Discordant Iron-Rich Ultramafic Pegmatites in the Bushveld Complex and their Relationship to Iron-Rich Intercumulus and Residual Liquids |
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| Authors: | SCOON ROGER N; MITCHELL ANDREW A |
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| Institution: | 1Trojan Exploration P.O. Box 784991, Sandton 2146, South Africa
2Department of Geology, University of Durban-Westville Private Bag X54001, Durban 4000, South Africa |
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| Abstract: | The iron-rich ultramafic pegmatites comprise a suite of coarse-grainedrocks that form discordant bodies within the layered sequenceof the Bushveld Complex. These pegmatites, which are considerablymore abundant than is generally realized, provide evidence forthe differentiation of iron-rich residual melts. The pegmatitesare composed largely of iron-rich olivine and clinopyroxene,together with Ti-magnetite and ilmenite. Feldspar is characteristicallyabsent, but paradoxically the pegmatites preferentially replaceanorthositic cumulates. Two subgroups are recognized, olivine-clinopyroxenepegmatite and Fe-Ti oxides pegmatite. With increased stratigraphicheight the pegmatites become richer in Fe-Ti oxides. Thus, olivine-clinopyroxenepegmatite is prevalent in the Upper Critical and Lower MainZones, whereas Fe-Ti oxide pegmatite is restricted to the UpperMain and Upper Zones. Zoned pegmatite, with a core of Fe-Tioxide pegmatite, is transitional between the two subgroups. New whole-rock and electron microprobe analyses of olivine-clinopyroxenepegmatite from the Upper Critical and Lower Main Zones provideconvincing evidence that their composition is directly relatedto height. Cryptic compositional variations are analogous tothose displayed by the layered cumulates, but for a given heightthe pegmatites are always more evolved. Compositions of clinopyroxenein the pegmatites reflect a near-linear relationship with height,whereas cumulus pyroxenes display upward iron-enrichment trendscomplicated by replenishment and reaction with trapped intercumulusliquid. The olivine-clinopyroxene pegmatite formed by magmatic replacementof earlier-formed cumulates in response to infiltration of iron-richmelts. Suitably differentiated melts comprised intercumulusand residual liquids derived from thick anorthosite layers.The absence of feldspar, although not fully understood, is attributedto an immiscible relationship between dense, iron-rich meltsand light, silica-alkali-rich liquids. The latter infiltratedupward to be reincorporated into the resident magma. The iron-richmelts, however, drained down into the crystallizing cumulatepile. Channelling along early-formed fractures and joints wassignificant, locally resulting in huge pipe-like bodies of pegmatite. The iron-rich melts became increasingly differentiated withheight, partly in response to the fractional crystallizationof more evolved cumulates. The olivine-clinopyroxene pegmatitesare related to infiltration of Fe-Ti oxide-rich silicate melt,whereas Fe-Ti oxide pegmatite is ascribed to Fe-Ti oxide liquid,as originally argued by Bateman (1951). The Bushveld Complexfollowed the Fenner trend of almost uninterrupted iron enrichment.Evidence of pronounced iron enrichment is, however, manifestedin the discordant iron-rich ultramafic pegmatites several thousandsof metres below the height at which iron-rich cumulates areobserved. |
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