Download Meta4-14PTTectonics

Temperature and Pressure Estimation
with
Tectonic Applications
Francis, 2014
Snow-ball
garnet
Classification of Regional Metamorphism on the Basis of P/T Ratio
1. Low P/T ratio:
•
•
andalusite → sillimanite series (cordierite before sillimanite)
In areas in which the shallow crust is heated by granitoid intrusions in volcanic arcs or
zones of continental collision, areas of crustal thinning, associated with high heat flows
and the rise of eithe basaltic and/or granitoid magmas.
Particularly common in Archean and Proterozoic terranes.
2. Medium P/T ratio: kyanite → sillimanite series (staurolite and sillimanite before cordierite)
•
•
•
In downward and/or lateral continuation with low P/T rocks in areas of lower geothermal gradient.
Abundant associated granitoid intrusions.
In lateral association with, though not necessarily continuous with, areas of high P/T metamorphic
rocks, in which case there are few or no granitoid intrusions.
In continental areas which have experience crustal thickening because of over thrusting.
3. High P/T ratio: blueschist series:
Na2Mg3Al2Si8O22(OH)2
Na2Fe2+3Fe3+2Si8O22(OH)2
Glaucophane  Crossite  Riebeckite
•
•
•
in accretionary prisms above subduction zones. Granitoids absent, abundance of metabasites and
meta-peridotites (oceanic crust), out of sequence blocks of eclogite common. They are commonly
“dirty”, fine-grained, poorly-equilibrated phyllitic rocks
Belts of high P/T rocks are characteristically diachronous - progressive, with temperature and age
increasing discontinuously towards the hinterland (typically a continent).
Largely restricted to Phanerozoic terranes, a few in Proterozoic terranes.
Pseudo-Invariant
Points
Dugald Carmichael, a
metamorphic petrologist at
Queens University, has
developed a scheme that
divides the pressures of
amphibolite
facies
metamorphism
into
6
bathozones separated by 5
metamorphic
invariant
points in model metapelite
systems:
Garnet
in
Muscovite
out
6 bathozones separated by 5 metamorphic invariant points in model metapelite
systems:
Bathozone 1 - garnet-in, musc-out, andalusite, sillimanite, partial melting
K-felds + and + vapor
qtz + musc + sill
F=1-3+2 = 0
Bathozone 2 - garnet-in, andalusite, sillimanite, musc-out, partial melting
bio + garn + and + vapor
qtz + musc + staur + sill
F=3-5+2 = 0
Bathozone 3 - andalusite, sillimanite, garnet-in, musc-out, partial melting
and
kyan + sill
F=1-3+2 = 0
Bathozone 4 - kyanite, sillimanite, garnet-in, musc- melting
qtz + musc + staur + sill
bio + garn + kyan + vapor
F=3-5+2 = 0
Bathozone 5 - garnet-in, kyanite, sillimanite, musc- melting
qtz + Na-felds + musc + sill
K-felds + kyan + granitic melt F=3-5+2 = 0
Bathozone 6 - garnet-in, musc- melting, kyanite, sillimanite
Bathozones – cont.
The beauty of this scheme
is that it depends only on
the
topology
of
discontinuous reactions in
PT space, and not on
mineral composition, and
thus can be used directly in
the field to estimate
pressures
simply
by
determining the order of
appearance
of
metamorphic phases within
increasing
metamorphic
grade.
XH20 ~ 0.5
Geothermometry
The best geothermometers are continuous exchange reactions that involve small volume changes
and are thus relatively insensitive to pressure. The most commonly used reaction is the exchange
of Fe and Mg between coexisting garnet and biotite:
Fe3Al2Si3O12 + KMg3AlSi3O10(OH)2`
Almandine
Phlogopite
Mg3Al2Si3O12 + KFe3AlSi3O10(OH)2
Pyrope
Annite
At equilibrium:
G
= 0 = GoTP + RTln ((aPy)( aAn)) / (aAl)( aPh))
where:
aPy = (XMgX)3(XAlY)2
aAl = (XFeX)3(XAlY)2
aAn = (XFeY)3
aPh = (XMgY)3
thus:
G
0
= 0=
=
Go
TP
Ho
T1bar
+ RTLn ((Mg/FeGa)3 / (Mg/FeBio)3)
- TSoT1bar + (P-1)∆V + RTLn (KD)
ln(KD) = - HoT1bar / RT + SoT1bar/R - ∆V(P-1)/RT
ln(KD) = a/T + b + cP, where a, b, c are ~
constant for solid-solid reactions and KD
is the Fe/Mg partition coefficient. If ∆V
(c) is small, then this is the equation of a
straight line in ln(KD) versus 1/T space:
ln(KD) = a / T + b
ln(KD )
= - 2089/T - 0.0096P/T + 0.782
with pressure term
Garnet – Biotite
Equilibria
Thermometry
garnet – biotite thermometer
Kd =(Mg/FeGa)3 / (Mg/FeBio)3
Geobarometry:
Most useful geobarometers are continuous net-transfer reactions. These are usually sensitive to both
temperature and pressure, and require an independent geothermometer for accurate pressure determination.
The most commonly used reaction is the equilibria between coexisting feldspar and garnet:
3CaAl2Si2O8
Anorthite
Ca3Al2Si3O12 + 2Al2SiO5 + SiO2
Grossularite + Kyanite + Qtz
At equilibrium:
G
= 0 = GoTP + RTLn ((aGr)( aKy)2(aQtz)/( aAn)3)
In natural systems, the feldspar and garnet are not pure endmembers, and the geobarometer requires the determination
of the activity of the end-member components in these minerals.
