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Magma replenishment, and the significance of poikilitic texlures, in the Lower Main Zone of the western Bushveld Complex, South Africa

Published online by Cambridge University Press:  05 July 2018

Andrew A. Mitchell ,
Hugh V. Eales  and
F. Johan Kruger
Andrew A. Mitchell
Affiliation:
Department of Geology, University of Durban-Westville, Private Bag X54001, Durban, 4000, South Africa
Hugh V. Eales
Affiliation:
Department of Geology, Rhodes University, Grahamstown 6140, South Africa
F. Johan Kruger
Affiliation:
Hugh Allsopp Laboratory, Bernard Price Institute for Geophysical Research, Private Bag 3, Wits 2050, South Africa
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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 up-sequence 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 replenishmentpoikilitic textureBushveld ComplexSouth Africa
Type
Research Article
Information
Mineralogical Magazine , Volume 62 , Issue 4 , August 1998 , pp. 435 - 450
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1998

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