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 Stable C and O isotope compositions have been compiled for the Kovdor, Sokli, Turiy Mys, Vuoriyarvi, Afrikanda, Khibiny, Sallanlatvi, Telyachiy Island, Tiksheozero and Siilinjärvi Carbonatite Complexes of the Kola-Karelia Region. Stable isotope relationships of various carbonate minerals (calcite, dolomite, rare-earth minerals) from several stages of carbonatite evolution have been investigated in order to determine primary compositions and the influence of secondary processes (Rayleigh fractionation in the fluid-carbonate-silicate system, hydrothermal alteration, degassing, crustal contamination). The phoscorite-carbonatite relationship has also been investigated. The data for some phoscorites (Sokli, Vuoriyarvi) are consistent with an origin by carbonatite-silicate liquid immiscibility, whereas those of the Turiy Mys phoscorites suggest the involvement of different sources. The primary carbon isotope compositions obtained for different complexes show a large variation at fairly constant oxygen isotope compositions. This is best explained by mantle heterogeneity, probably related to CO 2 metasomatism or by removal of a 12 C-enriched phase in the mantle.

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 tectonic position of the Palaeozoic alkaline complexes containing phoscorites and carbonatites is determined by the combination of three factors. These are (1) the presence of rift systems; (2) the presence of deep fracture zones; (3) the powerful energetic excitement of lithosphere with an epicentre in the region of the Khibiny complex, i.e. the action of a mantle plume or ‘a hot spot’. The ages of alkaline magmatism in the eastern part of the Baltic Shield are in general synchronous with the manifestation of alkaline magmatism in the whole of the North Atlantic Alkaline Province. Early Palaeozoic alkaline magmatism on the Kola Peninsula is small-scale compared with that in western Fennoscandia. It consists of diatremes and dykes of olivine melilitite-alnöite-carbonatites. The formation of the carbonatitic ultramafite-foidolite complexes started in the early Palaeozoic. The peak of alkaline magmatism, of volcanic activity, large intrusions, and dykes, took place in the late Devonian, forming the Devonian Kola Alkaline Province (KAP). As a whole, the carbonatites in their various different forms and sizes are related to six petrogenetic types and varieties of alkaline complexes and dykes. They are: (1) Proterozoic complexes of ultramafic/mafic rocks-foidolites-foidosyenites; (2) dykes and diatremes of the early Palaeozoic series of the Kandalaksha Graben; (3) Kandagubian seriesdykes; (4) Turiy series dykes; (5) Palaeozoic plutons of alkaline-ultramafic rocks; (6) agpaite nepheline syenite complexes. A quantitative model proposed for the formation of the carbonatites involves liquid immiscibility from an evolved carbonated nepheline melilitite melt.