New perspectives on the formation of the Boulder Bed of the western Bushveld Complex, South Africa

The Boulder Bed of the western Bushveld Complex is an m-scale unit of mottled anorthosite containing sub-circular dm-scale ‘boulders’ of pyroxenite, harzburgite, or norite. To better understand this unit, we have combined high-resolution element mapping and electron back-scatter diffraction analysis with electron-probe microanalysis of plagioclase crystals from the boulders and their host anorthosite. Several key features pertinent to understanding the formation of this unit have been described, including (i) anhedral olivine is concentrated at the base of boulders, whereas clinopyroxene is concentrated towards the tops; (ii) the upward decrease in grain size through the boulders; (iii) the occurrence of chromite along the base of boulders and seldom along the top; (iv) the presence of strongly reverse-zoned cumulus plagioclase (An_75-95) in the so-called marginal zone underlying boulders; (v) the absence of deformation in the host anorthosite but the prevalence of intra-crystalline deformation in intercumulus pyroxene of the marginal zone; (vi) that amphibole (± apatite ± phlogopite) partially line the base of some boulders; (vii) traces of pyrrhotite (± pentlandite ± chalcopyrite) occur within the lower halves of boulders. We propose that the boulders formed in response to the disaggregation of a locally PGE-rich pyroxenite, triggered by heat- and (or) volatile-induced partial melting of the noritic host rocks. Several of the petrologic features arose from the reaction between the boulders and the noritic partial melt prior to late-stage viscous compaction.