Mineralogical evolution of the Sokli phoscorite-carbonatite complex, Kola Peninsula, Finland

Abstract

There is a close mineralogical correspondance between phoscorites and paired calcite carbonatites, and the same evolution trend on major minerals is identified in both rock types. This trend involves a sustained increase in alkalinity and it is tentatively attributed to deep differentiation of a mixed carbonate-silicate-phosphate (Fe-rich) parental melt. However, detailed examination of mineral chemistry, especially, on late stage minerals, i.e., phlogopite and pyrochlore indicates that the chemical condition in which these minerals crystallized was clearly different in the two systems. The two associated rocks are considered to have been derived from a common parental magma enriched in Fe, Si, P and carbonates, and their geochemical contrasts to have been produced during the separation of two rocks from the parental magma.The mineralogical records suggest that each magmatic pulse segregates at upper levels to produce a phoscorite/calcite carbonatite pair with a distinct mineralogy. The liquid immiscibility process most simply explains a large chemical gap and different mineralogical records in late stage of phoscorites and conjugated carbonatites. Therefore, liquid immiscibility is favored for explaining the chemical fractionation observed in the Sokli PCC. Fe-Mn dolomite carbonatites, locally enriched in Sr, Ba and LREE, are the latest products from the residual (still Mg-rich) melt left after the crystallization of the Ca-carbonatites and phoscorites.