Abstract
The layered Bushveld Complex hosts a number of chromitite layers, which were found to contain significant amounts of zircon grains compared with adjacent silicate rocks. Cathodoluminescent-dark, partially metamict cores and transparent rims of composite zircon grains were analyzed for trace elements with SIMS and LA-ICPMS techniques. The cores are enriched in REE, Y, Th and U and are characterized by distinctly flatter REE patterns in contrast to those of the rims and transparent homogenous crystals. Zircon from the different stratigraphic units has specific Th/U ratios, the highest of which (1.5–4) occurs in a Merensky Reef zircon core. The Ti content of Bushveld zircon ranges from 12 to 52 ppm correlating to a crystallization temperature range of 760–930 °C. The geochemical characteristics of the first zircon generation are consistent with its high-temperature crystallization as the first major U, Th and REE acceptor from a highly-evolved residue of the high-Mg basalt magma, whereas the rims and coreless crystals have crystallized from percolating intercumulus liquid of new influx of the same magma. U-Pb SHRIMP dating of zircon cores and rims does not reveal a distinguishable difference between their ages indicating the absence of inherited zircon. Concordia ages of 2,051 ± 9 Ma (2σ, MSWD = 0.1) and 2,056 ± 5 Ma (2σ, MSWD = 0.05) for zircons from the Merensky Reef and the Upper Platreef located equally near the top of the Critical Zone are in agreement with published ages for the Merensky Reef. Zircon from the deeper-seated Lower Group, Middle Group and Lower Platreef chromitites yields younger concordia ages that may reflect prolonged late-stage volatile activity.
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Acknowledgements
We thank Adrian Colloty, James Winch and Fiona Scholes for the help with our sampling and logging. We are grateful to Gordon Chunnett for support of the study. Anglo Platinum is thanked for a permission to publish this paper. Marina Kuznetsova and Andrey Mokhov are acknowledged for the analytical assistance at IGEM RAS. Detailed comments by J. Scoates and T.R. Ireland on the earlier version of the manuscript were extremely useful. The authors would like to thank E. G. Grosch, J. Kosler and an anonymous reviewer for their helpful and insightful reviews. This study was financially supported by Anglo-Platinum through the THRIP (Technology and Human Resources for Industry Programme, South Africa), the NRF of South Africa and partly by the Russian Foundation for Basic Research through grant 11-05-00,014 to MY.
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Yudovskaya, M., Kinnaird, J., Naldrett, A.J. et al. Trace-element study and age dating of zircon from chromitites of the Bushveld Complex (South Africa). Miner Petrol 107, 915–942 (2013). https://doi.org/10.1007/s00710-013-0269-3
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DOI: https://doi.org/10.1007/s00710-013-0269-3