DISCORDANT IRON-RICH ULTRAMAFIC PEGMATITES IN THE BUSHVELD COMPLEX AND THEIR RELATIONSHIP TO IRON-RICH INTERCUMULUS AND RESIDUAL LIQUIDS

The iron-rich ultramafic pegmatities comprise a suite of coarse-graine d rocks that form discordant bodies within the layered sequence of the Bushveld Complex. These pegmatities, which are considerably more abun dant than is generally realized, provide evidence for the differentiat ion of iron-rich residual melts. The pegmatities are composed largely of iron-rich olivine and clinopyroxene, together with Ti-magnetite and ilmenite. Feldspar is characteristically absent, but paradoxically th e pegmatities preferentially replace anorthositic cumulates. Two subgr oups are recognized, olivine-clinopyroxene pegmatite and Fe-Ti oxide p egmatite. With increased stratigraphic height the pegmatities become r icher in Fe-Ti oxides. Thus, olivine-clinopyroxene pegmatite is preval ent in the Upper Critical and Lower Main Zones, whereas Fe-Ti oxide pe gmatite is restricted to the Upper Main and Upper Zones. Zoned pegmati te, with a core of Fe-Ti oxide pegmatite, is transitional between the two subgroups. New whole-rock and electron microprobe analyses of oliv ine-clinopyroxene pegmatite from the Upper Critical and Lower Main Zon es provide convincing evidence that their composition is directly rela ted to height. Cryptic compositional variations are analogous to those displayed by the layered cumulates, but for a given height the pegmat ites are always more evolved. Compositions of clinopyroxene in the peg matites reflect a near-linear relationship with height, whereas cumulu s pyroxenes display upward iron-enrichment trends complicated by reple nishment and reaction with trapped intercumulus liquid. The olivine-cl inopyroxene pegmatite formed by magmatic replacement of earlier-formed cumulates in response to infiltration of iron-rich melts. Suitably di fferentiated melts comprised intercumulus and residual liquids derived from thick anorthosite layers. The absence of feldspar, although not fully understood, is attributed to an immiscible relationship between dense, iron-rich melts and light, silica-alkali-rich liquids. The latt er infiltrated upward to be reincorporated into the resident magma. Th e iron-rich melts, however, drained down into the crystallizing cumula te pile. Channelling along early-formed fractures and joints was signi ficant, locally resulting in huge pipe-like bodies of pegmatite. The i ron-rich melts became increasingly differentiated with height, partly in response to the fractional crystallization of more evolved cumulate s. The olivine-clinopyroxene pegmatites are related to infiltration of Fe-Ti oxide-rich silicate melt, whereas Fe-Ti oxide pegmatite is ascr ibed to Fe-Ti oxide liquid, as originally argued by Bateman (1951). Th e Bushveld Complex followed the Fenner trend of almost uninterrupted i ron enrichment. Evidence of pronounced iron enrichment is, however, ma nifested in the discordant iron-rich ultramafic pegmatites several tho usands of metres below the height at which iron-rich cumulates are obs erved.