Petrology, geochemistry, and zircon U–Pb‐Lu–Hf isotopes of granitoids from the Ivindo Basement Complex of the Souanké Area, Republic of Congo: Insights into the evolution of Archean continental crust. (22nd July 2021) - Gatsé Ebotehouna, Chesther
Abstract : The Ivindo Basement Complex (IBC) comprises the largest exposure of Meso‐ to Neoarchean rocks in the northwestern segment of the Congo Craton in the Republic of Congo. The age and petrogenetic setting of the IBC are not clear due to a lack of detailed studies. In the Souanké area, which is on the northeastern margin of the IBC, the most exposed rocks are charnockites, tonalite/granodiorite, and trondhjemite. Based on geology, geochemistry, and zircon U–Pb‐Lu–Hf isotopes, this paper constrains the petrogenesis and tectonic significance of these granitoids for the evolution of the IBC. Poor‐quality zircons did not allow for age determination and Lu–Hf isotopic characteristics for the charnockite. However, zircon U–Pb dating of the tonalite/granodiorite and trondhjemite yielded the weighted mean 207 Pb/ 206 Pb ages of 2, 895 ± 9.4 Ma and 2, 889 ± 9.2 to 2, 897 ± 5.0 Ma, respectively. Zircon Hf isotopic ( ε Hf [ t ]) values of +1.22 to +4.55 and Th/La >0.12 for the tonalite/granodiorite and trondhjemite indicate that these rocks were derived from partial melting of an older mafic reservoir ( T DM2 ages range from 3, 028 to 3, 230 Ma), possibly triggered by mantle upwelling or plume activity. All the studied IBC granitoids are peraluminous, calcic‐alkalic, relatively sodium‐rich (Na2 O/K2 O > 1), and contain low Ni, Cr, and Yb contents, with strongly fractionated rare‐earth element (REE) patterns. On standard‐normalization diagrams, the IBC rocks exhibited similar distribution patterns in the trace element geochemistry as average Archean TTGs, except for high U, Nb, and Ta concentrations in the IBC trondhjemite. These high values may indicate amphibole dehydration melting (which sequesters Ta and Nb) at low‐pressure conditions at the source for the trondhjemite generation, while the other IBC rocks experienced high‐pressure melting. The 2, 900–2, 800 Ma tectono‐thermal event registered in the IBC from our study was also previously documented for the Ntem Complex TTGs of Cameroon (northwestern equivalent of the IBC) and elsewhere in other cratons such as in the Bação Complex (São Francisco Craton) of Brazil and, therefore, represents an important period for the Archean crustal evolution in geologic history. Abstract : The trondhjemite and tonalite/granodiorite samples from the Souanké area of the Ivindo Basement Complex (IBC) were emplaced during 2, 897–2, 889 Ma. Our investigation suggests that the setting during magma emplacement was likely to have been syn‐ to post‐collisional. Zircon Hf isotope compositions shows that tonalite/granodiorite and trondhjemite of IBC were derived from an older mafic crust with no significant mantle involvement. We suggest a conceptual model for continental crust evolution and the emplacement of the IBC granitoides in the Souanké area of the IBC.