Year: 2015
Type: dissertation
Abstract: The Bushveld complex is the world’s largest mafic intrusion and located on the Archean Kaapvaal craton, southern Africa. The complex’s host rock has anomalous quantities of platinum-group elements (PGE), chromite and other valuable metals of high demand. The origin of the complex remains to this day discussed, despite a plethora of geochemical and petrological studies. There are in general four proposed models for Bushveld’s formation. One hypothesis propose that the magmatism was triggered by an impact event (from asteroids and/or comets) and are supported by the circular form, PDFs and impact melt breccia. Another model suggests a formation from a back-arc as structural features, flow direction of magma from the feeder dyke TML and high δ37Cl and Cl/F ratio support this. A third model suggests a mantle plume, this is supported by the geomorphological structure, the geochemistry and the contamination of the magma by the crust. A later model is a combination between the mantle plume and a delamination of the mantle. Arguments for this model are the spatial relationship between the Transvaal Supergroup and the Bushveld Complex, seismic analysis and dykes that converge towards the complex. Lack of representative macro and microscopic indicators, along with paleomagnetism that suggests that the strata was horizontal, makes the meteorite impact model unlikely. Same goes for the mantle plume theory, due to the absence of radiation dyke swarms from 2.06 Ga BP. The delamination hypothesis explains the formation of the Bushveld complex well, however the age difference between the swarms is a counter-argument. Other counter-arguments are the presence of dykes south of the complex with a differing orientation and diamonds on shallow depths. There are arguments against the subduction hypothesis as well. The Bushveld complex is the only intrusion found in a sedimentary environment in which no calc-alkaline bedrock has been found. High δ18O could be explained by influx of sediment. No other model can explain the high chlorine ratios which leads to the conclusion that the Bushveld complex is related to some type of subduction processes.
Author: Filip Ivarsson
Language: Swedish
Subfield: Geochemistry and Petrology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W1700108695