The layered marker sequence, main zone, Eastern Bushveld Complex - a model for modal layering.

Abstract

This project involved the study of the Layered Marker Sequence of interbedded melanocratie and leucocratic rocks that occurs in the Main Zone of the Eastern Limb of the Bushveld Complex, The main aim was to develop a model for the formation of the observed layering. To achieve this a wide range of data has been gathered. In the field the layered package can be followed for a considered distance along strike. Individual layers however tend to terminate along strike. Layer contacts are sharp and there is a distinct lack of grading within layers. Modal variations between layers caused primarily by a change in the relative proportion of'plagioclase, are pronounced, whilst the ratio of clinopyroxene to orthopyroxene remains reasonably constant. The rock types making up the sequence exhibit a planar fabric defined by the bedding parallel orientation of the tabular plagioclase crystals. These 'crystals also appear orientated within this plane. Mineral compositions vary between layers and can be ascribed to trapped liquid shift effects. Whole rock geochemistry varies strongly between rock types. The variation in the concentrations of the more compatible elements can be explained by modal variation. The trends shown. by the incompatible elements indicate the importance of the trapped liquid shift effects within these rocks. From these lines of evidence, the Layered Marker Sequence under discussion cannot be explained by models such as in situ crystallization, rhythmic nucleation, magma injection, pressure fluctuations, or the mass transfer of material due to differences in grain surface energies. The evidence collected may best be explained by a physical separation of phases out of a density current flowing into the chamber after detachment ofthe boundary layer from the sidewalls of the chamber.