10 How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution

This session integrates structural and petrologic views of plutonic systems, exploring the crustal response to pluton formation, and processes that control the physical and chemical characteristics of pluton-host rock systems from initial emplacement to solidification.

Sunday, 19 October 2025
8:00 AM - 12:00 PM
HBGCC, 216AB

How Thick is a Layer? Insights into the Construction of the Upper Zone of the Bushveld Igneous Complex, South Africa

Paper number: 10-13
Program: Technical Sessions
Time: 11:10 AM – 11:25 AM

Authors

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 Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413963

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