Neoproterozoic kimberlite and lamproite magmatism of the Man Craton, Liberia, from a common sub-lithospheric source

Abstract It has been proposed that, rather than the lithosphere, cratonic lamproites may be derived from convecting mantle sources like those of kimberlites, but with extensive subsequent melt modification via melt-rock reaction with metasomatized, phlogopite-rich sub-continental lithospheric mantle (SCLM). Here we explore this model using samples from kimberlite (Camp Alpha) and olivine lamproite (Weasua) localities in Liberia, West Africa. U–Pb dating of perovskite, performed using in-situ Pb isotope compositions of coexisting (low U/Pb) mica, provides broadly coeval Neoproterozoic ages for Camp Alpha (762 ± 9 Ma) and Weasua (779 ± 6 Ma and 754 ± 7 Ma), indicating emplacement during break-up of the supercontinent Rodinia. The mineralogy and mica compositions along with bulk-rock geochemistry of Camp Alpha kimberlites are consistent with derivation from a sub-lithospheric mantle source. The Weasua lamproite contains perovskite with trace element concentrations (e.g., Sr < 3000 µg/g), trace element ratios (e.g., Th/U, Th/Nb, and La/Nb), and 87 Sr/ 86 Sr values (0.7029 to 0.7030) that overlap those of perovskite in the Camp Alpha kimberlites ( 87 Sr/ 86 Sr = 0.7028 ± 0.0002). These data are also similar to those of perovskite in worldwide Neoproterozoic to Cambrian-aged kimberlites but distinct from typical cratonic lamproites, which exhibit perovskite with high Sr contents (> 4000 µg/g) and Sr isotope signatures typical of the enriched lithospheric mantle (bulk-rock 87 Sr/ 86 Sr i generally > 0.7050). A possible petrogenetic model for the Weasua lamproites entails derivation from a sub-lithospheric source similar to that of the Camp Alpha kimberlites with mineralogical, and hence major-element, variations between these two proximal localities driven by variable assimilation of heterogeneous SCLM material.