Magma Replenishment, and the Significance of Poikilitic Textures, in the Lower Main Zone of the Western Bushveld Complex, South Africa

Andrew A. Mitchell, Hugh V. Eales and F. Johan Kruger
Department of Geology, University of Durban-Westville, Private Bag X54001, Durban, 4000, South Africa
Department of Geology, Rhodes University, Grahamstown 6140, South Africa
Hugh Allsopp Laboratory, Bernard Price Institute for Geophysical Research, Private Bag 3, Wits 2050, South Africa

Abstract: Petrographic and compositional variations in the Lower Main Zone (LMZ) of the western Bushveld Complex indicate changing regimes of magma replenishment. The lowermost unit of the LMZ, designated N-I, is an enigmatic sequence of leuconoritic cumulates, characterized primarily by upsequence increases in both orthopyroxene Mg# and whole-rock Sr isotope initial ratio. The Sr isotope profile of N-I is ascribed to injection and progressive integration of small influxes of fresh magma with high (Main Zone-type) Sr isotope initial ratios. The basal Fe-enrichment in N-I, on the other hand, is ascribed to a separate, later mechanism involving the downward migration of late-stage Fe-rich liquids. The overlying two units, N-II and G-I, delineated chiefly in terms of basal Mg-enrichment of orthopyroxene, are ascribed to injections of fresh magma into the chamber. Poikilitic orthopyroxene grains in the basal parts of both N-II and G-I suggest entrainment and partial resorption of plagioclase grains from the semi-crystalline resident material into which the fresh magma was intruded.

Keywords: magma replenishment • poikilitic texture • Bushveld Complex • South Africa

Mineralogical Magazine; August 1998 v. 62; no. 4; p. 435-450; DOI: 10.1180/002646198547783
© 1998, The Mineralogical Society
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