U-Pb SHRIMP dating of Detrital Zircons from the Nzilo Group (Kibaran Belt): Implications for the Source of Sediments and Mesoproterozoic Evolution of Central Africa

Abstract

The Kibaran Supergroup, a >3000-km-long belt of metasedimentary and igneous rocks in the southeastern Congo, is in a critical location between the Congo Craton (sensu stricto) and the Tanzania-Bangweulu Block. Understanding its tectonic evolution will shed much-needed light on the amalgamation history of sub-Saharan Africa. This study presents U-Pb SHRIMP age data for 150 detrital zircons from four metasedimentary formations of the Nzilo Group, the middle lithostratigraphic unit within the Kibaran Supergroup in Katanga Province. These samples yielded dates between 3214 ± 7 and 1329 ± 32 Ma. Prismatic Mesoproterozoic detrital zircons (dated at 1499 ± 49 to 1329 ± 32 Ma, with a peak at 1380 Ma) occur in all samples and are inferred to be derived from the Mitwaba orthogneisses, which intrude the Kiaora Group, the oldest lithostratigraphic unit of the Kibaran Supergroup. More than three-quarters of the zircon population is composed of recycled grains, yielding dates between 2434 ± 5 and 1696 ± 18 Ma, with peaks at 2050 and 1850 Ma. These peaks overlap with the timing of geological events in the adjacent Paleoproterozoic Bangweulu Block, including the Ubendian-Usagaran belts. Archean zircons were not found in the oldest Nzilo units and are restricted to the youngest Nzilo rocks, where they form only a minor component (<6%) of the zircons recovered. The Archean zircons are inferred to be derived from the unroofing of successively older crust in the East African lithosphere. U-Pb data support field observations and indicate that the Nzilo Group sediments postdate the ∼1.38-Ga Kibaran syn-D1 igneous rocks and received some detritus from them. The lithostratigraphic and geochronological data, coupled with the regional geology, indicate that a substantial portion of the Nzilo Group detritus came from interbasinal reworking of the underlying Kiaora Group and its associated 1.38-Ga orthogneisses, with a significant contribution from the Tanzania-Bangweulu Block. The data support a previously proposed subductional model for the Kibaran belt and constrain the paleotectonic environment during the deposition of the Nzilo Group.