Comparative mineralogy, geochemistry and petrology of aillikites and carbonatites, ijolites, melteigites and Ne-syenites of Beloziminsky massif- insights to origin

Abstract

Beloziminsky alkaline ultramafic carbonatite massif (BZM) includes carbonatites, ijolites, meltegites, syenites (CIMS) layers and bodies and aillikites intrusions (∼ 645 - 621 Ma) within massif and Yuzhnaya pipe (YuP), locating in 16 km eastward. Comparisons of mineralogy and geochemistry of 19 CIMS samples (>1100 analyses) and > 16 aillikites (>2300 analyses) locating within the BZM and accounting aillikite minerals from pipes and dykes outside the massif (Ashchepkov et al., 2020) (all together >5400) reveal great difference for rock-forming minerals and less for the accessories. The analyses of the pyroxenes in aillikite correspond to mantle Cr-diopside xenocrysts and megacrystic augites. The low-Na Ti-augites and diopsides as well as aegirines prevail in carbonatites-ijolites-melteigites –syenites intrusive bodies (CIMS suit). Amphiboles show long trend from hornblendes to richterites.The in dolomitic carbonates include also admixtures of Na, K, Ba and Ca- carbonatites often contain Sr, The aillikitic carbonates are rich in Mg or Ca and CIMS rocks together with Ca-Mg carbonatites also often include siderites.Thermobarometry for YuP and Yuzhnaya pipe outside BZM containing Cr-diopsides, Cr-phlogopites, Cr-spinels (P ∼ 4–2 GPa, T ∼ 800–1250°C) and xenocrysts of augites with elevated HFSE, U, Th. Al-augites tracing 90 mW/m2 geotherm.The huge thermal impact of plume produced Rodinia breakdown series of ultramafic-alkaline-carbonatite massifs. Initially aillikites in mantle originated due to melting of carbonated metasomatites containing ilmenite, perovskites, apatites, amphiboles and phlogopites created by subduction related melts. Additional enrichment in ore components may occurred in low crust due to liquation. The aillikites inside BZM contain low-temperature clinopyroxenes tracing steep advective geotherm (0.4–1.5 GPa) and clots related to intermediate magma chambers together with CIMS pyroxenes and amphiboles. This suggests that the liquation of aillikites was accompanied later by density separation and AFC fractionation with the participation of crust material.Trace elements (especially REE) in silicate minerals, carbonates, apatites, and accessories (perovskites, pyrochlores, monazites, columbites zircons, ancylites etc). show general rise of the REE levels and La/Ybn rations from aillikites to ijolites and later to Fe-cabonatites. Presence of zircons, monazites, columbite- tantalites and other Zr-Hf and Ta-Nb minerals (perovskite, tantalites etc) in BZM aillikites occurred due to the mixing with the silicate melts with the carbonate-rich magmas in deep levels and later in massif. In aillikites any of them are of xenocrysts.Apatites and perovskites show high REE levels. The carbonate-silicate magmas passed through the system of polybaric magmatic chambers and liquated carbonatites. Late aillikites captured and mixed all varieties and xenocrysts.