Paper Number: 3671
Multi-dating approaches
applied to the Proterozoic Mbuji-Mayi Supergroup, (D.R. Congo) to
constrain the diversification of early eukaryotes in Central Africa
François, C.1, Baludikay, B.K. 1, Storme, J-Y.1, Baudet, D.2, Paquette, J-L. 3, Fialin, M.4, Debaille, V. 5, Birck, J-L. 6 & Javaux, E.J. 1
1PPP, UR GEOLOGY, University of Liège, Allée du six Août, 14, B-4000 Liège, Belgium, c.francois@ulg.ac.be
2Geodynamics and Mineral Resources, Royal Museum for Central Africa, Tervuren, Belgium.
3Laboratoire ‘‘Magmas et Volcan’’ (UMR 6524 CNRS), Université B. Pascal, F-63038 Clermont-Ferrand, France
4Camparis, Université Paris 6, 4 place Jussieu, 75252 Paris Cedex 5, France
5Laboratoire G-Time, ULB, 50 Avenue F.D. Roosevelt, 1050 Brussels, Belgium
6Laboratoire de Géochimie http://www.ipgp.fr/fr/node/322 des enveloppes externes (UMR 7154 CNRS), IPGP, 1 rue Jussieu, F-75005 Paris
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The Mbuji-Mayi Supergroup in the Democratic Republic of Congo (DRC) is located between the Archean-Paleoproterozoic Kasai Craton and the Mesoproterozoic Kibaran Belt. This sedimentary sequence, unaffected by regional metamorphism, preserves a large diversity of well-preserved microfossils, evidencing the evolution of complex life (early eukaryotes) for the first time in Meso-Neoproterozoic record of Central Africa [1].
The lithostratigraphy of the Supergroup consists of two Groups: (i) BI Group: a lower siliciclastic sequence poorly constrained (ca. 1174 Ma to ca. 1055 Ma [2, 3, 4, 5, 6]) unconformably overlying the ca. 2.82-2.56 Ga granitoid Dibaya Complex [7, 8, recent notice on DRC geological map] and (ii) BII Group: an unconstrained upper carbonate sequence intercalated with sparse shales. Basaltic lavas overlying the Mbuji-Mayi Supergroup were dated around 950 Ma [5, 8].
In order to better constrain the age of this Supergroup in the Meso-Neoproterozoic limit, we used a combination of different geochronological methods. For the BI Group, results of in situ U-Pb dating with LA-ICP-MS (Laboratoire Magmas et Volcans, Clermont-Ferrand, France) provide 207Pb/206Pb ages between 2911 and 1284 Ma for zircons and between 2783 and 1029 Ma for monazites and xenotimes.
New results of in situ U-Th-Pb chemical dating of well-crystallized monazites and xenotimes with Electron MicroProbe (Camparis, UPMC, Paris), highlight that some crystals display zonations with an inherited core older than 1125 Ma and diagenetic rims around 1040-1065 Ma. This suggests that the diagenesis of BI Group is younger than 1175 Ma [6] and probably around 1030-1065 Ma, coherent with ages on two syngenetic galenas with a model age around 1055 Ma for the top of BI Group [2, 3, 4, 5]. Re-Os datings (Laboratoire Géochimie http://www.ipgp.fr/fr/node/322 des enveloppes externes, IPGP, Paris, France) are underway to constrain the age of fossiliferous shales, in particular from the unconstrained BII Group.
We also re-evaluate the age of basaltic lavas overlying the Mbuji-Mayi Supergroup (previously dated around 948 ± 20 Ma [5, 8] with Sm-Nd and Rb-Sr techniques (Laboratoire G-Time, ULB, Bruxelles, Belgium) to constrain the end of deposition of this Supergroup.
References:
[1] Baludikay B.K et al., (2016) Prec Res in review
[2] Cahen L et al. (1954) Bull de la Soc Geol Belg 77: 268–281
[3] Holmes A and Cahen L (1955) Colon. Geol. Miner. Ressources 5: 3–38.
[4] Raucq P (1957) No Sciences Geol 18
[5] Cahen L (1974) Centenaire Soc Geol Belg: 57–77
[6] Delpomdor F et al. (2013) Pal3 389: 4–34
[7] Delhal J et al. (1976) Ann Soc Geol Belg 99: 211–226
[8] Cahen L et al. (1984) Geological and Evolution of Africa: Clarendon Press, Oxford.