Features of the evolution of the Karagwe-English orogenic belt (Burundi, Central Africa)

Abstract

Introduction. Kibaran orogenic belt and its fragmentations were studied, and specific features of the internal structure of the KAB were established. Geological exploration of the Republic of Burundi was briefly analysed. The goal of the work was to determine the issues the solution of which will allow effectively exploring the mineral resources of the country.

Main features of the geological structure. Among the three identified structural levels, we thoroughly studied stratified and unstratified formations of the Kibaran stage of orogenesis. Stratified formations are represented by the Akanyaru supergroup which includes four groups of different ages, that were developed in the western part of the country, and by the Kagera supergroup which includes two groups typical for the east. Formations of the pan-African stage of development are also well-presented in the eastern part. Intrusive magmatism is represented by mafic-ultramafic and granitoid complexes. Their description and age were presented, and issues of their origin were considered. In the section of geochronology of intrusive complexes and the issue of the suture zone, an evolution option of the KAB as a superplume was proposed.

Issues of regional geology in Burundi. Based on the analysis of cartographic material from medium and large-scale surveys of the area of the Republic of Burundi, as well as a large array of published data, we identified the main issues of a fundamental and applied nature. The ambiguity in determining a single belt of the Kibaran orogeny was shown. Its northern part should be considered as a superplume that has been developing for a long time (between 1,800 million years up to the present) in the intracratonic mode under the influence of a plume. This was confirmed by the entire set of geological, lithological, and petrological data on stratified and intrusive complexes, as well as the results of their geochronological studies. An important impulse of bimodal magmatism at 1,375 million years cannot be considered as the one completing the Kibaran orogeny, as there are also later datings of magmatic activity at ~1,200, ~900, and ~600 million years. Granitoids classified as S-type differed significantly from similar classical granites typical for collision structures and had features of convergence with the A-type. Taking into account the polychronicity of granitoid within-plate magmatism, it is possible to obtain the figures that differ from the age of the main Kibaran event when detailing the age study of mafic-ultrabasic bodies.

Conclusion. It is recommended to solve all the above-mentioned issues in the nearest future in the course of creating a large-scale new-generation digital map of the Republic of Burundi.