ABSTRACT Phoscorites are plutonic ultramafic rocks, comprising magnetite, apatite and one of the silicates, forsterite, diopside or phlogopite. They almost always occur in close association with carbonatites. Common minor minerals are calcite, dolomite, phlogopite, tetraferriphlogopite and richterite. Key accessories are baddeleyite, pyrochlore, pyrrhotite and chalcopyrite. Phoscorites are rare rocks and only 21 occurrences have been found worldwide. They often form multiphase phoscorite-carbonatite complexes and field relationships include girdle, ring or arcuate zones and veins around or in the carbonatite cores of many complexes, steeply dipping veins that form stockwork pipe-like bodies, or oval, lens-like or isometric bodies up to 100s of metres wide. Five complexes in the Kola Alkaline Province: Kovdor, Vuoriyarvi, Turiy Mys, Seblyavr and Sokli, provide examples of phoscorite diversity. There is no accepted nomenclature for the subdivision of phoscorite varieties. Three possible systems are: (1) the addition of mineral prefixes; (2) the scheme of Yegorov (1993) based on modal (volume) abundances of minerals; and (3) the RHA method of Krasnova et al. (2002) based on molecular abundances of minerals calculated from whole-rock chemistry. Petrogenetic studies of phoscorite emphasize the close relationship between carbonatites and phoscorites and suggest their origin from evolving melts by crystal fractionation and/or liquid immiscibility. Isotopic studies provide evidence of a mantle origin and in some cases argue for successive melt batches.

ABSTRACT The Sokli alkaline complex, ‘Finnish Lapland’, Finland, shows a rock association dominated by calcite carbonatites and phoscorites surrounded by altered alkaline rocks (clinopyroxenites), and cut by ultramafic lamprophyres. Calcite carbonatites and phoscorites are intimately associated on a metre scale, and they exhibit similar mineralogies. At the larger scale of the intrusive complex, variations in mineral assemblage delineate a broad concentric structure, with three main groups of calcite carbonatites and associated phoscorites evolving towards later REE-depleted dolomite carbonatites and finally LREE-Sr-Ba-Mn-Fe-enriched dolomite carbonatites. Bulk-rock chemical data allow additional intrusive bodies to be discriminated. They underline the dominance of magnetite (over silicate and phosphate) in phoscorites and the general lack of gradation towards calcite carbonatite compositions. Large variations in mineral chemistry are documented as a function of intrusive stage, in particular for pyrochlore. Carbonatite and phoscorite from the same stage share many mineralogical similarities, but the zoning patterns of micas and pyrochlores record a longer-lived crystallization history in most phoscorites, thus not favouring the idea that phoscorites are cumulates and calcite carbonatites, residual liquids. The segregation between calcite carbonatite and phoscorite is more likely to be driven by immiscibility between Fe-rich and carbonate-rich melts, a possibility which is yet to be explored experimentally. The late-stage evolution towards dolomite carbonatites is best seen as an in situ differentiation of the phoscorites, complicated by hydrothermal redistribution of the REE .

ABSTRACT The Vuoriyarvi Alkaline Ultramafic Complex, northern Karelia, consists of a stock of clinopyroxenites surrounded by ijolitic rocks. It is intruded by stockworks of calcite carbonatites, associated with silicate-apatite-magnetite rocks (phoscorites), that evolve towards later calcite-dolomite carbonatites containing Sr-Ba-LREE minerals. Three main stages of emplacement of carbonatite-phoscorite associations are distinguished, characterized successively by forsterite-phlogopite, phlogopite-tetraferriphlogopite, and magnesio-arfvedso- nite-richterite as the main silicate minerals. The succession of mineral assemblages and the mineral composition trends reflect a global enrichment, from stage to stage, in fluorine, sulphur, alkali elements, Sr and Ti, while the evolution of phlogopite from VI Al-bearing compositions to tetraferriphlogopite reflects a depletion in Al. The carbonatite- phoscorite associations may represent the successive products of the fractional crystallization of an alkali-bearing carbonatite magma. The differentiation, within each association, into carbonatites and phoscorites is considered to result mainly, in the early stage, from an efficient separation between coeval cumulate products, while, in later stages, the phoscorites consist mainly of pegmatitic accumulations and replacement rocks produced by fluids related to the crystallization of the associated carbonatites.