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Jean-François Moyen and Gary Stevens

Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications). Jean-François Moyen and Gary Stevens. Stellenbosch University, South Africa. TTG are. Orthogneisses Tonalites, Trondhjemites & Granodiorites (Na-rich series) Fractionnated REE, etc.

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Jean-François Moyen and Gary Stevens

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  1. Partial melting of amphibolites and the genesis of Archaean TTG(and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch University, South Africa

  2. TTG are... • Orthogneisses • Tonalites, Trondhjemites & Granodiorites (Na-rich series) • Fractionnated REE, etc. • Largely homogeneous throughout the Archaean • Originated by partial melting of amphibolites (hydrated basalts), in garnet stability field

  3. Trace elements features of Archaean TTGs Nb-Ta anomaly Sr contents Y & HREE depletion

  4. Les « gneiss gris »

  5. Minéralogie

  6. Eléments majeurs

  7. REE

  8. Melting of hydrous basalt Gt/melt KD = 10 - 20 Yb In Garnet stability field (Gt in residue) (other minerals ≤ 1) Conditions for making TTGs Experimental melts

  9. Geodynamic site ? Gt-in Subduction Gt-in • Intermediate cases: • Shallow subduction • (± underplating) • Stacked oceanic crust Gt-in Gt-in Thick (oceanic or continental) crust (e.g. Oceanic plateau) Gt-in

  10. Partial melting of amphibolites 15-20 « modern » studies (1990-2000) + Phase diagrams (1970-80) 114 exp. fluid present or saturated 209 exp. « dehydration melting »

  11. Goal of the study • Review and compilation of published data on experimental melting • Elaboration of a global model for amphibolite melting • Implications for trace element contents • Geological/geodynamical consequences

  12. Review and compilation of published work • Starting materials • Solidus position & melt productivity • Mineral stability fields (Moyen & Stevens, subm. to AGU monographs)

  13. Starting materials

  14. Fluids and melting • Fluid-saturated (free fluid phase) • Fluid-present (yielded by breakdown of hydrous minerals in the near sub-solidus), limited availability • Fluid-absent (dehydration melting) • Dry

  15. Fluid saturated

  16. Dehydration melting

  17. Fluid-present

  18. Experimental solidus position

  19. Melt productivity: dehydration melting

  20. Melt productivity: water saturated (+ Qz)

  21. Melt productivity: fluid-present (- Qz)

  22. Mineral stability limits

  23. Control on amphibole stability

  24. Control on plagioclase stability

  25. Mineralogical models

  26. Mineralogical models

  27. Composition of experimental melts

  28. Na2O contents in experimental melts Very unlikely for amphibolite melting!

  29. K2O

  30. Major elements A linear model, of the form C/C0 = a F + b

  31. Modelled melts

  32. Model vs. TTGs

  33. Preliminary conclusions (1) • K2O content depends on the source. Only relatively K-poor sources (< 0.7 %) make TTGs … but really depleted sources won’t. • This means that K-rich amphibolites can indeed melt into granites (Sisson et al., 2005) • With appropriate sources, tonalites & trondjhemites occur for F = 20-40 % (900-1100 °C)

  34. C0 Cl = F + D (1 - F) Model for trace element Arbitrary Experimental data D = S Kdi. Xi Litterature

  35. Trace elements contents of the 3 sources

  36. Melt proportions

  37. Mineral proportions: amphibole and plagioclase

  38. Gt/melt KD = 10 - 20 Yb Mineral proportions: garnet

  39. Rt/melt KD = 25 - 150 Nb Rt/melt KD = 50 - 200 Ta Mineral proportions: rutile

  40. REE contents in (modelled) melts

  41. REE contents in (modelled) melts

  42. REE contents: La/Yb

  43. Y contents

  44. Sr contents and the role of residual plagioclase (Martin & Moyen, 2001, Geology 30 p 319-322; after Zamora, 2000)

  45. Sr/Y

  46. Nb/Ta

  47. Effect of pressure

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