Interactions between slab melts and mantle wedge in Archaean subductions: old and new views on TTG

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Interactions between slab melts and mantle wedge
in Archaean subductionsold and new views on TTG

• Jean-François Moyen1 Hervé Martin2
• 1- Univ. Claude-Bernard Lyon-I, France
• 2- Univ. Blaise-Pascal Clermont-Ferrand, France

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WHAT ARE TTG ?

• Geographic repartition
• Petrography
• Geochemistry
• Petrogenesis

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Archaean TTG are distributed all over the world
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Archaean TTG emplaced over a long period of time
? 2 Ga
From 4.5 to 2.5 Earth heat production decreased
by about 3 times
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Archaean TTG mineralogy
quartz
epidote
 Grey gneisses  Orthogneisses of tonalitic and
granodioritic composition
plagioclase
biotite
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Archaean TTG
Modern calc-alkaline
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ARCHAEAN
MODERN
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TTG define differentiation trends in Harker
diagrams
At least one part of this differentiation is due
to fractional crystallization
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Geochemical modelling for TTG parental magma
TTG source was basaltic Archaean tholeiites
Both garnet and hornblende were stable in the
melting residue
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Petrogenetical model for the TTG suite
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EXPERIMENTAL PETROLOGY MELTING OF BASALT
Experiments
TTG
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SECULAR EVOLUTION OF TTG

• The adakites
• MgO, Cr and Ni
• Sr, CaO and Na2O
• Interpretation

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Modern adakites analogues of Archaean TTG
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Modern adakites analogues of Archaean TTG
Adakites are found only when young, hot
lithosphere is subducted...
i.e., when Archaean thermal conditions are
(locally) recreated
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Evolution of Mg in TTG

• Fractional crystallization reduces Mg

• For each period the higher Mg represents TTG
  parental magma

• From 4.0 to 2.5 Ga Mg regularly increased in
  TTG parental magmas

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Evolution of Ni and Cr in TTG

• Fractional crystallization reduces Ni and Cr
  contents

• For each period the higher Ni and Cr contents
  represent TTG parental magma

• From 4.0 to 2.5 Ga Ni and Cr contents regularly
  increased in TTG parental magmas

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The MgO vs. SiO2 system

• MgO increases inTTG in course of time

• SiO2 decreases inTTG in course of time

• Adakites have exactly the same evolution pattern
  as TTG

• For the same SiO2, experimental melts are
  systematically MgO poorer than TTG

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PRELIMINARY CONCLUSIONS I
? magma / mantle interaction (reaction between
peridotite and slab melts)

• Mg, Ni and Cr enrichment
• (both in adakites and TTG)

• TTG source located under a mantle slice

? slab melting ? underplated basalt melting

• TTG are generated by

• Mg, Ni, Cr increased
• in course of time

? degree on interaction increases

• Degree on interaction
• increases in course of time

? slab melting depth augments
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Evolution of Sr in TTG

• Fractional crystallization reduces Sr contents

• For each period the higher Sr represents TTG
  parental magma

• From 4.0 to 2.5 Ga Sr regularly increased in TTG
  parental magmas

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Evolution of (Na2O CaO) and (Eu/Eu) in TTG

• For each period the higher (Na2O CaO)
  represent TTG parental magma

• From 4.0 to 2.5 Ga (Na2O CaO) regularly
  increased in TTG parental magmas

• From 4.0 to 2.5 Ga positive Eu anomalies appear
  in TTG parental magmas

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The Sr vs. (Na2OCaO) system

• Sr and (Na2OCaO) inTTG increase in course of time

• Adakites have exactly the same evolution pattern
  as TTG

• Sr content is directly correlated with stability
  of plagioclase in melting residue

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PRELIMINARY CONCLUSIONS II

• High Sr in TTG

? absence of residual plagioclase
? presence of residual plagioclase
Low Sr in TTG
Sr and (Na2OCaO) augmentation in TTG
? diminution of residual plagioclase

• ? Correlated with depth
• ? Shallow depth ? low Sr
• ? Great depth ? high Sr

Stability of plagioclase Residual
plagioclase No residual plagioclase
Increase of melting depth in course of time

• Sr and (Na2OCaO) augmentation in TTG

?
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INTERPRETATION
EARLY ARCHAEAN
LATE ARCHAEAN
TODAY
High heat production ? High geothermal gradients
? Shallow depth slab melting Plagioclase stable ?
Sr poor TTG Thin overlying mantle ? No or few
magma/mantle interactions ? Low Mg-Ni-Cr TTG
Lower heat production ? Lower geothermal
gradients ? Deep slab melting Plagioclase
unstable ? Sr-rich TTG Thick overlying mantle ?
important magma/mantle interactions ?
High-Mg-Ni-Cr TTG
Low heat production ? Low geothermal gradients ?
No slab but mantle wedge melting
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MORE EVIDENCES OF SLAB MELT - MANTLE INTERACTIONS

• Sanukitoids
•  Closepet-type  granites
• Petrogenesis
• Conclusion

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Sanukitoids geographic repartition
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Sanukitoids petrography
Diorites, monzodiorites and granodiorites
Lots of microgranular mafic enclaves
Qz Pg KF Bt Hb Cpx
Ap Ilm Sph Zn
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Sanukitoids geochemistry
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Making sanukitoids
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 Closepet-type  granites
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 Closepet-type  granites
Porphyritic monzogranite
Mixing between - mantle-derived diorite -
crustal, anatectic granite
Associated with dioritic enclaves
Qz KF Pg Bt Hb Cpx
Ap Ilm Sph Zn
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 Closepet-type  dioritic facies
Diorite and monzonites
eNd(T) -2 to 0 (enriched mantle)
Pg KF Bt Hb Cpx
Ap Ilm Mt Sph Zn All (all abundant)
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 Closepet-type  dioritic facies
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Making  Closepet-type  granites
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Petrogenetic relationships
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PRELIMINARY CONCLUSIONS III
Cooling of the Earth
? Low melt/peridotite ratio
Increased depth of melting
? Strong melt/mantle interactions sanukitoids
Low melt/peridotite ratio
? Complete assimilation of melts enriched
mantle (Closepet)
Even lower melt/peridotite ratio
Onset of sanukitoids and Closepet-type at the end
of the Archaean
?
Diminushing melt/peridotite ratio over time
(Earth secular cooling)
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CONCLUSIONS
TTG were generated by basalt melting, under a
mantle slice they were produced by subducted slab
melting
From 4.0 to 2.5 Ga depth of slab melting
increased At 4.0 Ga shallow depth
melting, plagioclase stable, no or few
mantle/magma interactions At 2.5 Ga great depth
melting, plagioclase unstable, strong
mantle/magma interactions Appearance of new types
of subduction-related rocks
These changes reflect the progressive cooling of
our planet