Comparative mineralogy, geochemistry and petrology of the Beloziminsky Massif and its aillikite intrusions

The Beloziminsky Massif (BZM) is an alkaline ultramafic carbonatite complex that includes carbonatites, ijolites, meltegites, and syenites (abbreviated as the CIMS suite) as well as aillikite intrusions that range in age from 645–621 Ma. Aillikite intrusions also occur in the Yuzhnaya Pipe (YuP), located about 16 km eastward of the BZM. Over 5400 analyses in total were conducted to compare mineralogy and geochemistry of different rock types in this study; of these, 24 CIMS samples (>1100 analyses) and about 16 aillikites (>2300 analyses) were collected from within the BZM; the rest are aillikite mineral samples from pipes and dykes outside the massif (>2000 analyses). The results suggest significant differences in sources for rock-forming minerals, less so for the accessories. The pyroxenes in aillikite correspond either to mantle Cr-diopside xenocrysts or megacrystic augites. Low-Na Ti-augites and diopsides as well as aegirines are prevalent in the CIMS intrusive suite. Amphiboles show a considerably long compositional trend, from hornblendes to richterites. Dolomitic carbonatites include admixtures of Na, K, and Ba while calcium carbonatites often contain Sr. The carbonate-rich aillikitics are enriched either in Mg or Ca. The CIMS rocks, particularly the Ca-Mg carbonatites, often include siderites. Thermobarometry for the YuP samples, collected from outside the BZM and containing Cr-diopsides, Cr-phlogopites and Cr-spinels, suggest a formation pressure of 2–4 GPa and a temperature of 800–1250°C; augite xenocrysts with elevated HFSE, U, Th, and Al-augites trace a 90 mW/m² geotherm.

The huge thermal impact of the plume that triggered the break-up of Rodinia also created a series of ultramafic–alkaline–carbonatite massifs. Initially, the aillikites in the mantle were likely produced by the plume-induced melting of carbonated metasomatites containing ilmenite, perovskites, apatites, amphiboles and phlogopites which, in turn, were created by subduction-related melts. Any additional enrichment in the ore components might have occurred subsequentlty in the lower crust, due to liquation. The aillikites inside the BZM contain low-temperature clinopyroxenes tracing a steep advective geotherm (0.4–1.5 GPa); they also contain clots, related to intermediate depth magma chambers, together with CIMS pyroxenes and amphiboles. This suggests that the liquation of aillikites was accompanied by density separation and assimilation and fractional crystallization (AFC) fractionation with the participation of crustal material. Trace elements (especially REEs) in silicate minerals, carbonates, apatites, and accessories (perovskites, pyrochlores, monazites, columbites, zircons, ancylites, etc.) show a general rise in REE levels and La/Yb_n ratios from aillikites to ijolites, and later to Fe- carbonatites. The presence of zircons, monazites, columbite-tantalites, and other Zr-Hf and Ta-Nb minerals like perovskites and tantalites in the BZM aillikites occurred due to the mixing of the silicate melts with carbonate-rich magmas at deeper levels in the crust and later, in the massif. In the aillikites, xenocrysts. Further, apatites and perovskites show high REE levels. The carbonate-silicate magmas likely passed through a system of polybaric magmatic chambers and liquated carbonatites. Consequently, the later-formed aillikites captured and mixed all varieties of xenocrysts.