Structure, Sr-Nd isotope geochemistry and zircon U-Pb geochronology of the granitoids of the Dabakala area (Côte d'Ivoire): Evidence for a 2.3 Ga crustal growth event in the Palaeoproterozoic of West Africa?

Structural, mineralogical and chemical studies show that the rocks from the Dabakala area, previously reported as gneisses and migmatites, are in fact syntectonic granitoid bodies. Three chemical associations are distinguished: (i) trondhjemitic (Dabakala tonalites and granodiorites), calc-alkaline (Finéssigué granodiorites and undifferentiated biotite granodiorites and granites) and peraluminous (Sarala hololeucocratic granites and Kowara muscovite trondhjemites). Most granitoids display deformation patterns typical of an interference between pluton emplacement and transpressional tectonics induced by a NNW-SSE to NW-SE compression which led to three major NS sinistral shear zones (N'Zi, Katidougou, Ouango-Fitini). Geochronological data (U-Pb ID-TIMS and SIMS zircon ages) yielded 2312 ± 17 Ma for the zircon cores from the Dabakala tonalite, 2154 ± 1 Ma for the zircon rims of the same tonalite, 2162 ± 32 Ma for Sarala, 2118 ± 2 Ma for Kowara, and 2103 ± 1 Ma for Finéssigué. Sr-Nd isotopic data further indicate that all these rocks are isotopically primitive (0.7000 < 87Sr/86Sr < 0.7026, + 1.8 < εNd(2.1Ga) + 3.7, 2039 < TDM < 2148 Ma, with the exception of the Dabakala tonalites: εNd(t) = -2.9, T DM = 2471 Ma). All the data suggest that these granitic rocks do not correspond to a reworked pre-Birimian crust as previously suggested. However, the genesis of the Dabakala tonalite involved recycling components of a 2.3 Ga pre-Birimian crust. A compilation of crustal residence ages for Birimian felsic sediments, volcanic and plutonic rocks at the scale of the whole Birimian crust in the Leo Rise could suggest that most of the Birimian felsic rocks arose, at least partly, from recycling of pre-Birimian differentiated crustal material. Therefore, it appears likely that the major Birimian s. s. (ca. 2170 Ma) and Bandamian (ca. 2105 Ma) growth events were preceded by an early Palaeoproterozoic crustal growth episode at ca. 2.3 Ga. © 2003 Elsevier B.V. All rights reserved.