Journal of Petrology | Volume 35 | Number 4 | Pages 881-917 | 1994
© Oxford University Press 1994
research-article |
Discordant Iron-Rich Ultramafic Pegmatites in the Bushveld Complex and their Relationship to Iron-Rich Intercumulus and Residual Liquids
1Trojan Exploration P.O. Box 784991, Sandton 2146, South Africa
2Department of Geology, University of Durban-Westville Private Bag X54001, Durban 4000, South Africa
Received October 23, 1990; Revised typescript accepted July 26, 1993
ABSTRACT
The iron-rich ultramafic pegmatites comprise a suite of coarse-grained rocks that form discordant bodies within the layered sequence of the Bushveld Complex. These pegmatites, which are considerably more abundant than is generally realized, provide evidence for the differentiation of iron-rich residual melts. The pegmatites are composed largely of iron-rich olivine and clinopyroxene, together with Ti-magnetite and ilmenite. Feldspar is characteristically absent, but paradoxically the pegmatites preferentially replace anorthositic cumulates. Two subgroups are recognized, olivine-clinopyroxene pegmatite and Fe-Ti oxides pegmatite. With increased stratigraphic height the pegmatites become richer in Fe-Ti oxides. Thus, olivine-clinopyroxene pegmatite is prevalent in the Upper Critical and Lower Main Zones, whereas Fe-Ti oxide pegmatite is restricted to the Upper Main and Upper Zones. Zoned pegmatite, with a core of Fe-Ti oxide pegmatite, is transitional between the two subgroups.
New whole-rock and electron microprobe analyses of olivine-clinopyroxene pegmatite from the Upper Critical and Lower Main Zones provide convincing evidence that their composition is directly related to height. Cryptic compositional variations are analogous to those displayed by the layered cumulates, but for a given height the pegmatites are always more evolved. Compositions of clinopyroxene in the pegmatites reflect a near-linear relationship with height, whereas cumulus pyroxenes display upward iron-enrichment trends complicated by replenishment and reaction with trapped intercumulus liquid.
The olivine-clinopyroxene pegmatite formed by magmatic replacement of earlier-formed cumulates in response to infiltration of iron-rich melts. Suitably differentiated 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 latter infiltrated upward to be reincorporated into the resident magma. The iron-rich melts, however, drained down into the crystallizing cumulate pile. Channelling along early-formed fractures and joints was significant, locally resulting in huge pipe-like bodies of pegmatite.
The iron-rich melts became increasingly differentiated with height, partly in response to the fractional crystallization of more evolved cumulates. The olivine-clinopyroxene pegmatites are 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 Complex followed the Fenner trend of almost uninterrupted iron enrichment. Evidence of pronounced iron enrichment is, however, manifested in the discordant iron-rich ultramafic pegmatites several thousands of metres below the height at which iron-rich cumulates are observed.
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