The Pan-African West Congo belt in the Republic of Congo (Congo Brazzaville): Stratigraphy of the Mayombe and West Congo Supergroups studied by detrital zircon geochronology

The African part of the Neoproterozoic Araçuai-West Congo orogen forms a more than 1400. km long belt, running from Gabon in the north to Angola in the south, parallel to the South Atlantic coast. It consists of the West Congo fold belt in the west and a largely undeformed sedimentary foreland basin in the east. In the Republic of Congo (Congo Brazzaville) these are known as, respectively, the 'Mayombe fold belt' and the 'Niari basin'. The Mayombe fold belt is largely made up of strongly deformed metasedimentary rocks of the Mayombe Supergroup and its reworked granitoid basement complex. The Niari basin comprises the sedimentary strata of the West Congo Supergroup, which contain a unit of diamictite and associated cap-carbonate, related to the Marinoan (ca. 635. Ma) glaciation. In this paper detrital zircon U-Pb data are employed to investigate the relationships between the Mayombe and West Congo Supergroups in Congo Brazzaville, as well as the relationships with similar stratigraphic units in the Democratic Republic of Congo (DR Congo). Unfortunately, major differences in stratigraphical nomenclature exist between Congo Brazzaville and DR Congo which, together with geological differences, complicate comparisons. The terminology used in this paper follows the Congo Brazzaville tradition.U-Pb data have been obtained by LA-ICP-MS on 1674 zircon grains from samples of the Mayombe and West Congo Supergroups (seven and eight samples, respectively). Two of the samples have Palaeoproterozoic ages. The other samples are of Neoproterozoic age. Zircons from the latter fall into four main age groups: Group 1, 500-800. Ma, forming 8% of the total population; Group 2, 900-1200. Ma (29%); Group 3, 1800-2300. Ma (19%); and Group 4, 2500-3100. Ma (31%). Only ca. 13% of the zircons fall outside of these groups. Most zircons of Group 1 (600-800. Ma) are interpreted to have been derived from late Neoproterozoic magmatic arc rocks and syn- to post-collisional granites of the Araçuai orogen in Brazil, since rocks of these ages are rare in the Congo craton to the east. Most of Group 2 zircons have probably been derived from early Neoproterozoic (1000-900. Ma) rhyolitic and granitic rocks exposed in DR Congo. The Palaeoproterozoic and Archaean zircons of Groups 3 and 4 may have been derived both from eastern and western source regions.None of the stratigraphical units beneath the Marinoan diamictite contain the 500-800. Ma zircons of Group 1. This is interpreted to indicate deposition of these units during the earlier Neoproterozoic rift and drift phases in the history of the Araçuai-West Congo orogen, while the strata that do contain Group 1 zircons were deposited during the collisional phases and post-collisional uplift. A significant hiatus is likely to be present beneath the Marinoan diamictite. Based on these results, a revision of the stratigraphy of the Mayombe and West Congo Supergroups is recommended.Three granitoid rocks and two metasedimentary samples from the reworked basement complex have also been investigated, yielding Palaeoproterozoic ages, 2050-2100. Ma for the granites, and Palaeoproterozoic and Archaean detrital zircon ages for the metasedimentary rocks.Sm-Nd isotope data have been obtained on a number of (meta-)argilitic and siltstone samples. All samples yielded Palaeoproterozoic model ages, indicating mixing of sediments of different ages, as also observed in the zircon age distributions.