U-Pb ages and provenance of detrital zircon from metasedimentary rocks of the Nya-Ngezie and Bugarama groups (D.R. Congo): A key for the evolution of the Mesoproterozoic Kibaran-Burundian Orogen in Central Africa

Abstract

The Kibaran Belt is one of the major Mesoproterozoic orogens formed between 1.4 and 1.0 Ga during the Rodinia assembly. Despite several decades of geologic investigations, correlations, evolution, and geodynamic models are still under debate. Modern studies distinguish the north-eastern Karagwe-Angkole Belt (KAB) and the southern Kibaran Belt (KIB). A further distinction of the KAB and the poorly studied northwestern Kivu Belt (KVB) has to be suggested. Latest researches in the KAB agree with a separation between a lower unit formed prior to 1375 Ma, and late tectono-magmatic events around 1250 Ma. The KIB model considers a continental collision starting at ∼1.25 Ga with latest tectonic events at ca. 1.00 Ga. A more precise correlation was achieved by U-Pb age determinations on detrital zircon from six samples of the metasedimentary Nya-Ngezie and Bugarama groups. Both of them occur along the Congo, Rwanda and Burundi borders and have frequently been used for interregional correlations despite their unknown age. Detrital zircon grains indicate Archaean to Mesoproterozoic inheritance for the Upper Nya-Ngezie Group (ca. 2800 to ca. 1120 Ma) and a similar age range for the lower Bugarama Group (ca. 2850 to ca. 1207 Ma). Accordingly, the two latest Kibaran-Burundian events of sedimentary deposition and tectonic overprint are younger than ca. 1207 and ca. 1120 Ma. This is inconsistent with prior age estimates for the bulk of formations in the vicinity. Thus, the time frame for the deposition of the numerous formations that have previously been correlated to the Nya-Ngezie and Bugarama groups has to be re-examined. New radiometric data indicate two formations in the western KVB that are younger than in the eastern KAB. Additionally, the detrital zircon record indicates significantly changing source rocks of the Nya-Ngezie deposits through time. Resulting correlations with the belts of the Kibaran-Burundian Orogen, suggest four geological “units”, seven tectonic events, five tectono-sedimentary cycles and two different geodynamic “models”. Additionally, three early, middle and late Kibaran belts are proposed. This allows proposing a reconciling model including a plate tectonic model to the West (KVB and KIB) and a coeval extensive model in the early stage of this orogen (KAB). Finally, the relationships between the N-S elongated structures belonging to the late Kibaran, late pan-African and modern rifting events in the same area are discussed.