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The birth, growth and ageing of the Kaapvaal subcratonic mantle TEXT SIZE: A A A
The Kaapvaal craton and its underlying mantle is probably one of the best studied Archean entity in the world. Despite that, discussion is still vivid on important aspects. A major debate over the last few decades is the depth of melting that generated the mantle nuclei of cratons. Our new evaluation of melting parameters in peridotite residues shows that the Cr2O3/Al2O3 ratio is the most useful pressure sensitive melting barometer. It irrevocably constrains the pressure of melting (melt separation) to less than 2GPa with olivine (ol), orthopyroxene (opx) and spinel (sp) as residual phases. Garnet (grt) grows at increasing pressure during lithosphere thickening and subduction via the reaction opx+sp grt+ol. The time of partial melting is constrained by Re-depletion model ages (T-RD) mainly to the Archean (Pearson and Wittig 2008). However, only 3% of the ages are older than 3.1Ga while crustal ages lie mainly between 3.1 to 2.8Ga for the W- and 3.7 to 2.8Ga for the E-block. Many T-RD-ages are probably falsified by metasomatism and the main partial melting period was older than 3.1Ga. Also, Nd- and Hf- model ages of peridotitic lithologies from the W-block are 3.2 to 3.6Ga old. The corresponding very negative epsilon Nd (-40) and epsilon Hf values (-65) signal the presence of subducted crustal components in these old mantle portions. Subducted components diversify the mantle in its chemistry and thermal structure. Adjustment towards a stable configuration occurs by fluid transfer, metasomatism, partial melting and heat transfer. Ages of metasomatism from the Lu-Hf isotope system are 3.2Ga (Lace), 2.9Ga (Roberts Victor) and 2.62Ga (Finsch) coinciding with the collision of cratonic blocks, the growth of diamonds, metamorphism of eclogites and of Ventersdoorp magmatism. The cratonic lithosphere was stabilized thermally by the end of the Archean and cooled since then with a rate of 0.07 degrees C/Ma.

Publication name

 MINERALOGY AND PETROLOGY, 112 23-41; 1 10.1007/s00710-018-0577-8 1 DEC 2018


 Brey, Gerhard P.; Shu, Qiao

Corresponding author(s) 

 Brey, Gerhard P. 
 Goethe Univ Frankfurt, Mineral, Inst Geowissensch, Altenhoferallee 1, D-60438 Frankfurt, Germany.

Author(s) from IGCAS   SHU Qiao

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