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Archaean magmatism

Archaean magmatism. NB- Arch ea n (US spelling) or Arch aea n (UK spelling). Why?. Somehow different from modern magmas Interesting to test our understanding of petrogenetic processes Not that rare, and good South African examples (Barberton) Economic interest

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Archaean magmatism

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  1. Archaean magmatism NB- Archean (US spelling) or Archaean (UK spelling)

  2. Why? • Somehow different from modern magmas • Interesting to test our understanding of petrogenetic processes • Not that rare, and good South African examples (Barberton) • Economic interest • Gold (large part of world’s gold + secondary deposits) • PGE bearing sulphides • Nickel • Department’s research interests

  3. Barberton gold fields

  4. Two characteristic rock types • Komatiites = ultra-mafic, Mg-rich lavas • TTGs = Tonalites, Trondhejmites & Granodiorites • Link with Archaean geodynamic style?

  5. The Archaean

  6. Oldest crustal remnants

  7. Jack Hill (Australia) zircon = 4.404  0.008 Ga

  8. The oceanic crust is young…

  9. 75 % of the crust was formed at ca. 2.5 Ga The Archaean is a major crust-forming period

  10. Earth’s heat production ►A 2- to 4-fold decrease from the Archaean to now

  11. Effects of higher Archaean heat production? • Shape of convection • Partitioning of heat flux • Effects on the continents thermal structure and behaviour • Petrogenesis?

  12. Shape of convection ? (Ra > 105) (Ra = 103 - 104) Ra = function of many things, including DT (or heat production)

  13. Archaean dome-and-keel patterns Vertical tectonics (“sagduction”) Zimbabwe (2.7 Ga) Pilbara (3.5 Ga)

  14. Superior Province

  15. Bimodal Archaean terranes • Greenstone belts (commonly dominated by greenschist facies amphibolites) • Mafic and ultramafic (= komatiites) lavas • Some intermediate lavas (andesites) • Detrical sediments • Some chemical sediments (BIFs) or biogenic formations (stromatholites) • Gneissic « basement » or plutons • Late plutons

  16. 2.9 – 2.7 Ga granites 3.1 Ga granites & syenites Moodies Fig Tree Onverwacht Ca. 3.2 Ga TTG Ca. 3.4 Ga TTG « Ancient gneisses » (3.6 – 3.4 Ga)

  17. 1. Komatiites Viljoen, M. J. and R. P. Viljoen (1969). "The geology and geochemistry of the lower ultramafic unit of the Onverwacht group and a proposed new class of igneousrocks."Geological Society of South Africa Special Publication 2: 55-86. A truly South-African rock type!

  18. Onverwacht group, BGB The original komatiites in Komatii formation (~1.5 km from type locality)

  19. Komatiites composition

  20. Structure of komatiites flows • Origin of komatiites • Komatiites and the Archaean mantle

  21. Subdivision of komatiite flows (Arndt et al. 1977) Polysutured top Random spinifex Orientated spinifex parallel blades of spinifex solid subhedral olivine B4 Basal chill, polysutured

  22. Chilled/brecciated top Subaquatic emplacement

  23. Spinifex textured layer(s) • Random spinifex • Orientated spinifex • Plate spinifex Spinifex grass, Western Australia (Barnes 1990)

  24. Random spinifex

  25. Orientated spinifex

  26. Plate spinifex

  27. Polyedral olivine

  28. Origin of komatiites • High Mg contents require high degree of mantle melting (40-60 %) • This implies very high temperatures and fast rise

  29. What are the implications of komatiites? • Probably formed in hot-spot like situations (difficult to arrive to > 1600° else) • Even though, this is hotted than modern hotspots • At least some parts of the Earth were very hot • At least part of the GSB formed from hotspots (intraplate situation)

  30. Komatiites and the history of the Archaean mantle

  31. 3 groups of komatiites, from the shape of their HREE pattern (or Gd/Yb ratios) Role of garnet

  32. Correlation with Al (and also Ca) • Al depleted (grp II) vs. Al-undepleted (grp. III) • Only grp I komatiites exist in the late Archaean

  33. Early differenciation of the Earth mantle (completed at 3.80 Ga) • Deep origin of Late-Archaean komatiites (or locally non-differenciated bits of mantle?) • Maybe due to a cooler Earth, hot temperatures found only very deep?

  34. 2. TTG • Archaean TTG (Tonalite, Trondhjemites and Granodiorites) • ≈ grey gneisses (although in details, some TTGs are not grey gneisses and some grey gneisses are not TTG…)

  35. Archaean grey gneisses Some relatively simple orthogneisses Stolzburg pluton (Barberton, 3.45 Ga)

  36. Commonly complex, migmatitic, polydeformed orthogneisses

  37. The Sand River Gneisses Ca. 3.1 Ga TTG gneisses in Messina area, Limpopo Belt, South Africa (R. White, Melbourne, for scale)

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

  39. Mineralogy

  40. Major elements

  41. REE

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