CALCITE - AMPHIBOLE - CLINOPYROXENE ROCK FROM THE AFRIKANDA, KOLA PENINSULA, RUSSIA: MINERALOGY AND A POSSIBLE LINK TO CARBONATITES. III. SILICATE MINERALS

Abstract

A calcite - amphibole - clinopyroxene rock (CAPR) occurs as branching veins and segregations in ultramafic alkaline lithologies of the Afrikanda complex, Kola Peninsula, in Russia. The rock is composed predominantly of diopside, magnesiohastingsite, calcite, titanite, chlorite and oxide phases. In the present study, a complete characterization of morphology, interrelations, and compositional variation of the oxide minerals is given, and eleven phases are described at Afrikanda for the first time. Three parageneses are distinguished among the oxide minerals, based on their geochemical features and relative position in the crystalization history. These are (in order of formation): primitive (early magnetite, lamellar ilmenite, primary perovskite, baddeleyite, calzirtite, zirconolite), evolved (secondary perovskite, loparite, lueshite, betafite, late magnetite, discrete ilmenite, thorutite, thoruminite) and replacement (rutile, hematite, fersmite, pyrochlore, an unidentified REE-Ti oxide) associations. The primitive and evolved associations formed in the temperature range 500-550°C at f(O2) approximately 10-20-10-22 bar, and a(SiO2) generally below 10-1.2. The replacement assemblage crystallized immediately prior to the precipitation of calcite under increasing a(SiO2) and f(O2) (? decreasing temperature). This assemblage is closely associated with late silicate minerals, including titanite, chlorite and Zr-bearing silicates. The complex mineralogy of CAPR cannot be explained in terms of simple differentiation of a parental silicocarbonatitic melt. The textural and mineralogical features indicate that the evolution of CAPR involved substantial interaction between a CO2-rich liquid (? fluid) and the ultramafic wallrocks.