The geochronology and geochemistry of Karoo volcanics in the Lebombo and adjacent areas

Doctoral Thesis


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University of Cape Town

Karoo volcanics of Jurassic to Cretaceous age crop out in the eastern areas of southern Africa. These volcanics are well preserved in the Lebombo monocline which straddles the borders of South Africa, Swaziland and Mozambique, and in the Nuanetsi and Tuli synclines and Sabi monocline of south-east Zimbabwe. Geochronological and geochemical data have been used to evaluate the geochemistry of the rock types found in the above areas, particularly with respect to the mafic volcanics of the Lebombo. In the eastwardly dipping Lebombo monocline, nephelinites overlain by picrite basalts occur in the northern part of the volcanic succession whereas over the remaining area olivine-poor basalts are dominant. Rhyolitic volcanics in the form of massive flows overlie the basaltic rocks and attain their maximum thickness in Swaziland and the southern Lebombo. The rhyolites are notably thinner to the north of Swaziland and are overlain by an upper sequence of olivine-poor lavas with interdigitated rhyolite flows. Basic dykes are ubiquitous in the Lebombo whereas acid dykes, though present, tend to be localised. In Nuanetsi, picrite basalts, olivine-poor basalts and rhyolite flows with interbedded basic lavas crop out ina major synclinal structure. Dyke rocks are common and late Karoo intrusive complexes cut the lavas. Volcanic rocks in the Tuli syncline consist predominantly of picrite basalts and olivine poor lavas, whereas nephelinites, picrite basalts and olivine-poor lavas crop out in the Sabi Monocline.

Includes bibliographical references.