Chemical and isotopic variations in plagioclase from the Upper and Main Zones, Northern Limb, Bushveld Complex
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The in-situ major element, trace element, and Sr-isotopic compositions of plagioclase in the broadly gabbroic cumulates from the Upper and Main Zones of the Northern Limb of the Bushveld Complex, as obtained from the Bellevue (BV-1) and Moordkopje (MO-1) drill cores have been determined by means of electron microprobe and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). The results show the existence of inter and intra-crystal initial 87Sr/86Sr disequilibrium of coexisting plagioclase, a phenomenon which has received rather rudimentary attention not only in the Bushveld Complex, but in other layered intrusions as well. This disequilibrium is particularly striking in samples from the Lower Main Zone, an interval which also records a lack of differentiation, as exemplified by the An% of plagioclase. In the rest of the stratigraphy, up to the top of the Rustenburg Layered Suite, there is less, if any, inter and intra-crystal initial 87Sr/86Sr disequilibrium of plagioclase coupled to more prominent differentiation trends. These results are in support of a model for the petrogenesis of this part of the Bushveld Complex, which involves the Lower Main Zone forming through the repeated intrusion of crystal mushes derived from a deeper seated, sub-compartmentalized staging chamber, into the Bushveld main magma chamber, with fractionation processes being key in the formation of the Upper Main and Upper Zones. Based on the initial 87Sr/86Sr composition of plagioclase, the Northern Limb of the Bushveld Complex can also be divided into a putative integration and differentiation stages, similar to the Western Limb of the Bushveld Complex. The integration stage in the Western Limb included the Lower, Critical, and Lower Main Zones, whereas the differentiation stage included the Upper Main and Upper Zones. The integration stage in this study incorporates the lower reaches of the Lower Main Zone, with the rest of the stratigraphy up to the top of the Upper Zone covering the differentiation stage. Furthermore, the Lower Main Zone of the Northern Limb of the Bushveld Complex has very consistent concentrations of most trace elements investigated, and these concentrations tend to increase upwards into the Upper Main and Upper Zones. This also suggests repeated intrusion for the Lower Main Zone with differentiation processes dominating in the upper parts of the Rustenburg Layered Suite.