d paths
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3. What does a typical P-T-t-d path look like?
Why?
clockwise loop petrological reference
frame (Winter Ch. 25.4)
Pressure (depth)
Temperature
typical form of P-T-t-d path resulting from
crustal thickening (most orogenic belts)
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3. What does a typical P-T-t-d path look like?
Why?
Temperature
geophysical reference frame (P increasing
downwards)
Pressure (depth)
typical form of P-T-t-d path resulting from
crustal thickening (most orogenic belts)
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3. What does a typical P-T-t-d path look like?
Why?
T
stable geotherm
100 C
crust
depth (z)
300 C
500 C
thermal structure of stable crust depends
on internal heat sources, mantle heat flux heat
capacity, conductivity, diffusivity density,
crustal thickness .......etc...
mantle
consider changes to crustal column (1D) resulting
from tectonic processes
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3. What does a typical P-T-t-d path look like?
Why?
T
A
stable geotherm
100 C
crust
depth (z)
300 C
500 C
instantaneously double crustal thickness by
thrusting!! what happens to geotherm?? what
happens to point A??
mantle
consider changes to crustal column (1D) resulting
from tectonic processes
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3. What does a typical P-T-t-d path look like?
Why?
T
100 C
crust
depth (z)
300 C
500 C
perturbed geotherm (unstable)
A
100 C
300 C
500 C
immediately after thrusting what happens to P and
T ?
mantle
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3. What does a typical P-T-t-d path look like?
Why?
T
T
t0 (pre-thrusting)
crust
P (z)
A
t1 (immediately post-thrusting)
instantaneous P increase initially no change in T
(why?) what happens next?
mantle
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3. What does a typical P-T-t-d path look like?
Why?
T
T
t0 (pre-thrusting)
crust
t2 (relaxation)
P (z)
A
t1 (immediately post-thrusting)
thermal relaxation gradual return to stable
geotherm T increases at constant P
mantle
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3. What does a typical P-T-t-d path look like?
Why?
T
T
t0 (pre-thrusting)
crust
t2 ? tn (relaxation)
P (z)
A
t1 (immediately post-thrusting)
thermal relaxation gradual return to stable
geotherm T increases at constant P
mantle
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3. What does a typical P-T-t-d path look like?
Why?
what about exhumation??
T
T
t0 (pre-thrusting)
crust
t2 ? tn (relaxation)
P (z)
A
t1 (immediately post-thrusting)
thermal relaxation gradual return to stable
geotherm T increases at constant P
mantle
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3. What does a typical P-T-t-d path look like?
Why?
what about exhumation??
T
T
erosion
t0 (pre-thrusting)
crust
t2 (relaxation)
P (z)
A
t1 (immediately post-thrusting)
mantle
crustal thickening creates topography ? erosion
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3. What does a typical P-T-t-d path look like?
Why?
what about exhumation??
T
T
erosion
crust
t0
(exhumation and cooling)
t3
P (z)
A
t1
t2
mantle
erosion ? exhumation P and T decrease together as
rocks approach cold surface
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3. What does a typical P-T-t-d path look like?
Why?
Temperature
Note constraints on early prograde
history normally absent peak P normally precedes
peak T why??
t0
retrograde
t4
?
?
Pressure (depth)
prograde
peak T
t3
t1
t2
peak P
consider gradual rather than instantaneous changes
in burial, exhumation, heating, cooling ?
typical P-T-t loop
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3. What does a typical P-T-t-d path look like?
Why?
What controls Temperature?
crustal heat production (U, Th, K) amount and
distribution
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3. What does a typical P-T-t-d path look like?
Why?
What controls Temperature?
temperature ? viscosity ? deformation
cool, stable crust
orogenesis buries and/or thickens heat- producing
material hot, weak crust
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3. What does a typical P-T-t-d path look like?
Why?
What controls Temperature?
time-scale for internal self-heating 20-25 My
cool, stable crust
orogenesis buries and/or thickens heat- producing
material hot, weak crust
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4. How are P-T-t-d paths determined from rocks?
petrography standard thermobarometry
Temperature
relative age
a prograde assemblage (inclusions, gnt
core, S1)
d
b peak P assemblage (gnt core-rim, early
matrix, S2)
Pressure (depth)
c peak T assemblage (gnt rim, main
matrix, S3)
c
a
d retrograde assemblage (fluid
inclusions, S4)
b
P-T calculation from thermobarometry lab exercise
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4. How are P-T-t-d paths determined from rocks?
petrography standard thermobarometry
thermochronology
Temperature
absolute age
a prograde assemblage (inclusions, gnt
core, S1)
410 Ma
d
b peak P assemblage (gnt core-rim, early
matrix, S2)
Pressure (depth)
420 Ma
c peak T assemblage (gnt rim, main
matrix, S3)
c
a
d retrograde assemblage (fluid
inclusions, S4)
440 Ma
425 Ma
b
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4. How are P-T-t-d paths determined from rocks?
Temperature
Fe
Mg
Ca
500 mm
garnet zoning profiles X-ray maps from EMP
Al
Mn
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4. How are P-T-t-d paths determined from rocks?
garnet zoning profiles
Temperature
Given final P-T conditions (gnt rim
matrix) garnet zoning profile reaction
history thermodynamic parameters Calculate P, T
as a function of t
Pressure (depth)
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4. How are P-T-t-d paths determined from rocks?
calculate from first principles
Temperature
Given starting conditions (T,z) thermal
parameters mechanical parameters tectonic
parameters Calculate P, T as a function of
t deformation history
Pressure (depth)
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4. How are P-T-t-d paths determined from rocks?
ideally, combine approaches
Temperature
Test calculations against P-T-t-d data Interpret
in terms of thermal-tectonic processes
d
Pressure (depth)
c
a
Note all of these are model-dependent!
b
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4. How are P-T-t-d paths determined from rocks?

