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

WHAT ARE TTG ?

•Geographic repartition

•Petrography

•Geochemistry

•Petrogenesis

Archaean TTG are distributed all over the world

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

plagioclase biotite

« Grey gneisses »:

Orthogneisses of tonalitic and granodioritic composition

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

TTG

Experiments

EXPERIMENTAL PETROLOGY: MELTING

OF BASALT

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

•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 interactionincreases 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

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

•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 (Na2O+CaO) augmentation in TTG

Stability of plagioclaseResidual plagioclaseNo residual plagioclase

Sr and (Na2O+CaO) augmentation in TTG

Low Sr in TTG

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 ARCHAEANTODAY

INTERPRETATION

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

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

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

« Closepet-type » dioritic facies

Making « Closepet-type » granites

Petrogenetic relationships

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

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 interactionsAppearance of new types of subduction-related rocks

These changes reflect the progressive cooling of our planet