Mechanisms of sill and dyke intrusion require an understanding of fracture growth,stress distributions and intensities, dilation,intrusion rates, hydraulic pressure,host-rock effects,en echelon fracture arrays,and flow direction. The methods of previous studies have been applied to natural sill and dyke examples of the Karoo Igneous Province in northern Natal . An en echelon array of Jurassic dolerite sills occurs within Permian Ecca sediments along the Mhlatuze River, west of Empangeni. Dolerite emplacement occurred as two intrusive phases. The first phase resulted in thick, . coarse-grained dolerite sills. The second phase produced relatively thinner, fme-grained sills. The intrusion of fmegrained dolerite into older sills is demonstrated by abrupt variations in the whole-rock and mineral geochemistry profiles across the sills. Syn-crystallisation effects such as crystal settling and fractionation, and post-crystallisation hydrothermal activity is also manifested in the mineralogical and geochemical changes across the sills. The fine grained doleriteis associated with xenolithic dolerite which represents a contaminated magma propagation front of the fine-grained dolerite. The higher viscosity of the xenolithic dolerite hindered propagation, and was thus overtaken and engulfedby the mainmagmapulse. Consistent sinistral off setting of sill segments is interpreted to be the result of a fingered sill periphery intruding an en echelon fracture array. Dilation of individual segments, or fingers, occurred simultaneously. Subsequent interaction of near-tip stresses induced inwardly propagating curvature of adjacent segmentsin the array.Resultant linkage has produced a stepped-sill geometry; sill propagation and flow directions were orthogonal to the plane of linkage. The flow direction is confirmed by shape preferred-orientations of acicular mineral grains within the chilled margins of the sills, indicating the direction of flow to be perpendicular to the plane of the en echelon array, and parallel to strike directions of offset surfaces that link adjacent sill segments. Multiple dyke intrusion is examinedat an outcrop of the Rooi Rand Dyke Swarm, along the Pongola River. Individual intrusive episodes are identifiable on the basis of chill-zone relationships. The pattern along the Pongola River suggests that younger intrusive episodes frequently intrude through the centres of older dykes. A three dimensional analysis of en echelon dyke let segments allows a re-construction of the dilation history,and provides an explanation for the development of blunt-ended intrusion segments. Mineral geochemistry anomalies around dyke tips suggests possible facilitation of incipient fracture via decreases in mineral strengths manifested by geochemical changes. A statistical digital analysis of micro-phenocryst orientations within chilled dyke margins is shown to provide a viable method to ascertain magma flow directions within dykes, and may thus be a useful tool for future investigations. / Thesis (M.Sc.)-University of Natal, 1994.

The Jurassic Rooi Rand dolerite dyke swarm was emplaced sub-parallel to the Lebombo Monocline during the initial stages of Gondwana breakup. The dykes extend northwards from the southern Lebombo region in northern KwaZulu-Natal, into central Swaziland, spanning a distance of approximately 200 kilometres with a width between 10 and 22 kilometres. Detailed mapping of a 600m-Iong section on the Pongolo River, established at least eleven phases of intrusion. Each dyke age was systematically sampled and analysed for whole-rock major, trace and rare earth element composition, as well as mineral chemistry. Selected samples were analysed for stable isotopes. In addition to notable intra-dyke chemical variations, there is also a high degree of inter-dyke mineralogical and geochemical variation, each dyke age bearing distinct geochemical characteristics. The apparent geochemical trend is not one of simple fractionation with time. Dyke chemistries are closely linked to magma genesis and magma volumes with time. Evolution of the magmas may be described in terms of varying degrees of partial melting and fractional crystallization, with a small degree of crustal contamination. Major, trace and rare earth element data indicate a lithospheric mantle source for the majority of dyke phases, and an asthenospheric source for only two of the eleven ages. Contrary to this, isotopic data (oxygen and radiogenic) indicate an enriched asthenospheric source for all the dolerites. This suggests that all ages may have originally been derived from the asthenosphere, with the majority of ages being intruded into the lithospheric mantle to later undergo partial melting and fractional crystallization, with some contamination. Previous studies assumed an asthenospheric source with depleted MORB-like rare earth element profiles to be representative for the majority of Rooi Rand dolerites. The Rooi Rand dolerites appear to display a geochemical link with the southern Sabie River Basalt Formation, as well as the Lebombo rhyolites. Magmatic evolution of the dykes was intimately linked to the initial rifting processes of lithospheric stretching and asthenospheric upwelling, which in this case concluded in a classic failed rift situation. / Thesis (M.Sc.)-University of Natal, 1996.