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Using trace elements to define the PT conditions of TTG genesis.

Using trace elements to define the PT conditions of TTG genesis. Jean-François Moyen Gary Stevens University of Stellenbosch South Africa. Key ideas. At least part of the geochemistry of TTG plutons can be interpreted as reflecting melting conditions

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Using trace elements to define the PT conditions of TTG genesis.

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  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

  4. 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 -- 3.45 Ga)

  5. Gt-in Melting of hydrous basalt In Garnet stability field (Gt in residue) Gt-in Conditions for making TTGs Little or no direct tectonic meaning!

  6. Using published experimental data • Review and compilation of published data on experimental melting ( > 320 runs described in 15-20 studies) • Building of a global model for amphibolite melting • Implications for trace element contents (Moyen & Stevens, AGU monographs 164 pp.149-175)

  7. Melt composition

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

  9. 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

  10. Effect of pressure

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

  12. Eu anomaly HREE depletion

  13. 900-1000 °C = trondhjemite and tonalite composition • 15-20 kbar = proper trace elements signature • Typical geotherm 15-20 °C/km (Comparable to Barberton HP amphibolites)

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

  15. Kenogamissi (2.74-2.71 Ga) Stolzburg (3.44 Ga) Theespruit (3.44 Ga) Steynsdorp (3.56 Ga)

  16. 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.

  17. 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

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