Concentrations of rare earth elements in silicate rocks of the Lower, Critical and Main Zones of the Bushveld Complex

Abstract

Concentrations of rare earth elements in the Lower, Critical and Main Zones of the Bushveld Complex at Union Section show distinct variation with stratigraphic height, in that Lower and Lower Critical Zone cumulates are LREE and Th enriched (Ce/Sm: 10–25, Ce/Sm N : 3.65, Th/Sm N : 6.17) over Main Zone cumulates (Ce/Sm: 4–10, Ce/Sm N : approximately 2, Th/Sm N : 1.57). The Upper Critical Zone constitutes a transitional interval in terms of REE concentrations (Ce/Sm: 9–17). This pattern is broadly coupled, albeit in a contraposed manner, to that in initial Sr isotopic ratio and confirms earlier studies that the Bushveld Complex crystallized from at least two compositionally distinct parental magmas. Mixing between the two magmas occurred as early as in the Lower Zone and is sensitively reflected by variation in Ce/Sm, Th/Sm, modal plagioclase and chromite. While REE contents of the Lower and Critical Zones are approximately 15% those of the corresponding B1 and B2 marginal rocks, the Main Zone cumulates have similar REE contents to the B3 marginal rocks. This suggests that the Main Zone rocks either crystallized from a highly viscous crystal mush without appreciable fractionation, or from magma strongly REE enriched over the marginal rocks, which would imply that the Main Zone is not represented in the suite of marginal rocks.