Calcite-bearing dolomite carbonatite dykes from Veseloe, North Transbaikalia, Russia and possible Cr-rich mantle xenoliths

Abstract

Carbonatites were discovered at Veseloe, North Transbaikalia, Russia in 2002. They are fluorapatite and calcite-bearing dolomite carbonatites and chemically classified as magnesiocarbonatites with high P, Ti, Cr and Ni. The Cr content in some parts of the Veseloe carbonatites is 3–4 times higher than average magnesiocarbonatite and results from the presence of partially resorbed polymineralic 3–5 mm diameter aggregates of Cr-rich minerals (magnetite, rutile, titanite, ilmenite, phengite, chlorite, chromite, titanite and talc). These aggregates may well be relict mantle xenoliths. They contain a rutile-magnetite intergrowth observed for the first time in nature and have compositions consistent with minerals from eclogite, kimberlite and mantle xenoliths. In the Veseloe carbonatites dolomite crystallized first, and then calcite, which is the inverse of the order of crystallization seen in most carbonatite complexes. Strontium isotope ratios are consistent with a Riphean mantle source. Carbon and O isotope ratios rule out some processes of alteration (metamorphic dehydration and high temperature hydrothermal alteration by meteoric waters) but do not plot in the accepted box for primary igneous carbonatite. Mineral geothermometers, inclusion measurements and the relict mantle xenoliths suggest petrogenesis by independent production of magnesiocarbonatite melt in the mantle rather than crustal differentiation of a carbonated-silicate parental melt.