The Sr-isotopic stratigraphy of the Eastern Limb of the Bushveld Complex

Motaung, Thapelo Sidwell

The Sr-isotopic stratigraphy of the Eastern Limb of the Bushveld Complex

dc.contributor.advisor	Roelofse, Frederick
dc.contributor.advisor	Magson, Justine
dc.contributor.author	Motaung, Thapelo Sidwell
dc.date.accessioned	2022-05-24T11:56:16Z
dc.date.available	2022-05-24T11:56:16Z
dc.date.issued	2021-10
dc.description.abstract	Data on the modal mineralogy, whole-rock geochemistry, mineral chemistry and Sr-isotopic compositions of plagioclase from the broadly pyroxenitic Lower Critical Zone to the gabbroic Upper Zone in the Eastern Limb of the Bushveld Complex are presented in this study, covering a stratigraphic interval of approximately 5000 m. The data were determined using transmitted light microscopy, X-Ray Fluorescence Spectrometry (XRF), Electron Probe Micro-Analyzer (EPMA) and Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS) to produce a continuous and relatively high-resolution profile of variations in mineral chemistry, whole-rock major and trace elements and Sr-isotopic composition of plagioclase. The results show Sr-isotopic trends that are similar to those observed in the Western Limb, with the first isotopically heterogeneous stage named the ―Integration Stage‖, characterised by fluctuating initial 87Sr/86Sr compositions from the Lower Critical Zone (~0.7051) to the Lower Main Zone (~0.7084). These variations are attributed to repeated influxes of compositionally distinct magmas. The Lower / Upper Critical Zone boundary is characterized by a sharp increase in initial 87Sr/86Sr ratios from 0.7054 to 0.7062 while the Critical Zone / Main Zone boundary is characterized by a sharp increase from 0.7065 to 0.7080. The isotopically homogeneous ―Differentiation Stage‖ includes the Upper Main and Upper zones. This interval is characterized by relatively uniform initial 87Sr/86Sr composition of ~0.7073. Limited variation in Sr-isotopic compositions suggests that this stratigraphic interval was formed through fractional crystallization with limited or no injections of new magma. The last major addition of magma into the Bushveld magma chamber is recorded by fluctuations in plagioclase An% and initial 87Sr/86Sr ratios at the level of the Pyroxenite Marker, which occurs at the boundary between the Lower and Upper Main zones. There is a decrease in initial 87Sr/86Sr ratios at the level of the Pyroxenite Marker, from 0.7079 to 0.7075. Within the ―Integration Stage‖ where significant variations in initial 87Sr/86Sr ratios of plagioclase are recorded, there is limited variation in plagioclase An% compared to the ―Differentiation Stage‖, which is characterized by a dominantly normal differentiation trend shown by decreasing plagioclase An% with increasing stratigraphic height and limited variation in initial 87Sr/86Sr ratios of plagioclase. This trend between plagioclase An% and initial 87Sr/86Sr ratios of plagioclase is also observed in the Western and Northern Limbs of the Bushveld Complex. Inter and intra-crystal initial 87Sr/86Sr disequilibrium is locally observed in the Lower Main and Upper Critical zones and is limited in the Upper Main and Upper Zones. This disequilibrium supports a model of petrogenesis which suggests that the Critical and Lower Main Zones formed through intrusion of variably contaminated crystal mushes derived from a sub compartmentalized, sub-Bushveld staging chamber that underwent different degrees of contamination with crustal rocks of the Kaapvaal craton. The Upper Main and Upper Zones revealed uniform initial 87Sr/86Sr compositions with a decreasing plagioclase An% trend, suggesting normal fractionation as the main petrogenetic process.	en_ZA
dc.description.sponsorship	Department of Science and Innovation (DSI)	en_ZA
dc.description.sponsorship	National Research Foundation (NRF)	en_ZA
dc.description.sponsorship	Centre of Excellence for Integrated Mineral and Energy Resource Analysis (CIMERA)	en_ZA
dc.identifier.uri	http://hdl.handle.net/11660/11634
dc.language.iso	en	en_ZA
dc.publisher	University of the Free State	en_ZA
dc.rights.holder	University of the Free State	en_ZA
dc.subject	Dissertation (M.Sc. (Geology))--University of the Free State, 2021	en_ZA
dc.subject	Rocks -- South Africa -- Bushveld -- Analysis	en_ZA
dc.subject	Geology -- South Africa -- Bushveld	en_ZA
dc.subject	Sr-isotopic trends	en_ZA
dc.title	The Sr-isotopic stratigraphy of the Eastern Limb of the Bushveld Complex	en_ZA
dc.type	Dissertation	en_ZA