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Orthopyroxene oikocrysts in the MG1 chromitite layer of the Bushveld Complex: implications for cumulate formation and recrystallisation

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Abstract

Two typical mineral textures of the MG 1 chromitite of the Bushveld Complex, South Africa, were observed; one characterised by abundant orthopyroxene oikocrysts, and the other by coarse-grained granular chromitite with only minor amounts of interstitial material. Oikocrysts form elongate clusters of several crystals aligned parallel to the layering, and typically have subhedral, almost chromite-free, core zones containing remnants of olivine. The core zones are surrounded by poikilitic aureoles overgrowing euhedral to subhedral chromite chadacrysts. Chromite grains show no preferred crystal orientation, whereas orthopyroxene grains forming clusters commonly share the same crystallographic orientation. Oikocryst core zones have lower Mg# and higher concentrations of incompatible trace elements compared to their poikilitic aureoles. Core zones are relatively enriched in REE compared to a postulated parental magma (B1) and did not crystallise in equilibrium with the surrounding minerals, whereas the composition of the poikilitic orthopyroxene is consistent with growth from the B1 magma. These observations cannot be explained by the classic cumulus and post-cumulus models of oikocryst formation. Instead, we suggest that the oikocryst core zones in the MG1 chromitite layer formed by peritectic replacement of olivine primocrysts by reaction with an upwards-percolating melt enriched in incompatible trace elements. Poikilitic overgrowth on oikocryst core zones occurred in equilibrium with a basaltic melt of B1 composition near the magma-crystal mush interface. Finally, adcumulus crystallisation followed by grain growth resulted in the surrounding granular chromitite.

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Acknowledgements

FEDK was supported by the German Federal Ministry of Education and Research (BMBF), Grant FKZ 033R119C, ZV by Marie Skłodowska-Curie Individual European Fellow grant. Cronimet Chrome Mining SA (Pty) Ltd. is thanked for provision of drill core material. We are grateful to M. Oeser-Rabe and S. Schuth for help with the LA-ICP-MS analyses, also assistance by K. Born and P. Czaja at the SEM and microprobe at the Museum für Naturkunde Berlin are greatly acknowledged. Fruitful discussions with M. C. Hoffmann and I. V. Veksler were highly appreciated. Helpful reviews by two anonymous reviewers and editor Chris Ballhaus greatly improved the original paper.

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  1. Museum für Naturkunde Berlin, Leibnitz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstrasse 43, 10115, Berlin, Germany

    Felix E. D. Kaufmann & Lutz Hecht

  2. Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, 12449, Berlin, Germany

    Felix E. D. Kaufmann & Lutz Hecht

  3. Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK

    Zoja Vukmanovic & Marian B. Holness

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  1. Felix E. D. Kaufmann
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  2. Zoja Vukmanovic
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Correspondence to Felix E. D. Kaufmann.

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Kaufmann, F.E.D., Vukmanovic, Z., Holness, M.B. et al. Orthopyroxene oikocrysts in the MG1 chromitite layer of the Bushveld Complex: implications for cumulate formation and recrystallisation. Contrib Mineral Petrol 173, 17 (2018). https://doi.org/10.1007/s00410-018-1441-x

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