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Interactions between slab melts and mantle wedge in Archaean subductions: old and new views on TTG Jean-François Moyen 1 & Hervé Martin 2 1- Univ. Claude-Bernard.

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Presentation on theme: "Interactions between slab melts and mantle wedge in Archaean subductions: old and new views on TTG Jean-François Moyen 1 & Hervé Martin 2 1- Univ. Claude-Bernard."— Presentation transcript:

1 Interactions between slab melts and mantle wedge in Archaean subductions: old and new views on TTG Jean-François Moyen 1 & Hervé Martin 2 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 quartzepidote plagioclasebiotite « Grey gneisses »: Orthogneisses of tonalitic and granodioritic composition

6 Archaean TTGModern calc-alkaline

7 ARCHAEANMODERN

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 TTG Experiments EXPERIMENTAL PETROLOGY: MELTING OF BASALT

12 SECULAR EVOLUTION OF TTG The adakites MgO, Cr and Ni Sr, CaO and Na 2 O 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. SiO 2 system MgO increases inTTG in course of time SiO 2 decreases inTTG in course of time Adakites have exactly the same evolution pattern as TTG For the same SiO 2, experimental melts are systematically MgO poorer than TTG

18 Mg, Ni and Cr enrichment (both in adakites and TTG) TTG are generated by Mg, Ni, Cr increased in course of time TTG source located under a mantle slice Degree on interaction increases in course of time PRELIMINARY CONCLUSIONS I  magma / mantle interaction (reaction between peridotite and “slab melts”)  slab melting  underplated basalt melting  degree on interaction increases  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 (Na 2 O + CaO) and (Eu/Eu*) in TTG For each period the higher (Na 2 O + CaO) represent TTG parental magma From 4.0 to 2.5 Ga (Na 2 O + 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. (Na 2 O+CaO) system Sr and (Na 2 O+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 High Sr in TTG PRELIMINARY CONCLUSIONS II  absence of residual plagioclase  diminution of residual plagioclase  Correlated with depth  Shallow depth  low Sr  Great depth  high Sr Increase of melting depth in course of time  presence of residual plagioclase Sr and (Na 2 O+CaO) augmentation in TTG Stability of plagioclase Residual plagioclase No residual plagioclase Sr and (Na 2 O+CaO) augmentation in TTG Low Sr in TTG 

23 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 EARLY ARCHAEANLATE ARCHAEAN TODAY INTERPRETATION

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 Porphyritic monzogranite Associated with dioritic enclaves Qz + KF + Pg + Bt + Hb ± Cpx Ap + Ilm + Sph + Zn Mixing between : - mantle-derived diorite - crustal, anatectic granite « Closepet-type » granites

31 Diorite and monzonites  Nd (T) = -2 to 0 (enriched mantle) Pg +KF + Bt + Hb ± Cpx Ap + Ilm + Mt + Sph + Zn + All (all abundant) « Closepet-type » dioritic facies

32

33 Making « Closepet-type » granites

34 Petrogenetic relationships

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

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


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