Browsing by Author "Logan, C. T."

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Aspects of Karroo vulcanicity in the Komatipoort area Lebombo.
(1979) Logan, C. T.
The petrology, mineralogy and geochemistry of the Karroo-age basaltic and rhyodacitic volcanics present in the Lebombo Belt near Komatipoort, Eastern Transvaal, Republic of South Africa are described. The volcanics comprise a succession of extrusives within the Komatipoort area, consisting of: (3) Rhyodacitic lavas and tuffs (2) Basalts (1) Olivine Basalts. These volcanics dip eastwards at angles between 10° and 40°, with dips increasing as the Eastern boundary of the area, the Mozambique border, is approached. Further Karroo-age volcanics, constituting the upper part of this succession, lie across the border in neighbouring Mozambique. Various intrusives, similar in composition to the extrusives, are also present in the area, commonly as north-south trending dykes forming part of a large dyke-swarm. These, as else\Olhere in the Lebombo, are considered to be feeders to the extrusives. Three relatively major intrusions occur in the area, the largest being the Komatipoort Intrusion, here interpreted as a 700m thick, sub-concordant, composite sheet-like body, ~onsisting of five major lithological units. From the base upwards these are:- . unit 1 - olivine gabbro, unit 2 - igneously laminated gabbro, unit 3- granophyric gabbro, unit 4 - granophyre, unit 5 - feldspathic gabbro. The olivine gabbro, the granophyre and the feldspathic gabbro appear to form separate intrusive bodies, but the igneously laminated gabbro and the granophyric gabbro, could represent the products of in situ differentiation. If so, the granophyric gabbro has suffered subsequent disturbance as shown by evidence including a homogeneous composition, reaction and corrosion textures and the presence of deformed pyroxene grains, which are described in detail. Another major intrusive of probable Karroo age is the Crocodile River Intrusion, which occurs to the north-east of the main mapped, and represents the southern- most end of a northsouth trending line of mafic intrusives, which parallel the Lebombo for approximately 200 km. The intrusion here has a dykelike form, and shows evidence of fractionation by crystal settling. A smaller, obviously composite intrusion occurs near the base of the basaltic sequence in the Komatipoort area, (the Basal Intrusion). Representative samples of a variety of extrusive and intrusive rock types have been analysed. These analyses include major and trace element determinations of a series of samples of the major units of the Komatipoort Intrusion. In 'addition, analyses of a number of minerals from several different rock types occurring in the Komatipoort Intrusion, are presented. The analytical data available for the Komatipoort volcanics confirms the previously known southward variation in the geochemistry of the basalts and the presence of bath a high and a low-Ng basalt series in the Komatipoort area. The majority of the basic rocks in the Komatipoort area belong to the low-Mg series. Compositional variation in this series may be explained largely by low-pressure fractionation of olivine and pyroxene only, despite the presence of abundant plagioclase phenocrysts. Some of the variation in the high-Mg series , basalts can be explained by the fractionation of olivine, and what appear to be relatively highly fractionated rocks formed in this manner occur in the area. The Lebombo volcanics ln general display a bimodal silica distribution and rocks with an intermediate silica content are rare. In the Komatipoort area intermediate rocks do occur in the, form of two classes of granophyre, (high and low silica varieties), present in the granophyre unit of the Komatipoort Intrusion. Liquid immiscibility is a possible mechanism for the formation of the two types of granophyre, and a widespread development of this process in intermediate magmatic liquids could provide an explanation for the ,scarcity of rocks of this composition in the Lebombo belt. Relatively few analyses of the rhyolitic volcanics are presented in this study, but it appears possible that those available could be representative of two processes, firstly, partial melting of the lower crust or upper mantle, and secondly, fractionation by crystallisation of the commonly observed phenocryst phases,(feldspar, pyroxene, quartz, magnetite), or addition of these phases to the magma.