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.

ABSTRACT A set of criteria has been established that we use to assess whether carbonatites are generated as primary mantle melts, or whether they are the products of magma differentiation (crystal fractionation, liquid immiscibility) of a parental, carbonated silicate melt. On the basis of these criteria, in conjunction with the vast amount of published field information and geochemical data from the Kola Alkaline Province (KAP), no clear-cut pattern emerges that favours any of these processes over the others. Any evidence for liquid immiscibility seems to be restricted to some members of the dyke swarms (Kandaguba, Turiy), while robust evidence for the generation of primary carbonatitic magmas seems to be lacking. Potential candidates for primary melts include the carbonatites from Turiy Mys and the older dyke swarms associated with the Kandalaksha Deep Fracture Zone. The spatial and temporal association of most carbonatites with silicate rocks at Kola, along with the presence of olivinites, clinopyroxenites and other cumulate rocks in some complexes favour crystal fractionation as an important process in generating some of the KAP rocks. We are left with the impression that all three processes may be responsible for carbonatite generation, even within the same complex. The isotopic evidence suggests the involvement of at least three distinct mantle sources, one of which is common to all of the complexes and perhaps indicative of a deep-seated, primitive mantle at least 3 Ga old. Overall, we propose an integrated plume-related model for the Devonian alkaline and carbonatitic magmatism that characterizes much of the KAP. Low-degree partial melting within the volatile-rich, and cooler parts of a plume head accompanied by the mixing of small-volume magma batches, their subsequent differentiation, and interaction with entrained materials and continental lithosphere may help explain some of the problems associated with unravelling the genesis of carbonatite magmas.

ABSTRACT The oldest alkaline silicate rocks known are Archaean (2700–2600 Ma) lamprophyres and alkali syenites. Several Proterozoic alkaline gabbroic intrusions which in part also contain carbonatites occur between 2000 and 1800 Ma. Intensive alkaline magmatic activity formed more than 20 complexes (Kola Alkaline Province, KAP) during the Palaeozoic between 410 and 362 Ma with the majority of ages being between 382 and 362 Ma. The data show no systematic geographical distribution of ages within the KAP so that timing of magmatic activity cannot be correlated to the major tectonic structures. A distinction of the intrusions into Caledonian and Hercynian groups is not supported. A lithologically controlled variation of ages within single complexes can neither be seen betweencarbonatites and associated alkali silicate rocks nor between carbonatites and phoscorites.

Abstract The (Mid–) Late Devonian to Early Carboniferous was a time of widespread rifting on the East European Craton (EEC) and its margins. The most prominent basin among these and, accordingly, the best documented is the Dniepr–Donets Basin (DDB) in Ukraine and southern Russia. The DDB is associated with voluminous rift-related magmatism and broad basement uplift. Two other large, extensional, basin systems developed along the margins of the EEC at the same time: the East Barents Basin (EEB) and its onshore prolongation the Timan–Pechora Basin (TPB), and the Peri-Caspian Basin (PCB). Rifting, associated magmatism, and possible domal basement uplift are also reported elsewhere within the EEC, suggesting a common, ‘active’, rifting process, involving a cluster of thermal instabilities (or generalized thermal instability) at the base of the lithosphere beneath widely separated parts of the EEC by Mid–Late Devonian times. The DDB is an intracratonic rift basin, cutting across the Archaean–Palaeoproterozoic structural grain of its basement and, as such, differs from the EBB–TPB and PCB, which are pericratonic rift basins developed on reworked and juvenile crystalline basement accreted to the EEC during the Neoproterozoic. The DDB opened into a deep basin, possibly having oceanic lithospheric affinity, to the SE, in the area where it adjoins the southern PCB, suggesting the possibility that rifting led to (limited?) continental break-up in this area at this time. Post-rift compressional tectonic reactivations and basin inversion in the DDB, leading to the formation of its prominent Donbas Foldbelt segment, are related to Tethyan events (Cimmerian and Alpine orogenies) occurring on the nearby southern margin of the EEC. Post-rift compressional inversions in the PCB and TPB, which lie closer to the Urals margin of the EEC, are related to Uralian tectonics.

ABSTRACT Complex Ti, Nb and Zr oxides (baddeleyite, zirconolite, perovskite-, pyrochlore- and ilmenite-group minerals) are primary HFSE hosts in early calcite carbonatites and phoscorites. Appreciable amounts of Hf and Ta are concentrated in baddeleyite and uranoan pyrochlore, respectively. In their absence, minor Ta may be sequestered in perovskite, lueshite or ilmenite, and Hf in zirconolite. Ilmenite (as ‘exsolution’ lamellae in magnetite and discrete crystals) and perovskite are the principal Ti minerals, but a significant proportion of this element is also bound in rock-forming silicates. Primary zircon is of limited significance; it is characteristically poor in Hf. Titano-, niobo- and zirconosilicates are restricted to deuteric and metasomatic parageneses developed after and at the expense of the primary HFSE minerals during the late stages of carbonatite evolution, or as a result of metasomatic overprint in the wallrocks. Depending on the activity of silica, Na and other cations in deuteric (metasomatic) fluids, these parageneses may also contain complex oxides enriched in Sr, Ba and light REE (e.g. loparite and bariopyrochlore), as well as TiO 2 polymorphs, late-stage baddeleyite, and poorly characterized Ti-Nb-Zr phases. Evolution of fluids causes systematic changes in modal mineralogy (e.g. replacement of gittinsite by catapleiite) and/ or composition of minerals (e.g. enrichment of perovskite in REE). The activity of HFSE in these environments is determined by the nature and relative stability of their (hydroxo-)carbonate complexes.

ABSTRACT The Afrikanda pluton (Kola Peninsula) comprises predominantly olivine- and clinopyroxene-dominant cumulate ultramafic rocks, with subordinate melteigites and ijolites. On the basis of petrographic and geochemical evidence, these rocks are interpreted to have formed at mid-crustal depths from a Ca-rich melanephelinitic magma. Olivinites (+wehrlites) and clinopyroxenites precipitated from separate batches of magma, whereas the foidolites are related to the clinopyroxenites by crystal fractionation. In both cases, the parental magma was derived by partial melting of a metasomatized lithospheric source enriched in pargasite and phlogopite. The melting occurred just above the amphiboleperidotite solidus, thus producing a liquid depleted in K, Ba and Rb, but enriched in light REE , Nb, Ta and Th. Plutonic carbonatitic rocks occur predominantly as branching veins and nests in the clinopyroxenites. They show variable modal composition in terms of both principal (diopside, magnesiohastingsite and calcite) and minor (perovskite, magnetite, titanite, chlorite and ilmenite) rock-forming phases, delineating a series from calcite-amphibole clinopyroxenite to calcite carbonatite. These rocks crystallized from an alkalisilica-rich carbonatitic magma of mantle provenance. Solidification of this magma and accompanying processes (reaction with the wallrock, xenocryst assimilation, fractionation of a Na-rich fluid, and subsolidus re-equilibration) led to the formation of several distinct parageneses comprising over 50 mineral species.