Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet

摘要

The major element composition of plagioclase, pyroxene, olivine, and magnetite, and whole-rock 87Sr/86Sr data are presented for the uppermost 2·1 km of the layered mafic rocks (upper Main Zone and Upper Zone) at Bierkraal in the western Bushveld Complex. Initial 87Sr/86Sr ratios are near-constant (0·7073 ± 0·0001) for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation. The 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite) layers and changes up-section from gabbronorite (An₇₂ plagioclase; Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayalite ferrodiorite (An₄₃; Mg# 5 clinopyroxene; Fo₁ olivine). The overall fractionation trend is, however, interrupted by reversals characterized by higher An% of plagioclase, higher Mg# of pyroxene and olivine, and higher V₂O₅ of magnetite. In the upper half of the succession there is also the intermittent presence of cumulus olivine and apatite. These reversals in normal fractionation trends define the bases of at least nine major cycles. We have calculated a plausible composition for the magma from which this entire succession formed. Forward fractional crystallization modeling of this composition predicts an initial increase in total iron, near-constant SiO₂ and an increasing density of the residual magma before magnetite crystallizes. After magnetite begins to crystallize the residual magma shows a near-constant total iron, an increase in SiO₂ and decrease in density. We explain the observed cyclicity by bottom crystallization. Initially magma stratification developed during crystallization of the basal gabbronorites. Once magnetite began to crystallize, periodic density inversion led to mixing with the overlying magma layer, producing mineralogical breaks between fractionation cycles. The magnetitite and nelsonite layers mainly occur within fractionation cycles, not at their bases. In at least two cases, crystallization of thick magnetitite layers may have lowered the density of the basal layer of melt dramatically, and triggered the proposed density inversion, resulting in close, but not perfect, coincidence of mineralogical breaks and packages of magnetitite layers.