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Differentiation of tholeiitic Karroo magma at Birds River, South Africa

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Strongly fractionated residua derived by partial crystallization of tholeiitic gabbroic magma were emplaced at shallow depth within sediments of the uppermost part of the Karroo succession. Field evidence suggests that these residua were tapped from a deeper intrusion during cauldron subsidence, but were subsequently engulfed by later intrusion of olivine gabbro on a much larger scale. Ferrogabbro and pegmatoids were produced by differentiation in place of the younger gabbro, but the older intrusion was already chemically evolved when emplacement occurred. The product, a quenched, porphyritic rock containing ferroaugite-ferrohedenbergite, fayalitic olivine and sodic plagioclase in a microcrystalline to glassy matrix, is chemically comparable with the coarse-grained, iron- and alkali-enriched facies of other well known differentiated bodies. Regular trends in the contents of Rb, Sr, Y, Zr and Nb correlate well with fractionation indices based on major-element chemistry.

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Eales, H.V., van Robey, J.A. Differentiation of tholeiitic Karroo magma at Birds River, South Africa. Contr. Mineral. and Petrol. 56, 101–117 (1976). https://doi.org/10.1007/BF00375423

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  • Iron
  • Crystallization
  • Fractionation
  • Olivine
  • Mineral Resource