242-11 Felsic Rocks in the Eastern Bushveld Complex: Indicators of Early Underplating of the Kaapvaal Basement?

Abstract:

The Bushveld Complex, best known for the mafic-ultramafic Rustenburg Layered Suite, contains many examples of granitic rocks commonly attributed to the Lebowa Granite Suite. In the eastern Steelpoort region, such units are found at Magnet Heights, Tigershoek, and Steelpoort Park, and are attributed to the Nebo Granite, the most voluminous sub-unit of the Lebowa Granite Suite. Models for their petrogenesis include fractional crystallization, assimilation, and crustal melting, but geologic and petrologic connection between the granites and the underlying Rustenburg Suite remain uncertain. Samples from Magnet Heights, Tigershoek, and Steelpoort Park localities were petrographically and geochemically characterized and selected samples were dated using LA-ICPMS U-Pb methods.

The three felsic units are geochemically and lithologically similar to the Bushveld Nebo Granite and likely share a common history. But no direct connection between the individual units was established. In addition, the Magnet Heights samples have evidence for magma contributions from a non-Bushveld magmatic system. Inherited xenocrystic cores in zircon from Magnet Heights and Steelpoort Park suggest mass contributions to these granites from (meta)-sedimentary host-rocks. The U-Pb ages of the Steelpoort Park and Magnet Heights samples are 2060 to 2055 Ma, and skew older than 2056 Ma Bushveld magmatism. Their ages conflict with their lithological and geochemical affinity to the Bushveld Nebo Granite, but may be reconciled by a model that involves widespread underplating of the Kaapvaal basement producing early, low-volume partial melts prior to the main phase of mafic Bushveld magmatism.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-18137

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