Abstract

Syn-Rustenburg Layered Suite deformation in the Spruitfontein inlier of the western Bushveld Complex metamorphic aureole provides an insight into the emplacement mechanisms of the sill-like Rustenburg Layered Suite. East—northeast-trending ductile structures were formed during north—northwest- to south—southeast-oriented compression caused by the lateral propagation of east—northeast- or east-trending Marginal Zone finger-like sills. These fingers appear to have been largely co-planar in the west of the inlier, such that folding occurred in the region of compression between adjacent sills. With progression to the east, however, the fingers apparently diverged more and the wedging effect of the magma at the lower level of the finger system initiated rigid body rotation of Pretoria Group rocks, and the formation of a bridge structure between the dilating fingers. The orientation of syn-Rustenburg Layered Suite deformational features, coupled with Rustenburg Layered Suite facies variations across the inlier, suggest that magma flow was directed towards the east or east—northeast. Field relations do not elucidate whether or not the Spruitfontein inlier is still attached to the floor; however agenetic model whereby the inlier was initially deformed by intrusion-related processes and then became slightly detached from the floor may provide the most satisfactory explanation of Rustenburg Layered Suite compartmentalization across the inlier.

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