Massive chromitites of the Bushveld Complex, South Africa: A critical review of existing hypotheses

The controversy over the origin of massive chromitites in layered intrusions has recently become more contentious than ever before. At issue is whether they are produced via gravity settling/in situ crystallization of chromite directly on the chamber floor or by kinetic sieving, metasomatic replacement, or sill-like intrusions beneath the chamber floor, i.e., in deep parts of the cumulate pile. The latter group of models implies that massive chromitites have never been on the chamber floor. In this paper, we show that decisive clues to the ‘chamber floor dilemma’ come from field relations of massive chromitite with magmatic dropstones in the Bushveld Complex, South Africa - the largest fossilized magma chamber in the Earth's continental crust. The roof sequences in such basaltic magma chambers are known to be inherently unstable; therefore, crustal instability results in their breakdown and collapse as angular blocks (i.e., magmatic dropstones) through the resident melt column onto the upward-growing chamber floor. We have discovered such magmatic dropstones in the sequences that host massive UG1, UG2, and MG2 chromitites in the Critical Zone of the Bushveld Complex. The dropstones are easily recognizable in the field because they are composed of fine-grained melanorite/orthopyroxenite, which are texturally dissimilar from and have sharp contacts with adjacent cumulate rocks (chromitite, anorthosite, norite, etc.). The dropstones range in shape from angular to lenticular fragments a few centimetres in size to large tabular dropstones of ∼0.5–1.0 m across and ∼10 m long. The dropstones indent pre-existing layers of chromitite/anorthosite beneath them and are covered by subsequently deposited layers of the same rocks above them. Some dropstones appeared to have knifed into chromitite/anorthosite layers and cut them off, and a few dropstones appeared to have been driven into the floor cumulates and crumpled the layers outwards and upward. Also, some beheaded dropstones indicate the truncation of the floor cumulates by planar erosional surfaces. The physical relationships of dropstones with their host rocks indicate that: (1) there was almost invariably a sharp interface between the top of the inward-growing cumulate pile and the overlying resident melt; (2) the uppermost part of the cumulate pile was coherent and igneous layering, involving chromitites, was generally fully developed right up to the crystal-liquid interface; (3) by the time of dropstones's landing, the chromitite layers were already formed on the chamber floor, with the resident melt immediately overlying them; (4) the existence of chromitite layers at the crystal-liquid interface implies their formation through the deposition of chromite alone directly onto the chamber floor; and (5) such crystal deposition requires the resident melt in the chamber to be chromite-only saturated (i.e., no other liquidus phases in the chamber). The occurrence of magmatic dropstones thus rules out the formation of massive chromitites by processes that operate at some depth in the cumulate pile (e.g., kinetic sieving, metasomatic replacement, or sill-like intrusions) and rather indicate their deposition via in situ crystallization from a chromite-only-saturated magma directly on the upward-growing floor of a Bushveld magma chamber. A thorough review of this concept demonstrates its high potential in explaining most field, textural and chemical features of massive chromitites in the Bushveld Complex.