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

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

3. Archaean TTG are distributed all over the world

4. 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. 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  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 (Na2O+CaO) augmentation in TTG 

23. 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

24. 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