Phlogopite and tetraferriphlogopite from phoscorite and carbonatite associations in the Sokli massif, Northern Finland

Abstract

The phoscorite—carbonatite complex (PCC) in the Sokli massif, northern Finland, is divided into 5 stages according to mineral assemblages and occurrences. The earlier three stages comprise phoscorites and calcite carbonatites (P1 to P3 and C1 to C3, respectively), and they usually occur as pairs with the same mineral assemblage (calcite, apatite, megnatite, olivine, and mica). The latter two stages consist of only dolomite carbonatites called D4 and D5. All micas investigated from the Sokli PCC fall in the range of the phlogopite-tetraferriphlogopite series. Tetraferriphlogopite begins to crystallize from late stage 2 and becomes a dominant silicate in the P3C3 rocks and D4-D5 dolomite carbonatites. Although tetraferriphlogopites occur as primary or secondary products, discrete and euhedral (magmatic) tetraferriphlogopites are considered to have crystallized from a melt strongly depleted in aluminum and saturated in Ti-bearing phases under low temperature condition. The chemical variation of phlogopites shows that Fe and F contents increase, whereas Al, Ba, Ti and Mg contents decrease from stage 1 to stage 5. The progressive depletion in aluminum and the enrichment in fluorine towards the later stages seem to be a specific feature of the Sokli phlogopite. The difference of phlogopite rim compositions between phoscorites and paired carbonatites indicates that there was a slight difference of elemental partitioning into the interstial melts during the segregation of the rocks from a parental magma.