The Lower Zone of the Bushveld Complex comprises an up to 2-km-thick package of different ultramafic rock types with an approx. 90-cm-thick, sulphide-bearing noritic interval that occurs in the western and eastern limbs. The distribution and geometry of the zone are highly variable across the Complex, showing pronounced, yet laterally discontinuous layering on different scales. Together with the ubiquitous lack of large-scale fractionation in the Mg# of orthopyroxene, variable Sr isotope compositions and erratic Pt/Pd ratios, these observations strongly suggest an emplacement of the Lower Zone as a sill complex, as these contrasting geochemical characteristics are difficult to account for in a large Bushveld magma chamber, as previously suggested. It is more likely that these sills were episodically fed from a sub-Bushveld staging chamber, and variably contaminated, while passing through the crust before their final emplacement in the Lower Zone. Detailed mineralogical and microtextural work based on high-resolution elemental mapping of a set of samples, covering the entire Lower Zone stratigraphy of the western Bushveld shows that the variations in the late crystallising interstitial mineral mode are different from what would be expect, if all phases crystallised from a fixed initial mass of interstitial liquid. The interstitial mineral mode, represented by plagioclase, clinopyroxene and other late stage phases, shows variable ratios of these minerals ranging from ca. 21:15:64 to 75:17:8. In comparison to modelled expected ratios, most of the analysed rocks have higher amounts of early crystallising interstitial phases (e.g. plagioclase, clinopyroxene), relative to late crystallising phases (e.g. quartz, alkali feldspar). Therefore, interstitial melt must have migrated at different stages of fractionation during cumulate solidification, as a consequence of either compaction or displacement by convecting interstitial liquids. Two samples, however, show the opposite: late phases are relatively more abundant than early ones, which is consistent with a convection-driven replacement of primitive interstitial liquid by more evolved liquid. These results have important implications for the interpretation of the Lower Zone and, by extension, for layered intrusions in general: (1) interstitial sulphide mineralisation may be introduced into a cumulate through infiltrating melts, i.e. the liquid components of a sulphur-saturated crystal mush are not withheld from further migration, upon interaction with a cumulate pile; (2) most importantly, late stage minerals, such as zircon, rarely crystallise from trapped liquid that was initially in equilibrium with the cumulate. Therefore, dating of interstitial zircon from cumulates is unlikely to record the actual timing of emplacement, but merely the crystallisation of a later episode of residual melt that migrated through the cumulate.
Contributions to Mineralogy and Petrology – Springer Journals
Published: Jul 7, 2017
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