A review of Karoo vulcanicity in Southern Africa |
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| Authors: | J W Bristow E P Saggerson |
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| Institution: | (1) Department of Geology, University of New Mexico, 87131 Albuquerque, New Mexico, U.S.A.;(2) Present address: P.O. Box 47, 8300 Kimberley, South Africa;(3) Department of Geology, University of Natal, 4001 Ducban, South Africa |
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| Abstract: | Vocanic rocks of Karoo age which today cover more than 140,000 km2 of the southern African sub-continent occur as scattered outliers representing eroded remnants of an originally more extensive
volcanic province. The rocks are best preserved in central southern Africa including Lesotho, and the continental margin areas
of Namibia in the west and Mozambique. Zambabwe, Swaziland and South Africa in the east. Extensive lava fields (yet few volcanoes)
dykes, sills, layered intrusions and at least two major dyke swarms characterise the region. Volcano-stratigraphic and geochemical
mapping have been used to subdivide the volcanic successions found in the different areas and recently adopted nomenclature
is presented. Considerable more variability and complexity occurs in the volcanic succession than was previously recognised:
geochemical variations and stratigraphic relationships indicating that four major provinces can be recognised. Rocks from
the central Karoo areas are primarily of basaltic composition whereas those from the western and eastern marginal areas include
mafic basic, intermediate and acid types. Emplacement of rocks such as carbonatites, nephelinites, and picrite basalts enriched
in incompatible elements, indicate that derivation from a heterogeneously enriched source played a significant role in the
petrogenesis of a large proportion of the Karoo mafic and basic rocks. Age relationships of the volcanic rocks reveal that
vulcanicity extended over a period of 130 m.y. from mid-Triassic to early Cretaceous time. |
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