Some geochemical constraints upon models for the crystallization of the upper critical zone-main zone interval, northwestern Bushveld complex

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Abstract
Ratios between elements Mg, Fe, Co, Cr, Ni, V, and Sc are consistently different in mafic rocks of the upper critical zone, and those above the Bastard unit. Within the 300 m section above the Merensky Reef, 87Sr/86Sr ratios increase from c.0.7063 to c.0.7087, irrespective of rock type. Decoupling of Mg/(Mg + Fe2+) ratios and the Ca contents of plagioclase, and wide variations in the proportions of anorthosite within the Bastard, Merensky, and Merensky Footwall units, are inconsistent with anorthosite formation by simple fractional crystallization of magma batches of limited volume. Conversely, significant differences in Sr-isotope ratios show that these anorthosites could not have shared a common parental liquid. These data are used to develop a model whereby (a) the 300 m column above the critical zone represents the mixing of liquids of isotopically and geochemically discrete upper critical and main zone lineages, (b) mafic layers of the Bastard, Merensky, and Merensky Footwall units crystallized from discrete injections of primitive, mafic liquid while (c) the leucocratic upper parts of these units crystallized during progressive hybridization of liquid residua, which remained after significant separation of mafic phases, with a supernatant column representing the liquid residua of earlier cycles, and (d) the buoyancy of plagioclase, and enlargement of the primary phase volume of plagioclase consequent upon an increase in An/Ab ratio of hybrid liquids, were significant factors in the generation of anorthositic layers.