Browsing by Author "Basson, I"

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    Structural overview of selected Group II kimberlite dyke arrays in South Africa: implications for kimberlite emplacement mechanisms
    (Geological Society of South Africa, 2003) Basson, I; Viola, G
    Group II kimberlite dykes occur in small, dominantly en-echelon dyke- fracture arrays, with individual dyke-fractures showing small angular variations from their array trends (5° to 15°). The analysed dyke systems are characterized by closely matching opposing dyke contacts, "in-situ" breccia, multiple kimberlite stringers within a dilated dyke-parallel fracture cleavage, wedge-shaped apophyses in bent bridges at dyke- fracture offsets/overlaps, kimberlite-free offset/overlap areas and calcite vein fibres orthogonal to dyke contacts. Commonly found microscopic structures include syn-emplacement/syn-crystallization calcite veinlets, containing high aspect ratio stretched fibrous calcite, and elongate phlogopite phenocrysts and serpentinized olivine phenocrysts growing across the width of these veins. Both macro- and microscopic structures support a model of orthogonal host rock dilation during kimberlite emplacement. Terminations of dyke-fracture segments show minimal curvature or overlap, suggesting that remote horizontal stresses dominated during their emplacement ("passive" intrusion), as opposed to magma overpressured systems wherein dyke or dyke-fracture overlaps curve strongly towards each other ("active" intrusion). The application of Mohr diagrams suggests that low differential stresses, with no or only a very minor shear component, prevailed at the time of emplacement. The dominance of remote horizontal forces, imparting small differential stresses to the brittle portions of the crust, a closely-spaced, dilating dyke-parallel fracture cleavage ahead of the dyke tip (imparting a local suction) and the low-volume, low-viscosity, highly volatile nature of kimberlitic magmas may explain their empirically-constrained high emplacement velocities. This, in turn, explains the means by which such magmas may entrain significant volumes of high specific-gravity mantle material. Mobile hydrofracturing in the fringe zones around dilated... [ABSTRACT FROM AUTHOR]
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    Tectonic implications from the geochemistry of Mfongosi Group metasediments, Natal Metamorphic Province, South Africa
    (2003) Basson, I; Watkeys, M K
    The formation of the Mesoproterozoic Natal Metamorphic Province occurred during the closing stages of the assembly of the supercontinent Rodinia (∼1150 to ∼950 Ma), forming part of the ∼1190 to ∼950 Ma Grenvillian orogeny. This was preceded by the accretion, at 1210 Ma, of Mesoproterozoic island arcs along the southern margin of the Archaean Kaapvaal Craton. The Mfongosi Group represents an enigmatic sequence of rocks found between the Kaapvaal Craton and the partially ophiolitic Natal Nappe Zone to the north of these accreted island arcs. Mfongosi metagreywackes adjacent to the Kaapvaal Craton have ocean island arc major element geochemical signatures. These contrast with metagreywackes approximately two kilometres further south, near the contact zone between the Natal Thrust Front and Natal Nappe Zone, which have active continental margin major element geochemical signatures. A third type of metasediment is represented in both areas by geochemically distinct low-Ca+Na, high-K meta-arkoses to meta-lithic arkoses, which were formed by relatively minor sedimentation from a passive continental margin. The Mfongosi Group is a fore-arc complex, incorporating elements of a fore- arc basin and an accretionary prism deposited in a trench, which filled during final oblique collision between the Kaapvaal Craton and an oceanic island arc to the south. The inclusion of an active continental margin signature in the metasediments is enigmatic, although the geochemistry of the metagreywackes suggests that the margin of the Grunehogna Province is a probable source. Subsequent closure of this basin resulted in inversion and accretion of the metasedimentary sequence onto the southern margin of the Kaapvaal Craton. [ABSTRACT FROM AUTHOR]