Enriched lithospheric contribution to the genesis of the Freetown Layered Complex (Sierra Leone): isotopic geochemistry study of a high-Ti CAMP layered intrusion.

We present a geochemical and geochronological study of a mafic layered intrusion set along the Atlantic coast of Sierra Leone - the Freetown Layered Complex (FLC). Isotopic ages (40Ar/39Ar on plagioclase: 201.7±0.7 and 202.3±2.3 Ma; U-Pb on baddeleyite: 198.794 ± 0.048/0.071/0.22 Ma) and crystal chemistry (high-TiO2 pyroxenes crystals) demonstrate connection of the FLC with the high-Ti magmatism of the Central Atlantic Magmatic Province (CAMP). Yet, measurements of Sr-Nd-Pb-Os and Hf isotopes reveal a peculiar signature cpmared to most previously studied CAMP occurrences, showing that the FLC originated from an independent magma type. Particularly distinctive of the FLC rocks are the low 206Pb/204Pb and high 207Pb/204Pb, calling for the geochemical contribution of an ancient component in the genesis of these melts. Some lower crustal assimilation is isotopically confirmed and suggested by the finding of a granulite xenolith in one of the sampled rocks, yet it cannot be the sole responsible for the Pb isotopic signatures observed. We claim that the easiest way to confer these signatures to the FLC is by hybridization of a predominant (depleted) upper asthenospheric melt with small volumes (1-3%) of highly enriched alkaline melts derived from the sub-continental lithospheric mantle, possibly lamproites. This scenario is also supported by the geodynamic setting of the FLC, emplaced within a Proterozoic mobile belt (Rokelide) and bordering an Archean craton (Man), and by the reported presence of lamproites and kimberlites in the area. Only high-Ti CAMP rocks found in regions once contiguous with Sierra Leone show similar isotopic compositions, though less extreme.