Stable (C, O) and radiogenic (Sr, Nd) isotopic evidence for REE-carbonatite formation processes in Petyayan-Vara (Vuoriyarvi massif, NW Russia)

Abstract

A study of radiogenic (Sr, Nd) and stable (C, O) isotopic data for rare earth carbonatites from the Petyayan-Vara field of the Devonian Vuoriyarvi alkaline-ultrabasic massif is presented. The cumulative evidence indicates that the primary igneous rocks of the Petyayan-Vara area are burbankite-bearing magnesiocarbonatites having isotopic signatures of the depleted mantle (εNd365Ma = 5.0, 87Sr/86Sr(i) = 0.7031, δ13C ca. –4‰, and δ18O ca. 11‰). Interaction of the primary carbonatite melt with the host silicate rocks produced high-Ti carbonatites with a mantle δ13C (ca. –4‰) and isotopically heavy δ18O (ca. 20‰). These rocks trapped K, Na, Mg, CO2, and rare earth elements (REEs) (mainly heavy REEs) from the melt and Si, Al, Fe, Ti, and P from the host rocks. Early post-magmatic exposure of burbankite-bearing carbonatites to a mixture of fluids of crustal and orthomagmatic carbonatite origin caused redistribution of REEs, Ba, and Sr and formation of REE-rich carbonatites with abundant ancylite mineralization. This effect did not disturb the SmNd system but induced radiogenic Sr accumulation and a change in C and O isotopic composition towards heavier values. Later, but most likely before denudation, the Petyayan-Vara rocks underwent another metasomatic event involving crustal fluids infiltrating through fracture systems. This event triggered formation of bastnäsite-rich carbonatites with fewer REEs at the expense of ancylite-rich carbonatites, and changed all the isotopic systems in the affected rocks. This model successfully accounts for the evolution of all the carbonatite varieties discovered to date in the Petyayan-Vara field.