1.92 Ga kimberlitic rocks from Kimozero, NW Russia: Their geochemistry, tectonic setting and unusual field occurrence

Abstract

The Kimozero body is inferred to represent the oldest known diamond-bearing kimberlite. It was emplaced into a basaltic sequence as a gently dipping sheet-like unit, comprising several rock varieties with textures indicating a supracrustal origin. Minor subvolcanic bodies are represented by several narrow dykes and diatremes, which formed during at least two intrusive phases. The kimberlitic rocks have been exposed to extensive interactions with hydrous and CO2 rich fluids, which led to extreme changes in the chemical compositions of the rocks, making it difficult to identify their origin. Geochemical data indicate a within-plate anorogenic tectonic setting of the Kimozero magmatic body. Trace element compositions show that the rocks have a clear geochemical affinity to orangeite (micaceous group II kimberlite) and olivine lamproite. Nd and Sr isotope compositions of the Kimozero rocks show that they were derived from a mantle source resembling Bulk Silicate Earth. New zircon U–Pb isotope data indicate emplacement age of the kimberlite at 1.92Ga, suggesting that the rocks were associated with a late stage of Paleoproterozoic rifting of the Karelian Craton, related to the formation and growth of the Svecofennian Ocean.