• some details
• P-T calculations from thermobarometry
• lab assignment
• b) T-t histories from thermochronology

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4. How are P-T-t-d paths determined from rocks?
T-t histories from thermochronology
What are temperature-time (T-t) histories?
Age data from a cooling pluton Separation Point
Batholith, New Zealand (Harrison and McDougall,
1980)
Why are these ages different? What do the
differences tell us?
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4. How are P-T-t-d paths determined from rocks?
T-t histories from thermochronology
What is the link between temperature and age?
typical cooling curve
because diffusion rate depends strongly on T,
this graph also shows how diffusion rate
changes with time
time ?
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4. How are P-T-t-d paths determined from rocks?
T-t histories from thermochronology
What is the link between temperature and age?
typical cooling curve
because diffusion rate depends strongly on T,
this graph also shows how diffusion rate
changes with time
time ?
D/P ratio ( age!!) increases as diffusion rate
decreases
(Dodson, 1973)
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4. How are P-T-t-d paths determined from rocks?
T-t histories from thermochronology
What is the link between temperature and age?
typical cooling curve
because diffusion rate depends strongly on T,
this graph also shows how diffusion rate
changes with time
time ?
Tc closure temperature T at which host
closed to diffusion of D
D/P ratio ( age!!) increases as diffusion rate
decreases
(Dodson, 1973)
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4. How are P-T-t-d paths determined from rocks?
T-t histories from thermochronology
Crystallisation (growth) age vs cooling age
crystallisation age time at which the dated
crystal formed cooling age time at which the
dated crystal became closed
to diffusion
time at which the dated crystal cooled
through its
closure temperature (Tc)
crystallisation age cooling age if and only
if crystal in question formed at T lt Tc or system
quenched
crystallisation age ? event or stage in
process cooling age ? stage in a
process set of related ages ? duration rate of
process
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4. How are P-T-t-d paths determined from rocks?
T-t histories from thermochronology
What kinds of T-t data are available?
temperature ranges, selected isotopic systems
Tc closure temperature PAZ partial annealing
zone
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4. How are P-T-t-d paths determined from rocks?
T-t histories from thermochronology
What do T-t data tell us about processes?
Age data from a cooling pluton Separation Point
Batholith, New Zealand (Harrison and McDougall,
1980)
crystallisation age batholith intrusion
rapid cooling from intrusion temperature to
ambient conditions
slow cooling with surrounding rocks
What does this tell us about cooling rate?
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4. How are P-T-t-d paths determined from rocks?

• some details
• P-T calculations from thermobarometry
• lab assignment
• b) T-t histories from thermochronology

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5. How are P-T-t-d paths interpreted?
combine petrology with tectonic models
Temperature
Test calculations against P-T-t-d data Interpret
in terms of thermal-tectonic processes
d
Pressure (depth)
c
a
Note all of these are model-dependent!
b
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5. How are P-T-t-d paths interpreted?
representative path styles and first-order
interpretations
Temperature
Temperature
Pressure (depth)
Pressure (depth)
standard loop
isothermal decompression
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5. How are P-T-t-d paths interpreted?
representative path styles and first-order
interpretations
Temperature
Temperature
Pressure (depth)
Pressure (depth)
standard loop
isothermal decompression
isothermal decompression paths rapid exhumation
? decompression (P decrease) no time to cool ?
isothermal (T constant)
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5. How are P-T-t-d paths interpreted?
representative path styles and first-order
interpretations
Temperature
Temperature
Pressure (depth)
Pressure (depth)
isobaric heating/cooling
standard loop
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5. How are P-T-t-d paths interpreted?
representative path styles and first-order
interpretations
Temperature
Temperature
Pressure (depth)
Pressure (depth)
isobaric heating/cooling
standard loop
isobaric heating/cooling paths little or no
exhumation ? isobaric (P constant) amount/rate of
heating/cooling controlled by heat source(s)
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5. How are P-T-t-d paths interpreted?
representative path styles and first-order
interpretations
Temperature
Temperature
Pressure (depth)
Pressure (depth)
standard loop
hairpin
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5. How are P-T-t-d paths interpreted?
representative path styles and first-order
interpretations
Temperature
Temperature
Pressure (depth)
Pressure (depth)
standard loop
hairpin
hairpin paths peak P and peak T approximately
coincide rapid burial and exhumation (limited
thermal relaxation)
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5. How are P-T-t-d paths interpreted?

• models suggest
• 2 fundamental controls
• on orogenic style
• Magnitude (mass)
• Temperature
• real orogenic belts
• can be understood
• in terms of competition
• between erosion
• ( M, T)
• and orogenesis
• ( M, T)

(Beaumont et al. 2006)