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

[摘要] This project involved the study of the Layered Marker Sequence of interbedded melanocratie andleucocratic rocks that occurs in the Main Zone of the Eastern Limb of the Bushveld Complex, Themain aim was to develop a model for the formation of the observed layering. To achieve this awide range of data has been gathered. In the field the layered package can be followed for aconsidered distance along strike. Individual layers however tend to terminate along strike. Layercontacts are sharp and there is a distinct lack of grading within layers. Modal variations betweenlayers 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 typesmaking up the sequence exhibit a planar fabric defined by the bedding parallel orientation of thetabular plagioclase crystals. These 'crystals also appear orientated within this plane. Mineralcompositions vary between layers and can be ascribed to trapped liquid shift effects. Whole rockgeochemistry varies strongly between rock types. The variation in the concentrations of the morecompatible elements can be explained by modal variation. The trends shown. by the incompatibleelements indicate the importance of the trapped liquid shift effects within these rocks. From theselines of evidence, the Layered Marker Sequence under discussion cannot be explained by modelssuch as in situ crystallization, rhythmic nucleation, magma injection, pressure fluctuations, or themass transfer of material due to differences in grain surface energies. The evidence collected maybest be explained by a physical separation of phases out of a density current flowing into thechamber after detachment ofthe boundary layer from the sidewalls of the chamber.

[发布机构] University of the Witwatersrand