Abstract
Layered mafic–ultramafic intrusions, such as the Rustenburg Layered Suite (RLS) of the Bushveld Complex (South Africa), are the fossilized remnants of upper crustal magmatic plumbing systems and they can be studied to provide important insights into magma chamber dynamics. In this study, we describe the field and petrological relationships, as well as the chemistry (mineral, bulk-rock majors, traces and Sr–Nd isotopes) of 13 blocks that are hosted in a layered sequence of poikilitic anorthosites in the upper Main Zone of the RLS to constrain their petrogenesis. The blocks (olivine-bearing norite, norite and poikilitic norite) are 0.2 to ~ 6 m in size and are rectangular in shape. The blocks are distinct in atomic Mg# (average Mg# 73.9 mol%) and in Sr–Nd isotopes (average initial 87Sr/86Sr of 0.707344 and εNd of − 5.23 at 2.06 Ga) compared to their host anorthosites (average Mg# 61.2 mol%, initial 87Sr/86Sr of 0.707291 and εNd of − 8.31 at 2.06 Ga). We suggest that the blocks represent noritic material that formed in a conduit or staging chamber system below the RLS and were carried into the RLS by carrier magmas (plagioclase slurries). The blocks have Sr–Nd isotopic compositions that can be explained by the assimilation of upper crust, whereas the Sr–Nd isotopic compositions of the host anorthosites can be explained by the assimilation of lower crust. We also develop a numerical model in which we establish the crystallinity and magma flow rate of the plagioclase slurries that carried these cumulate blocks into the RLS.
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Acknowledgements
The authors thank Caiphas Majola and Sam Tshabalala (University of the Witwatersrand) for the preparation of thin sections, Glen Poirier (University of Ottawa-Canadian Museum of Nature) for microprobe analyses and Marlin Patchappa (EarthLab—University of the Witwatersrand) for geochemical analyses. We thank Linda Iaccheri, Kimberley Beaton (University of the Witwatersrand), and Henriette Ueckermann (University of Johannesburg) for help with the isotopic analyses. We would also like to thank Richard Montjoie (Lesego Platinum) and Paul Nex (University of the Witwatersrand) for providing crucial information regarding the geology of the study area. We thank Lesego Platinum for providing geological maps and raw data regarding the geology of the study area. We would like to thank Grant Cawthorn, Lew Ashwal, James Mungall, Allan Wilson, and Geoffrey Howarth, as well as two anonymous reviewers, for providing constructive feedback to the manuscript. We also thank Tim Grove for editorial handling.
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The study was supported by research grants to R. Latypov from the DSI-NRF Center of Excellence for Integrated Mineral and Energy Resource Analysis (CIMERA).
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Bourdeau, J.E., Hayes, B., Zhang, S.E. et al. Origin and significance of noritic blocks in layered anorthosites in the Bushveld Complex, South Africa. Contrib Mineral Petrol 177, 11 (2022). https://doi.org/10.1007/s00410-021-01872-8
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DOI: https://doi.org/10.1007/s00410-021-01872-8