The Karoo Igneous Province contains one of the world's most impressive array of dolerite dykes and sills. Previous investigations on these dykes have largely been petrological and geochemical, with interest being where the magma came from in the mantle rather than its movement in the crust. In general it has been assumed that, in any given area, the sills have been fed by the adjacent dykes. Precisely from which direction the magma has flowed has always been uncertain beyond the conclusion that the magma moved vertically upwards in the dykes and horizontally outwards in the sills.
In this study a technique has been developed whereby the flow direction may be established. It uses a two-fold approach, incorporating the morphology of the intrusion contacts and the textural features within the dolerite. The morphology of the contacts, combined with an understanding of the fracture mechanics, reveals the shape and orientation of the intrusion as it injected, limiting the possible flow directions. Restoration of this event also aids in unravelling the magma flow through the fissure. The most useful textural feature is the alignment of the phenocrysts. Digital analysis of orientated microscopic thin-sections enables a flow direction to be calculated statistically.
This technique has been applied to Karoo dolerites of northern Natal and shows that the calculated flow direction may be at right-angles to the traditional intuitively estimated flow direction.
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