Abstract

Correlations between the Proterozoic Buganda-Toro Sequence, the Ruwenzori fold belt of Eburnian (1,850 ± 250 m.y.) age, and other stratigraphic and structural units in East Africa are critically examined. The data herein are based upon a recent investigation of the Ruwenzori Mountains in western Uganda which lie astride the tectonic front between the Ruwenzori fold belt and gneisses older than 2,600 m.y. belonging to the Basement Complex of Uganda. Foliation in the Archean gneisses trends north-northeast in the northern half of Ruwenzori and is reworked into an easterly direction within the younger orogenic belt to the south. Metasedimentary rocks and low-K metatholeiites of the Toro Supergroup, which is part of the 2,000 to 2,600-m.y.-old Buganda-Toro Sequence, rest unconformably upon the gneisses and are preserved in elongate synclines within the Ruwenzori fold belt. Regional metamorphism within the belt is of low-pressure intermediate type and shows a rapid increase in grade away from the northern margin.

An extensive area of Archean gneisses, here named the “Central African Shield,” is recognized. Fragments of this shield, which appears to have existed as continental crust during the development of the Nyanzian and Kibalian greenstone belts during the later Archean, are preserved within the currently recognized pre- 1,500-m.y.-old “cratons.” Both the greenstone belts and shield acted as a cratonic area during development of younger, ensialic mobile belts such as the Ruwenzorian (±2,000 m.y.) and the Kibaran (±1,300 m.y.). The Buganda-Toro Sequence is thought to belong to the Ruwenzorian cycle rather than correlated with the older Nyanzian Sequence. Metasediments within the greenstone belts and mobile belts contain significantly different mineral deposits and show an apparent progression with age from low- to medium-pressure metamorphic facies.

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