Discordant RbSr and PbPb whole rock isochron ages for the Archaean basement of Sierra Leone

Abstract

Abstract Field mapping and structural studies in northern Sierra Leone by an I.G.S. team have established a stratigraphic sequence in this part of the Archaean of the West African Craton. An older “Leonian” granite-greenstone terrain is identified which experienced a tectonic-metamorphic event before the formation of the granite-greenstone terrain which ended with the Liberian tectonic-metamorphic event. Granite gneisses in the Fadugu district with Leonian structures yield statistically acceptable but discordant Pb-Pb and Rb-Sr whole-rock isochron ages of 2959±50 Ma and 2753±61 Ma, respectively (2 σ errors). These ages may be correlated with radiometric ages for the Leonian and Liberian structures elsewhere in Sierra Leone, and it is concluded that the Fadugu Rb-Sr whole-rock isochron has been reset by the Liberian event. The Pb-Pb whole-rock isochron for the Fadugu gneisses and a previously determined (but recomputed and partially checked) Rb-Sr whole-rock isochron age of 2980+80 Ma for granite gneisses from southeastern Sierra Leone provide a definitive age for the Leonian tectonic-metamorphic event at about 2970 Ma. Both the initial 87 Sr/ 86 Sr ratios and present-day first-stage model 238 U/ 204 Pb value for the Leonian granitoids are indistinguishable from mantle values, but do not preclude the possibility that these granitoids were derived from parental material with a short history in the crust or lower crust. The Rb-Sr whole-rock isochron age of 2753+61 Ma for the Fadugu granite gneiss provides a definitive age for the Liberian event in northern Sierra Leone. A succession of rocks older than the Leonian (i.e., older than 2970 Ma) has been identified in the field but not yet dated.