APATITE IN THE UPPER ZONE OF THE BUSHVELD (WESTERN LIMB)- EVIDENCE FOR A REJUVENATED MAGMA AT THE HEIGHT OF LAYER 21?

The occurrence of apatite and its trace element geochemistry in a borehole core through the Upper Zone from the Western Limb of the Bushveld is reported (BK1). Apatite displays cyclic occurrence in the upper portion of the Upper Zone, appearing and disappearing several times. Two “spikes” of apatite, where apatite appears in abundance and then disappears suddenly, occur below the magnetitite layer noted as Layer 21, and are marked by enrichment in light REEs. The apatite intervals above Layer 21 are marked by sudden appearance and gradual disappearance of apatite, and have a pronounced negative Eu anomaly. Previous studies of apatite in the Upper Zone in the Eastern Limb noted this difference in REE profiles and explained it as a consequence of the trapped liquid shift, or as an indication of massive liquid immiscibility in the chamber at the level of Layer 21. We propose an alternative solution, in which a rejuvenated magma is injected into the magma chamber at or just below the level of Layer 21. This new rejuvenated magma is likely genetically related to the previous magma but is much richer in Fe and depleted in V compared to the previous magma, and is responsible for the formation of Layer 21 (8m thick), a layer considerably thicker than any other magnetitite layer, including the Main Magnetite Layer. The influx of a new magma is supported by significant compositional shifts in the Upper Zone across layer 21, and reflected in the compositions of orthopyroxene (Mg#=25 below; Mg#=49 above), plagioclase (An#=47 below; #An=58 above), and olivine (Mg#=20 below; Mg#=40 above), and by evidence for liquid immiscibility only in the magma above Layer 21. The Eu anomaly above Layer 21 thus reflects a magma which has experienced significant plagioclase fractionation, whereas the magma below Layer 21 has not experienced the same level of fractionation.