Fabrics, internal zonation and magma flow in small gabbroic sills, Karoo, South Africa
Abstract
Layer-forming processes result in the development of anisotropic fabrics in gabbroic sills. Whereas this phenomenon is fairly well documented in large mafic-ultramafic sills (e.g. the Duluth Complex or the Bushveld Complex) fewer studies have been dedicated to smaller intrusions. Magmatic differentiation typically produces large-scale, macroscopic stratification of rock units but these variations are less visible in sills thinner than 100 m. Investigation of fabrics in these smaller tabular bodies could provide important clues on internal magmatic processes. The Karoo Large Igneous Province (LIP) offers unparalleled opportunities to investigate fabrics, internal zonation and magma flow in mafic sills of thicknesses ranging from 5 to 50 m, and Karoo sills can reach up to 1000 m in thickness (e.g., Insizwa). Regional-scale geochemical studies suggest that the intrustion’s tholeiitic magma composition across the Karoo LIP was remarkably homogeneous, especially as far as major elements are concerned. Sills are particularly extensive in the Ecca and Beaufort Groups. Magnetic fabric is generally considered a good proxy for magmatic fabric. The Anisotropy of Magnetic Susceptibility (AMS) has been measured on oriented hand specimens collected across vertical profiles and on non-oriented specimens collected at regular spacing along borehole cores through several representative sills. Scalar parameters such as magnetic susceptibility, degree of anisotropy and shape factor show variations as a function of stratigraphic height in the intrusion. The variations in these parameters can be interpreted in terms of magmatic processes such as convection, crystal settling or compaction. Vectorial parameters provide additional constrains regarding the dynamic vs static nature of fabric forming processes. Preliminary results suggest that AMS fabrics in some sills are anomalous in most specimens with the exception of the topmost part of the intrusion. The origin of anomalous magnetic fabrics is currently investigated with both Anisotropy of Anhysteretic Remanent Magnetization (AARM) measurements and 3-D image analysis. In addition, the average gabbro grain size tends to increase towards the center of these sills. The AMS fabric imbrication, often called upon to account for oblique fabrics with respect to the walls of a tabular intrusion, have not been observed in the studied intrusions. In contrast, other sills display very consistent normal AMS fabrics parallel to flow fabrics. Surprisingly the normal and anomalous sills are of similar composition and emplaced in the same area. The difference between normal and anomalous types can be explained in terms of isolated vs interconnected sills.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V21A1978M
- Keywords:
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- 1518 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic fabrics and anisotropy;
- 1519 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic mineralogy and petrology;
- 8414 VOLCANOLOGY / Eruption mechanisms and flow emplacement;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies