Date of Award
Master of Science
Gabbroic rocks in sills commonly display well organized magnetic fabrics that are generally attributed to magma flow. Yet in a number of cases, magnetic fabrics that are considered proxies for flow fabrics, form at a high angle to the intrusive - host rock wall. A symmetrical oblique magnetic fabric observed in opposite margins of a dike has been interpreted as the result of viscous drag against the margin, a model referred to as fabric imbrication. In this study tested if a similar viscous drag model could also apply to sills. The Karoo Large Igneous Province (LIP) of South Africa offers unparalleled opportunities to investigate fabrics, internal zonation and magma flow in mafic sills of thicknesses ranging from 5 to 50 m. Exceptionally; Karoo sills can reach a 1000 m in thickness (e.g., Insizwa). Geochemical studies suggest that the Karoo gabbro sills have a consistent tholeiitic magma composition across the entire Karoo Basin, especially as far as major elements are concerned. It is also practically undeformed and upon initial inspection unweathered, making it a well suited natural laboratory to test fabric models in gabbro sills. Magnetic fabrics are generally considered a good proxy for magmatic fabrics. The anisotropy of magnetic susceptibility has been measured on oriented hand specimens collected across vertical profiles through two representative sills. Scalar parameters such as magnetic susceptibility, degree of anisotropy and shape factor show variations with height in the intrusion. The variations in these parameters can be interpreted in terms of magmatic dynamic and static processes such as, for example, thermal convection, gravitational crystal settling or magmatic mush compaction. Directional parameters provide additional constrains on the dynamic vs. static nature of fabric-forming processes. Petrographic studies indicate that the primary carrier of the magnetic properties of the Karoo gabbros is titanomagnetite. Accessorily, pyrrhotite and hematite are present, but not in sufficient quantities to account for more than 1% of the sampled intrusions magnetic properties. Hydrous phyllosilicates are present in one sample, suggesting some alteration has occurred at the upper contact of the Country Club Road sill. Magnetic fabrics of the studied sills reveal complex susceptibility and remanence fabrics as a function of stratigraphic height. The proposed fabric development model does not sufficiently describe the observations, thus these fabrics were formed by a combination of mechanisms, both early and late in the cooling history of the intrusions. The observed magnetic fabrics display properties consistent with both the fabric imbrication model and the proposed viscous shear model. It is possible that crystal settling, melt segregation and possibly convection processes also were likely integral to the formation of the magnetic fabrics of the studied Karoo sills, though further studies are necessary.
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