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

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

Archaean TTG are distributed all over the world

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

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

Archaean TTG Modern calc-alkaline

ARCHAEAN MODERN

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

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

Petrogenetical model for the TTG suite

EXPERIMENTAL PETROLOGY: MELTING OF BASALT Experiments TTG

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

Modern adakites: analogues of Archaean TTG

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

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

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

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

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

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

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

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

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 

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

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

Sanukitoids: geographic repartition

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

Sanukitoids: geochemistry

Making sanukitoids

« Closepet-type » granites

« 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

« 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)

« Closepet-type » dioritic facies

Making « Closepet-type » granites

Petrogenetic relationships

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)

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