Origin of Anorthosite and Magnetitite Layers in the Bushveld Complex, Constrained by Major Element Compositions of Plagioclase

Abstract

The Bushveld Complex, a layered mafic intrusion in South Africa, shows extreme vertical differentiation in terms of mineral compositions and modal proportions from dunite to ferrodiorite. In a continuous borehole core drilled through the uppermost 2� 8k m of the intrusion, typical rocks range upwards from troctolite, through gabbronorite and ferrogabbronorite to ferrodiorite, with extreme examples of anorthosite, magnetitite and feldspathic pyroxenite. The An content of plagioclase has previously been determined for 420 samples and decreases upward from An78 to An36, with six minor, slow reversals. Variations in modal proportions of plagioclase have been calculated based on 2200 density determinations on whole-rocks. Forty-five anorthosite layers have been identified, ranging from 1 to 23 m thick. None of these layers is associated with the abovementioned reversals in An content in plagioclase and nearly all have leucocratic rocks below and above, with more than the likely cotectic proportions of plagioclase. These observations argue against an origin for anorthosite related to magma addition or to supersaturation and oscillatory nucleation. Rhythmically pulsed crystallization, possibly associated with pressure changes, followed by crystal settling and sorting of minerals of different densities is a hypothesis consistent with all the observations. Twenty layers of magnetitite have been identified. There is a significant reversal in An content in the overlying plagioclase compared with the underlying sample across only one such layer. Again, this observation challenges hypotheses that such layers result from magma addition, but is consistent with a pressure-change hypothesis for triggering magnetite crystallization. The upper contacts of magnetitite layers that grade into anorthosite over many centimetres possibly also reflect settling and sorting. Rocks forming the uppermost 100 m of the intrusion contain the most sodic plagioclase compositions, demonstrating that there is no downward crystallizing roof facies. Furthermore, this uppermost 100 m section is depleted in plagioclase relative to its cotectic proportions. Hence, we find no evidence supporting flotation or prolonged suspension of plagioclase.