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Strontium isotope disequilibrium of plagioclase in the Upper Critical Zone of the Bushveld Complex: evidence for mixing of crystal slurries

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Abstract

We report in situ Sr isotope data for plagioclase of the Bushveld Complex. We found disequilibrium Sr isotopic compositions on several scales, (1) between cores and rims of plagioclase grains in the Merensky pyroxenite, the Bastard anorthosite, and the UG1 unit and its noritic footwall, (2) between cores of different plagioclase grains within thin sections of anorthosite and pyroxenite of the Merensky unit, the footwall anorthosite of the Merensky reef and the footwall norite of the UG1 chromitite. The data are consistent with a model of co-accumulation of cumulus plagioclase grains that had crystallized from different magmas, followed by late-stage overgrowth of the cumulus grains in a residual liquid derived from a different level of the compacting cumulate pile. We propose that the rocks formed through slumping of semi-consolidated crystal slurries at the top of the Critical Zone during subsidence of the center of the intrusion. Slumping led to sorting of crystals based on density differences, resulting in a layered interval of pyroxenites, norites and anorthosites.

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Acknowledgments

We thank EA Mathez and AE Boudreau for constructive suggestions. S Forss is thanked for preparation of thin and thick sections.

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Authors and Affiliations

  1. Department of Geosciences, University of Oulu, P.O. Box 3000, 90014, Oulu, Finland

    Sheng-Hong Yang & Wolfgang D. Maier

  2. Geological Survey of Finland, Espoo, Finland

    Yann Lahaye & Hugh O’Brien

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  1. Sheng-Hong Yang
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  2. Wolfgang D. Maier
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Correspondence to Sheng-Hong Yang.

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Communicated by T.L. Grove.

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Yang, SH., Maier, W.D., Lahaye, Y. et al. Strontium isotope disequilibrium of plagioclase in the Upper Critical Zone of the Bushveld Complex: evidence for mixing of crystal slurries. Contrib Mineral Petrol 166, 959–974 (2013). https://doi.org/10.1007/s00410-013-0903-4

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