Major-, trace-element and Sr-Nd-Hf isotope geochemistry of diamondiferous dykes from Tonguma and Koidu, Sierra Leone: Highly micaceous kimberlites formed by assimilation of metasomatised lithospheric mantle rocks

The Man Craton in West Africa hosts two clusters of highly micaceous kimberlites at Koidu and Tonguma with mineralogical geochemical features transitional towards those cratonic lamproites. To develop a better understanding the genesis these peculiar rocks, which have also been documented elsewhere (e.g., South Africa, China, Russia, India), we undertaken isotopic study representative fresh samples from Tonguma. New mica RbSr dating by isotope dilution confirms Jurassic age (∼145 Ma) establishes that dykes are marginally younger (∼138 Ma). Bulk-rock analyses both localities show strong enrichment K2O (up to 4.4 wt%), but CaO CO2 24 wt% 18 wt%, respectively), is entirely consistent nature rocks their variable abundance calcite. Despite potassic compositions, incompatible trace element systematics Ba/Nb) typical archetypal unlike Sr-Nd-Hf composition bulk range between moderately depleted values (age-corrected 87Sr/86Sr ∼ 0.7032; εNd = +3.8; εHf +3.9) resembling common (Prevalent Mantle or PREMA-like) kimberlite component more geochemically enriched compositions (87Sr/86Sr 0.7058; −0.4; +0.3). direct relationship bulk-rock Nd isotopes average Mg# magmatic olivine, combined previously reported correlation xenocrystic olivine lithospheric mantle derivation kimberlites, suggests observed spread radiogenic stems following petrogenetic scenario. Primary melts similar asthenospheric origin unradiogenic Sr Hf interacted variably metasomatised (i.e., containing phlogopite) hence wall rocks. Mass balance calculations suggest transfer elements was mediated incongruent melting metasomatic components rather than addition. This reinforces previous suggestions interaction carbonate-rich sub-continental can generate magmas within cratons.