The geology and structural relationships of the southern Lebombo volcanic and intrusive rocks,South Africa |
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| Authors: | E P Saggerson J W Bristow |
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| Institution: | (1) Department of Geology, University of Natal, 4001 Durban, South Africa;(2) Department of Geochemistry, University of Cape Town, 7700 Rondebosch, South Africa |
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| Abstract: | A brief account is presented for the Lebombo volcanic succession which crops out in Natal, South Africa. The volcanic belt
is of late Karoo age and is composed of a thick sequence of basaltic lavas (Sabie River Formation) overlain by an equally
voluminous succession of acid-flows (Jozini Formation) erupted over a period of about 70 m.y. Field relationships indicate
that the Lebombo basalt pile consists of simple and compound flow units. The rhyolite succession consists of thick (80–284
m) flows units characterised by features found in both ignimbrites and rhyolitic lavas respectively. It is postulated that
they were extruded as high temperature, low volatile pyroclastic flows. The Bumbeni volcanic complex which crops out near
the southern termination of the Lebombo mountains, disconformably overlies the Jozini Formation and is characterised by a
suite of rocks that includes rhyolite lavas, air-fall and ash-flow tuffs, syenite intrusions and basic-intermediate lavas.
Dolerite dykes are ubiquitous throughout the succession and an extremely dense concentration of basic intrusions located along
the western margin of the belt gives rise to the Rooi Rand dyke swarm. Rare sill-forms are found associated with the mafic
volcanies. Acid intrusives are represented by simple and composite quartz-porphyry intrusions and rhyolite dykes. The structure
of the Lebombo is that of a faulted monocline, tilted to the east, developed prior to the fragmentation of eastern Gondwanaland.
The volcanic belt is located at the tectonic contact between two major Precambrian elements, the 3,000 m.y. Kaapvaal craton
to the west and the southerly extension of the 550 m.y. Mozambique belt to the east. It is bounded to the south by the 1,000
m.y. old Natal-Namaqua mobile belt. |
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