1. Interactions between slab melts and mantle wedge in Archaean subductions: old and new views on TTG
Jean-François Moyen1 & Hervé Martin2
1- Univ. Claude-Bernard Lyon-I, France
2- Univ. Blaise-Pascal Clermont-Ferrand, France

2. Geographic repartition Petrography Geochemistry Petrogenesis
WHAT ARE TTG ?
Geographic repartition
Petrography
Geochemistry
Petrogenesis

3. Archaean TTG are distributed all over the world

4. From 4.5 to 2.5 Earth heat production decreased by about 3 times
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

5. Archaean TTG: mineralogy
quartz
epidote
« Grey gneisses »:
Orthogneisses of tonalitic and granodioritic composition
plagioclase
biotite

6. Archaean TTG
Modern calc-alkaline

7. ARCHAEAN
MODERN

8. TTG define differentiation trends in Harker diagrams
At least one part of this differentiation is due to fractional crystallization

9. Geochemical modelling for TTG parental magma
TTG source was basaltic:
Archaean tholeiites
Both garnet and hornblende were stable in the melting residue

10. Petrogenetical model for the TTG suite

11. EXPERIMENTAL PETROLOGY: MELTING OF BASALT
Experiments
TTG

12. SECULAR EVOLUTION OF TTG
The adakites
MgO, Cr and Ni
Sr, CaO and Na2O
Interpretation

13. Modern adakites: analogues of Archaean TTG

14. 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

15. 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

16. 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

17. 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

18. TTG source located under a mantle slice
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

19. 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

20. 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

21. The Sr vs. (Na2O+CaO) system
Sr and (Na2O+CaO) 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

22. PRELIMINARY CONCLUSIONS II
High Sr in TTG
 absence of residual plagioclase
Low Sr in TTG
 presence of residual plagioclase
Sr and (Na2O+CaO) augmentation in TTG
 diminution of residual plagioclase
Stability of plagioclase
Residual plagioclase
No residual plagioclase
 Correlated with depth
 Shallow depth  low Sr
 Great depth  high Sr
Increase of melting depth
in course of time
Sr and (Na2O+CaO) augmentation in TTG 

23. INTERPRETATION TODAY LATE ARCHAEAN EARLY ARCHAEAN
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
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

24. SLAB MELT - MANTLE INTERACTIONS
MORE EVIDENCES OF
SLAB MELT - MANTLE INTERACTIONS
Sanukitoids
« Closepet-type » granites
Petrogenesis
Conclusion

25. Sanukitoids: geographic repartition

26. Sanukitoids: petrography
Diorites, monzodiorites and granodiorites
Lots of microgranular mafic enclaves
Qz + Pg + KF + Bt + Hb ± Cpx
Ap + Ilm + Sph + Zn

27. Sanukitoids: geochemistry

28. Making sanukitoids

29. « Closepet-type » granites

30. « 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

31. « 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)

32. « Closepet-type » dioritic facies

33. Making « Closepet-type » granites

34. Petrogenetic relationships

35. PRELIMINARY CONCLUSIONS III
Cooling of the Earth
 Low melt/peridotite ratio
Increased depth of melting
Low melt/peridotite ratio
 Strong melt/mantle interactions: sanukitoids
Even lower melt/peridotite ratio
 Complete assimilation of melts: enriched mantle (Closepet)
Onset of sanukitoids and Closepet-type at the end of the Archaean 
Diminushing melt/peridotite ratio over time (Earth secular cooling)

36. 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