10-11 How Thick is a Layer? Insights into the Construction of the Upper Zone of the Bushveld Igneous Complex, South Africa
Session: How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution
Presenting Author:
Richard RobertsAuthors:
Roberts, Richard James1, Rossouw, Tawnee2, Deysel, Adriana3, Hetherington, Callum J.4(1) University of Pretoria Department of Geology, Pretoria, None, South Africa, (2) University of Pretoria, Pretoria, South Africa, (3) University of Pretoria, Pretoria, South Africa; Council for Scientific and Industrial Research, Pretoria, South Africa, (4) Texas Tech University, Lubbock, TX, USA,
Abstract:
The mafic succession in the Bushveld Complex, South Africa, hosts remarkable layering, in numerous different forms. The Upper Zone, the final magma package to crystallise, hosts thick and laterally continuous magnetitite layers that stretch for 100s of km. In this study we present new handheld XRF data and trace element chemistry for apatite-bearing intervals above and below Layer 21 in the Upper Zone, taken from the Bierkraal boreholes drilled in the Western Limb. The handheld XRF data, taken on a cm by cm scale, shows that apatite rapidly appears and disappears in the succession over less than a meter. Close inspection of the data shows that each apatite-bearing interval truncates against an anorthosite layer at the top and features an anorthosite-magnetitite pair at the base of each cycle. Some of these anorthosite and magnetitite layers identified are less than 5 cm in thickness. The structure of a cycle (anorthosite-magnetitite- apatite-bearing gabbro) conforms to the model presented by Yuan et al. (2017) in which an incoming pulse of magma first dumps its load of transported plagioclase crystals to form an anorthosite, then proceeds to crystallise in situ. This hypothesis is backed up with new apatite chemistry analyses and field observations. Previous studies have not identified as many cycles as this study, and the authors contend that the cycles are thinner and more abundant than previously believed. The construction of the Upper Zone by numerous smaller magma pulses would facilitate rapid cooling, but raises questions as to the laterally continuity of many of the layers.
Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025
doi: 10.1130/abs/2025AM-9728
© Copyright 2025 The Geological Society of America (GSA), all rights reserved.
How Thick is a Layer? Insights into the Construction of the Upper Zone of the Bushveld Igneous Complex, South Africa
Category
Topical Sessions
Description
Session Format: Oral
Presentation Date: 10/19/2025
Presentation Start Time: 11:10 AM
Presentation Room: HBGCC, 216AB
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