The activities of kyanite and quartz are assumed to be 1.
Go
TP
=-
RTln (aGr /aAn3)
where:
aGr = (XCaX)3(XAlY)2
aAn = XCaW
ln (aGr /aAn3) = - HoT1bar / RT + SoT1bar/R - ∆V(P-1)/RT
Kequil = 48,357/T - 150.66 + 6.608(P-1)/T
The loci of constant equilibrium constant
are straight lines in P-T space.
Thermobarometry
Kd = (Mg/FeGa)3 / (Mg/FeBio)3
Kd = ((XCaX)3(XAlY)2 )/ XCaW
garnet – biotite thermometer
garnet – plagioclase barometer
P - T - Time paths and Tectonics
One of the most interesting applications of
metamorphic petrology is its use, in conjunction
with structure and radiometric age dating, in
unraveling the tectonic history of orogenic zones.
Zoned garnets play an especially important role in
this exercise.
Because the kinetics of metamorphic reactions and diffusion are thermally activated, obeying the
Arrhenius relationship (rate  e-E/RT), reaction rates increase exponentially with temperature and thus the
metamorphic assemblage we see preserved in a rock is dominated by the peak metamorphic temperature
it has experienced, and may not record the peak pressure, nor its prograde and retrograde metamorphic
history. More importantly, however, is the realization that a sequence of associated rocks of increasing
metamorphic grade does not define an instantaneous geothermal gradient or the actual thermal structure
of the crust at any time during metamorphism, but rather the locus of peak metamorphic temperatures of
all P-T paths, which is known as the metamorphic field gradient. Individual rocks have not followed this
path and it is thus not possible to determine the reaction history of a high grade metamorphic rock by
examining a sptially associated low grade metamorphic rock of similar bulk composition.
Metamorphic
Field Gradient
Combining P - T determinations
with
Isotopic Geochonometers
The closure temperature for isotopic
equilibrium in minerals decreases in the
order: zircon, allanite, monazite, titanite,
hornblende, biotite, and apatite. Thus the
different ages obtained on each mineral can
be correlated with the temperture and
pressure estimates obtained from zoned
minerals to define the P-T-t history of a
rock suite.
Appalachians
of
New England
1. The Taconic Orogeny in the
mid-Ordovician associated with
the accretion of a volcanic arc
with the North American
continent.
2. The Acadian Orogeny in the
Devonian associated with the
docking of the continental
Avalon terrane with North
America.
3. The Alleghenian Orogeny in the
Permian, whose effects are seen
mainly in the southern U.S.
Acadian Orogeny
After Spear, 1993
Eastern Acadian Terrane - Low pressure – high
temperature ”Buchan” style metamorphism with
the highest grade zones being cored by granitic
intrusions.
Counter clockwise P-T path is
interpreted to reflect crustal extension in the preAcadian continental margin.
Western
Acadian
Terrane
Regional
“Barrovian” style metamorphism with clockwise
P-T metamorphic paths interpreted to be the late
over thrusting of the Eastern Acadian terrane
during the main Acadian deformation.
Acadian Orogeny
Tibetan Plateau
Himalayan
Metamorphic Belt
Indian
continent
The Himalayan metamorphic belt is a spectacular example of “Barrovian” style
metamorphism associated with crustal thickening because the overthrusting that is
producing the thickening is still going on today as the Indian Continent drives North
into the Asian Continent.
The metamorphic isograds are
inverted in the neighbourhood
of the main central thrust along
which the higher Himalaya are
being emplaced on the lesser
Himalaya to the South. This
anomalous
decrease
in
temperature with increasing
depth would decay away if
active thrusting ceased.
After Spear, 1993
Thrusting and Inverted Geotherms
Mantle Xenoliths:
Oliv
Opx
Cpx
Spin or Garn
Cpx - Opx
cpx with exsolution
lamellae of opx
Temp
Opx dissolved in Cpx
Garn
Spin
Feld
Press
Al2O3 dissolved in Opx
coexisting with garnet
Mantle
Geotherms
Continental Collision Zones
Tauern Window – Austrian Alps
The Tauern window is an erosional window
through the Australoalpine nappes of the
Austrian Alps, revealing the contact within
the underlying “lower plate”. The contact is
lined with two schist belts and a central
eclogite facies belt. Their metamorphic
assemblages each define distinct P-T paths
that require that these three belts units have
only recently been juxtaposed, with the
eclogite belt having been injected between
the two schist belts. All three belts, however,
exhibit a post peak metamorphic phase of
isothermal decompression associated with
uplift and erosion.
Subduction Zones Metamorphic Belts
Characterized by high pressure – low temperature conditions with increasing metamorphic
grade corresponding to the following facies sequence:
zeolite  prehnite/pumpellyite  glaucophane  ecologite
Two types of retrograde paths:
Alpine-type
Retrograde path consists of isothermal decompression,
during which amphibolite and/or greenschist facies
mineral assemblages partially overprint the high
pressure mineral assemblages. High pressure aragonite
is typically not preserved, but converts to calcite. This
type of retrograde path is thought to reflect rapid
exhumation by uplift and erosion, possibly associated
with an attempt to subduct continental crust.
Franciscan-type
Retrograde path is essentially a return along the low
temperature prograde path, with little overprinting of
high pressure mineral assemblages and the preservation
of aragonite. Origin is less clear, but requires slow
exhumation possibly due to return flow in a subduction
accretionary prism.
These metamorphic belts are
typically
diachronous
progressive,
with
the
metamorphic grade and age increasing discontinuously
towards the hinterland. This metamorphic polarity
provides a way of determining the polarity of old
subduction zones.