Abstract

The petrogenesis of all the Mesozoic igneous rocks of south-eastern Africa is considered in relation to a single unifying thermotectonic event postulated as initiating the disruption of Gondwanaland. The cycle began with the rise of a body of potassium-rich picrific magma from a depth of at least 500 km. This is the predominating source material for most of the rocks in the northern part of the province (e.g. Rhodesia). Overlying and peripheral to the main magma body a zone of normal (sodic) magma was generated and gave rise to some of the northern rocks and most of the southern rocks (e.g. Karroo dolerites, basalts of Lesotho and Swaziland). Initial eruptive products were locally well fractionated but gave way rapidly to highly primitive picritic lavas representing what is termed the culminatory stage of the cycle. A subsequent stage in which the basalts emitted remained relatively uniform and moderately fractionated over a long period of time is referred to as the 'steady state' phase. A waning stage may be represented by more salic rocks such as the phonolites of Lupata (Mozambique) and the associated plutons of the Chilwa province (Malawi). The Cretaceous kimberlites of Southern Africa may represent the terminal igneous activity of the cycle. Superimposed on the cycle of mantle-derived rocks is a minor cycle of anatexis in the crust which gave rise to most of the acid rocks of the province, particularly the rhyolites of the Lebombo monocline. It is suggested that other provinces such as the Deccan Traps and the Tertiary of Greenland may show patterns of igneous activity which are similar to the Karroo cycle in important respects.

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