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.

西部布什维尔德杂岩体的巨石层是一个 m 尺度的斑驳斜长岩单元，包含亚圆形 dm 尺度的辉石岩、方晶石或紫晶石的“巨石”。为了更好地理解这个单元，我们将高分辨率元素映射和电子背散射衍射分析与来自巨石及其主体斜长石的斜长石晶体的电子探针显微分析相结合。已经描述了与理解该单元的形成相关的几个关键特征，包括（i）反面橄榄石集中在巨石的底部，而单斜辉石集中在顶部；(ii) 通过巨石的粒度向上减小；(iii) 铬铁矿沿着巨石底部出现，很少沿着顶部出现；(iv) 强烈反带积云斜长石的存在 (An_75-95 ) 在所谓的边缘地带下面的巨石; (v) 主斜长岩没有变形，但边缘带的辉石积云中普遍存在晶内变形；(vi) 闪石（±磷灰石±金云母）部分排列在一些巨石的底部；(vii) 磁黄铁矿（± 镍黄铁矿± 黄铜矿）的痕迹出现在巨石的下半部分。我们提出，巨石是响应于局部富含 PGE 的辉石岩的分解而形成的，这是由热和（或）挥发物引起的诺里海母岩的部分熔融引发的。一些岩石学特征是在后期粘性压实之前巨石与诺里海特部分熔体之间的反应产生的。