Detrital zircon U–Pb ages from the Paleoproterozoic Lulua and Luiza volcanosedimentary Groups in the Kasai Shield, Congo Craton: Implications for the source of sediments and the Kasai-Ntem and São Francisco Craton relationship

Abstract

The Kasai Shield contains the southern Mesoarchean nucleus of the Congo Craton, which coalesced into a major landmass by the end of the Archean. This study provides 193 U–Pb ages of Archean detrital zircons within two related intracontinental (volcano)-sedimentary groups within the Archean Kasai Shield, the Paleoproterozoic Lulua and Luiza Groups. The oldest zircons found in this study were dated at 3198 ± 18 Ma and 3167 ± 42 Ma, and three others were dated around 3100 Ma. These ages are consistent with a Paleo- to Mesoarchean origin for this continental nucleus, and the earlier Hf model ages and the oldest known rocks from the Upper Luanyi Granite Gneisses and the Kanda Kanda Tonalites constitute the probable source. Most zircons in the western Lulua quartzites have ages between 3000 and 2700 Ma, with subpeaks at 2935 Ma, 2850 Ma, and 2740 Ma. The samples from the more intensely metamorphosed Luiza Group have an age population subpeak around 2815 Ma, and a peak around 2735 Ma and the zircons are more euhedral, indicating a shorter transport distance or transport in a lower energy sedimentary environment. The oldest zircons are dated at 3198 ± 18 Ma, 3167 ± 42 Ma, and around 3100 Ma (n = 3). The obtained ages are consistent with the known Kasai ages, and a prominent local source component in these intracontinental basins seems likely. Zircon ages that correspond to the intense Kasai magmatism around 2560 Ma are scarce, indicating that Neoarchean Kasai plutons were not part of the sources, or had limited exposure during deposition of these sediments. Our results may indicate a relationship with the two-stage Neoarchean accretion of the Ntem Complex to the northern border of the Congo Craton between 2900 and 2800 Ma. Lastly, the similarity between these ages and those in the southern São Francisco Craton (Rio das Velhas I, II, and Mamoma events) suggests that this craton may also be a potential source for the sediments.