1. Using trace elements to define the PT conditions of TTG genesis.
Jean-François Moyen
Gary Stevens
University of Stellenbosch
South Africa

2. Key ideas
At least part of the geochemistry of TTG plutons can be interpreted as reflecting melting conditions
P-T conditions of amphibolites melting exert a strong control on the major and trace elements composition of TTG liquids
“True” TTG signature requires high pressure (15-20 kbar)
However, both low and high pressure TTGs can be identified
1 If there is fractionnation, the same logics could also apply using the most primitive liquid – it requires a little more work to determine its composition

3. TTG are... Orthogneisses Tonalites, Trondhjemites & Granodiorites
(Na-rich series)
Fractionnated REE, high La/Yb and Sr/Y, etc.
Largely homogeneous throughout the Archaean
TTGs, as I ’ll show in the next few slides, are sodic granitoids, defining a sodic series of differenciation. They also have a very typical REE pattern, with characteristic depletions in HREE.
It is now commonly accepted that they are generated by partial melting of hydrated basalts (amphibolites), within garnet stability field. However, details of their petrogenesis are still debatted; in particular, it is unclear whether it is, or not, possible to make TTGs through metling of eclogite, or if you need to be within amphibolite facies.

4. “gneisses” or “plutons” ?
TTG
“gneisses” or “plutons” ?
It is difficult to constrain good petrogenetical studies on complex, multiply injected and molten, possibly tectonic assemblages!
Sand River Gneisses
Ca. 3.1 Ga TTG gneisses in Messina area,
Limpopo Belt, South Africa
Stolzburg pluton (Barberton, South Africa Ga)

5. However, the most common component of the grey gneisses is relatively constant

6. Conditions for making TTGs
Melting of hydrous basalt
In Garnet stability field (Gt in residue)
TTGs, as I ’ll show in the next few slides, are sodic granitoids, defining a sodic series of differenciation. They also have a very typical REE pattern, with characteristic depletions in HREE.
It is now commonly accepted that they are generated by partial melting of hydrated basalts (amphibolites), within garnet stability field. However, details of their petrogenesis are still debatted; in particular, it is unclear whether it is, or not, possible to make TTGs through metling of eclogite, or if you need to be within amphibolite facies.
Gt-in
Little or no
direct tectonic meaning!
Gt-in

7. Stolzburg pluton (3.44 Ga, Barberton)
No clear fractionation trend
Apparently “compatible” Na
(prob. a melting reaction Ab + Amp = Melt + Grt)

8. Using published experimental data
Review and compilation of published data on experimental melting
( > 320 runs described in studies)
Building of a global model for amphibolite melting
Implications for trace element contents
(Moyen & Stevens, AGU monographs 164 pp )

9. Melt composition

10. C0 Cl = F + D (1 - F) Compilation of experimental data
Interpolated “maps” of modal composition Cl = C0
F + D (1 - F)
“Maps” of trace elements composition

11. Major elements

12. REE contents: La/Yb
KoB
ThB AB

13. Sr/Y
NB- 1.Similar maps can be established for all elements or ratios, e.g. La/Yb
2. Actually different models were built for dirrerent types of amphibolites

14. Effect of pressure

15. TTG composition as a depth indicator
Sr contents
Nb-Ta anomaly
and Nb/Ta
Y & HREE
depletion

16. Eu anomaly
HREE
depletion

17. 900-1000 °C = trondhjemite and tonalite composition
15-20 kbar = proper trace elements signature
Typical geotherm °C/km
(Comparable to Barberton HP amphibolites)

18. TTGs and TTGs? Kenogamissi (2.74-2.71 Ga) Stolzburg (3.44 Ga)
Steynsdorp (3.56 Ga)
Theespruit (3.44 Ga)

19. Stolzburg (3.44 Ga)
Steynsdorp (3.56 Ga)
Theespruit (3.44 Ga)
Kenogamissi ( Ga)

20. Low Y Yb etc. trondhjemites
Lower Y Yb
Higher Sr/Y La/Yb
Etc.
Tdj.
Subduction
Low Y Yb etc. trondhjemites
Ton.
Dio.
Intraplate (plume/orogenic collapse/etc.)
(relatively) high Y Yb etc. tonalites - diorites
Grd.

21. Key ideas
At least part of the geochemistry of TTG plutons can be interpreted as reflecting melting conditions
P-T conditions of amphibolites melting exert a strong control on the major and trace elements composition of TTG liquids
“True” TTG signature requires high pressure (15-20 kbar)
However, both low and high pressure TTGs can be identified
1 If there is fractionnation, the same logics could also apply using the most primitive liquid – it requires a little more work to determine its composition