THE HIGH-TI CAMP FREETOWN LAYERED COMPLEX (SIERRA LEONE) - LITHOSPHERIC IMPRINTING REVEALED BY ISOTOPE SYSTEMATICS

The mafic Freetown Layered Complex (FLC) cropping out along the Atlantic coast of Sierra Leone was the object of a combined geochemical and geochronological study. 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 contents in pyroxene crystals) demonstrate the affinity of the FLC with the high-Ti magma type of the Central Atlantic Magmatic Province (CAMP). Sr-Nd-Hf-Pb-Os isotope systematics show that the FLC has an unusual composition relative to most previously studied CAMP occurrences. In particular, its low ²⁰⁶Pb/²⁰⁴Pb, coupled with high Δ7/4 and Δ8/4, are distinctive features akin to a mild DUPAL signature. A granulite xenolith found in one of the sampled FLC rocks further indicates lo-wer crustal assimilation, but the effects are inferred to be minor and insufficient to explain the distinctive Pb isotopic signatures of the FLC. The most straightforward petrogenetic model for the FLC calls on hybridization of a predominantly 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 consistent with the geodynamic context of the FLC, which was emplaced within a Proterozoic mobile belt (Rokelide) that borders an Archean craton (Man). Less extreme, but similar isotopic compositions are observed only in other high-Ti CAMP rocks found in regions that were once contiguous with Sierra Leone, and in high-Ti basalts from two other igneous provinces of the Southern Atlantic realm (Gondwana-related) – the Karoo and the Paraná-Etendeka. A genetic model is proposed for the FLC where geochemical overprinting on volumetrically dominant asthenospheric melts was imparted by enriched domains residing within the lithosphere, in turn suggesting a lithospheric origin for the DUPAL signature.