Compositional Evolution of Calzirtite and Perovskite in Phoscorites and Carbonatites of the Guli Alkaline–Ultramafic Complex, Polar Siberia

Abstract

The paper presents data on the composition and phase heterogeneity of calzirtite Ca₂Zr₅Ti₂O₁₆ and perovskite CaTiO₃, which are HFSE oxides that crystallized during the early stages of formation of the carbonatite rock series of the Guli alkaline–ultramafic complex in Polar Siberia. The composition of HFSE minerals systematically changed during the evolution of the carbonatite melt from phoscorites to carbonatites. The calzirtite enriched up to 6 wt % Nb₂O₅, and the perovskite enriched up to 15 wt % Nb₂O₅, 7.7 wt % ZrO₂, and 6 wt % LREE₂O₃ in the phoscorites and early calcite carbonatites. Perovskite with low concentrations of admixtures crystallized in the late calcite carbonatites in association with U-, Th-, Ta-rich fluorcalciopyrochlore, thorianite, zirconolite, and baddeleyite. The composition of perovskite-group minerals evolved according to the following of isomorphic exchange schemes: Nb⁵⁺ + Fe³⁺ ↔ Ti⁴⁺ + Zr⁴⁺ and 2Ca²⁺ ↔ Na⁺ + REE³⁺. The enrichment of the early calzirtite and perovskite generations in HFSE is explained by the high Nb, Zr, and LREE partition coefficients in carbonatite melt–mineral equilibria. During the crystallization of the carbonatite melt, the activity of alkaline elements decreased, which is confirmed by a decrease in sodium content in the perovskite and a change in the composition of the solid inclusions. The early generations of perovskite and calzirtite from the phoscorites commonly host numerous polyphase inclusions of Ca, Na, K, Ba, and Sr carbonates, halides, and alkali metal sulfides, whereas calcite, fluorapatite, pyrophanite, and barite are found in the late generations of these minerals. It is shown that the crystallization of the phoscorites have crystallized from anhydrous melt that contained no water, and this was favorable for the preservation of alkaline carbonates as solid inclusions in minerals.