Abstract

Birimian (Paleoproterozoic) granitoids and associated volcanic and sedimentary rocks from the Kedougou-Kenieba-Inlier (KKI) of Eastern Senegal have been analyzed for their major, trace, REE concentrations and their Sr-Nd isotopic compositions. The calc-alkaline I-type granitoids were previously divided into several structural groups. The new geochemical data, however, do not give evidence for this subdivision into pre-, syn- and post-deformational intrusions. The results demonstrate the juvenile character of the granitic igneous rock suite and its isotopic similarities to the adjacent volcanic Mako and sedimentary Dialé-Daléma Supergroups. The Birimian rocks show uniform juvenile features such as low initial Sr isotopic composition (~0.700 to ~0.704), positive ∑Nd(t) values (2.1 to 4.3) and restricted Nd model ages (~2.0 to ~2.3 Ga). They preclude the contamination of the juvenile Birimian crust of the KKI by notable amounts of reworked Archean crustal material. Comparison with existing data helps to clarify the complex evolution of Birimian Supergroups of the WAC. It can be concluded that the Birimian rocks are the product of a large-scale crustal growth process with large amounts of juvenile crust formed from the depleted mantle in a geodynamic environment compared to modern volcanic arcs. These bimodal volcanic arcs were later accreted to the WAC.

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