Abstract

The Bushveld Complex follows well-established crystallization sequences on local and secular scales, although sequences indicated by chill-facies melting experiments are transiently represented in the earliest cumulates. Evidence for an early ultrabasic magma existing in the complex is presented.The earliest Critical Zone sedimentation produced thick isomodal orthopyroxene layers interspersed with chromite seams owing their origin to cyclic iron enrichment of the bottom magmas isolated from the main magma volume. By contrast, later accumulation was as cyclic mineral-graded units containing inherent cumulus chromite. A precursor to Main Zone-type cyclicity--pyroxene(s): plagioclase--is also represented. Rhythmic "mix-layering"--chromite: plagioclase--is postulated as being caused by tectonically triggered magma turbulence which mingles cyclicity end members and winnows them into pure cumulate layers.Magmatic sedimentation in the remarkably persistent mineral-graded Merensky unit was profoundly influenced by regional predepositional or contemporaneous footwall arching in the eastern and western compartments of the complex.In the uppermost Critical Zone, anorthosites show remarkable persistency in thickness. Evidence is presented that plagioclase floated repeatedly to temperature-density-compositional inversions, thereby forming anorthosite mats which entrapped successive magmas and volatiles beneath them. Plagioclase removal basified the entrapped magmas while the volatiles fostered rapid ferromagnesium mineral growth and aggregation into boulder-like masses. In the Merensky unit these boulders sank in the magma, dimpled the early-formed chromite layer and formed the basal pegmatoid reef. Chromite settling, interrupted by pyroxene crystallization, depleted iron from the magma, thus increasing f (sub S 2 ) and producing immiscible sulfides which collected the platinoids accumulated via the inter-cumulus fluid from the base of the complex.

First Page Preview

First page PDF preview
You do not currently have access to this article.