The Koidu kimberlite complex, Sierra Leone: geological setting, petrology and mineral chemistry.
Article
Tompkins, LA, Haggerty, SE. (1984). The Koidu kimberlite complex, Sierra Leone: geological setting, petrology and mineral chemistry.
. 83-105. 10.1016/b978-0-444-42273-6.50015-8
Tompkins, LA, Haggerty, SE. (1984). The Koidu kimberlite complex, Sierra Leone: geological setting, petrology and mineral chemistry.
. 83-105. 10.1016/b978-0-444-42273-6.50015-8
Koidu lies at the SW margin of the West African craton, is Mesozoic and consists of three pipes and a kimberlite dyke swarm intruded into Archaean basement. Diatreme facies, massive kimberlite and carbonatitic kimberlite are present. Eclogites dominate, peridotites are absent and there are discrete nodules of garnet, ilmenite, clinopyroxene, orthopyroxene and chlorite. A continuum in the chemistry of the nodule suite (chlorite excepted) is established: clinopyroxene and diamond eclogites formed at P >47 kbar; ilmenite + clinopyroxene intergrowths at P 47 kbar, coincident with the shield geotherm; garnet + ilmenite + clinopyroxene yield 1050oC at 47 kbar; discrete garnet was at 25 and 45 kbar; and ilmenite-spinel equilibrated at fO2's = FMQ at 1000o-1200oC. This suite and their host kimberlites were injected along explosive adiabatic paths in contrast to the chlorite-bearing nodules (precursor phlogopite) and the carbonated kimberlites which were non-adiabatic passive intrusions. -J.M.H.
