Convergent plate boundaries

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Convergent plate boundaries

• Most metamorphism is focused at plate boundaries
• Orogens are commonly poly metamorphic multiple
  episodes
• P-T-t paths
• P-T evolution tectonic evolution
• Normal crustal geotherm puts T at 500-6000C at
  20-30km (just above Moho)
• However, typical Bariovian (low P) metamorphism
  600-7500C at 20-30km.
• Metamorphism requires perturbation of geotherm.
• Possibilities underplating, intrusion, crustal
  thickening
• Crustal rocks are enriched in U, Th, K
  radioactive heat producing elements
• Because rocks are poor conductors fast tectonic
  processes can perturb the geotherm
• First order dependence P-T-t path dimensions,
  rates, time available for conductive
  re-equilibration, rate of erosion or tectonic
  unroofing and distribution of heat sources
• Second order dependence strain heating,
  advecting fluids and mineral reactions

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Thermal relaxation
Instantaneous doubling of the crust Can reach
temperatures of of Barovian metamorphism
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Alpine orogeny
Prograde path is governed by amount of thermal
relaxation and radioactive heating Retrograde
path determined by unroofing rates tectonic
extension and faulting, erosion, crustal
thinning Erosion in Himalayas as much as
5km/My. Through mineral equilibria temperature
and pressure can often be calculated. To obtain
the path the P-T points need to be dated t.
4
P-T-t paths
Pressure increases first followed by
temperature. Relict mineral assemblages can
provide prograde path depends on not achieving
equilibrium!!
Closure temperature
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Differences between orogens
Japan
Paired metamorphic belt South of MTL high P
facies including blueschist, north of MTL low P
facies
Arrangement similar in western US, close to the
trench high P facies further away higher T lower
P.
High P facies represent the accretionary
wedge. Further inland crustal thickening and
magmatism results in low P/T facies Question is
this original or tectonic emplacement of terrains?
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Alps
High P facies away from trench, Inverted facies,
high T on top
Apalachian
Apalachian 525-270Ma Several orogenic pulses
related to accretion of arc terranes, Barovian
and Buchan style
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Regional metamorphic terraines
Slate belts
Buchan, low T, P metamorphism, Highest P lower T
than Barovian In general these belts have a low
T metamorphism
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Mantled gneiss domes
In high grade metamorphic core of an
orogen Overlain concordantly be schists Sometimes
buoyant migmatite domes
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Metamorphic core complexes

• High grade metamorphic core
• Characteristics
• Tectonic denudation due to extensional faulting
• Unmetamorphosed rocks overlie the core of
  metamorphosed rocks which was intruded by granite
• Low dip folation parallel to faults
• Syntetctonic

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Tauern window
Classic domical structure Deppest part of the
orogen exposed
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Facies and mineral associations
Pelitic rocks
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Facies and mineral associations low T
Filled circle low Al-shale Open circleAl-rich
shale
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Facies and mineral associations intermediate T
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Facies and mineral associations high T
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Barovian P-T-t path
Wopmay, Orogen, NWT
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Buchan versus Barovian

• Buchan
• Andalusite and kyanite rather then sillimanite
• Garnet and staurolite less common, cordiorite
  wide spread (lower T)
• Migmatite less common.

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Mafic protolith
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Ocean ridge metamorphism
Oxidation Fe2SiO41/2O2?Fe2O3SiO2 19Fe2SiO44SO4
2-?18Fe2O32FeS219SiO2
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Convective circulation
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Ophiolite emplacement
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High pressure metabasite
1

1. Prehnite-pumpellyite Actinolite-pumpellyite-chlor
   ite-epidote-lawsonite(gt3kbar), 200-300oC, 1-4kbar
2. Blueschist sodic amphible glaucophane-lawsonite
   combination. Albite?JadieteQuartz
3. Eclogiteomphacite and garnet
4. UHP (ultra high pressure Coesite and diamond,
5. Dora Maira Massif in Alps, UHP as lenses in
   schists coesite, pyrope, kyanite

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UHP-T-t path