Correlations between olivine composition and groundmass mineralogy in Sierra Leone kimberlites provide constraints on craton-specific melt-lithosphere interactions

Abstract

Sierra Leone contains two Jurassic-aged diamondiferous kimberlite clusters, namely Koidu and Tongo-Tonguma (hereafter referred to as Tongo), consisting of eruptive pipes and NE-SW trending dikes. In this study, a combination of detailed petrography, and phlogopite, spinel, and olivine compositions in hypabyssal samples is presented to classify and constrain the petrogenesis of these kimberlites. Both the Koidu and Tongo rocks are predominantly macrocrystic and highly micaceous with phlogopite abundances, normalised to olivine-free, of 36–65 vol% in the groundmass. These phlogopite contents are comparable to those of some cratonic lamproites and significantly higher than any other kimberlites. Other groundmass minerals include spinel, perovskite, and apatite set in a base of carbonate and serpentine. Phlogopite and spinel have similar compositions in the Koidu and Tongo samples, displaying evolutionary trends consistent with those observed in worldwide kimberlites. Olivine macrocrysts and microcrysts display complex zoning with distinct cores, internal zones, and rims. The core compositions display a range in Mg# (81–95) and are interpreted to be derived from the disaggregation of lithospheric mantle xenoliths and proto-kimberlite related megacrysts. The Tongo olivine rims, interpreted to be primary magmatic crystallisation products, have similar compositions from various locations within the cluster whereas the Koidu samples display a range in rim compositions (Mg# 87–89) from different locations within the cluster. The average Koidu rim compositions display a strong positive correlation with the average core compositions, consistent with the trend formed by kimberlites worldwide and indicative of a strong control by melt-lithosphere interaction on melt compositions. Previously, it has been shown that the Mg# of olivine rim's negatively correlates with the abundance of groundmass phlogopite (± oxide minerals) in kimberlites. However, the Koidu and Tongo kimberlites are exceptionally phlogopite-rich given their olivine Mg#, fall outside of the worldwide kimberlite array and have olivine compositions and phlogopite abundances like some cratonic lamproites. This leads us to suggest that Koidu and Tongo represent a rare style of highly micaceous kimberlite magmatism, not previously reported in other cratonic regions, and are genetically linked to the assimilation of K2O-rich, metasomatic mantle lithologies. We further suggest that the K2O content, reflected by groundmass phlogopite abundances, of worldwide kimberlite and cratonic lamproite parent magmas ascending to the surface may be related to assimilation of craton-specific styles of metasomatic lithologies in the SCLM.