The Sr isotopic stratigraphy of the LCZ-UCZ transition in the Western Limb of the Bushveld Complex

Loading...
Thumbnail Image
Date
2021
Authors
Malatji, Mafete
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Data on the modal mineralogy, whole-rock geochemistry, plagioclase mineral chemistry and Sr isotopic compositions in lithologies covering an interval of ~100 m across the Upper Critical Zone (UCZ) and Lower Critical Zone (LCZ) transition in the Western Limb of the Bushveld Complex are presented in this study. The aims of this study were to (1) investigate the presence or absence of isotopic disequilibrium in plagioclase (2) to investigate differences between the LCZ and UCZ from a geochemical, petrological and Sr isotopic perspective and (3) to refine chromitite formation models using the data obtained over the course of the study. Samples were obtained from the BH7929 drill core donated by Impala platinum to the University of the Free State. Samples were analysed using transmitted light microscopy, X-Ray Fluorescence Spectrometry (XRF), Inductively Coupled Plasma Mass Spectrometry (ICPMS), Electron Probe Micro-Analyzer (EPMA) and Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA MC-ICP-MS) to produce whole-rock major and trace element geochemistry and plagioclase elemental and isotopic compositional profiles across the UCZ-LCZ transition. Results reveal that the LCZ is composed of orthopyroxene-dominated lithologies that display cryptic layering, hosting two chromitite layers (MG1-2), with plagioclase predominantly existing as an intercumulus phase. Plagioclase is predominantly cumulus in the UCZ, dominating the UCZ lithologies that display modal layering. Two chromitite layers were investigated in the UCZ (MG3-4). Compositional breaks in whole-rock major and trace elements are detected at the UCZ-LCZ transition and at the level of the chromitite layers, reflecting variations in the dominant mineral phases. Fractionation indices including whole-rock Mg# and Cr/V ratio reveal little variation throughout the study interval in silicate-dominated lithologies, with variations mostly detected at the level of chromitite layers. Plagioclase An% averages 82.10 ± 1.90% in the UCZ, whereas it averages 73.58 ± 2.60% in the LCZ. Chromitite layers in the LCZ reveal lower An% values in comparison with adjacent silicate lithologies, whereas the UCZ reveals very little to no variations between chromitites and silicate lithologies. Sri values in the UCZ average 0.7059 ± 0.0003, whereas Sri in the LCZ averages 0.7054 ± 0.0004. Decreases in the Sri value of plagioclase are observed at the level of the chromitite layers in the LCZ, whereas the UCZ reveals a constant Sri up the stratigraphy. The data provide credence to the importance of magma mixing (i.e. Irvine, 1977) as a process operational in the formation of chromitite layers within the LCZ and UCZ and argue against models suggesting variations in intensive parameters or in-situ crystallization as dominant processes in the formation of chromitite layers. It is proposed that the UCZ-LCZ transition displays credible evidence for the repeated intrusion of batches of isotopically distinct magmas, with chromitite layers in the LCZ forming in response to the mixing of newly introduced and resident magma in a manner analogous to that envisaged by Irvine. The MG3 layer in the UCZ also appears to have formed as a direct consequence of mixing between newly intruded UCZ magma and the residual LCZ magma. The MG4 layer does not preserve Sr-isotopic evidence for magma mixing as it has similar Sri as that of adjacent silicate lithologies. In order to account for the mass balance of Cr, it is argued that at the level of chromitite layers, intruded magma pulses were chromite-laden, with additional chromite formation occurring in response to magma mixing. The MG1 chromitite layer provides potential evidence in support of such an argument in the form of multiple isotopically distinct populations of plagioclase that may have been intruded along with suspended chromite crystals.
Description
Keywords
Dissertation (M.Sc. (Geology))--University of the Free State, 2021, Isotopic disequilibrium in plagioclase, Upper Critical Zone (UCZ), Lower Critical Zone (LCZ), Refine chromitite, Rocks -- South Africa -- Bushveld -- Analysis, Geology -- South Africa -- Bushveld
Citation