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Tectonic analysis reveals a previously unidentified link between alkaline rocks and carbonatites (ARCs) forming the Kola Alkaline and Carbonatite Province (KACP) in Arctic Russia, and ARCs of the Seiland Igneous Province (SIP), in the northern Caledonides of Finnmark, Norway. We here conclude that the rocks of the two provinces have been derived by decompression melting of deformed alkaline rocks and carbonatites (DARCs) under rifts and within the underlying lithospheric mantle preserved within the Paleoproterozoic Lapland-Kola Suture Zone (LKSZ) and its extension beneath the allochthons of the Norwegian Caledonides. The LKSZ projects under the Caledonide allochthons to the postulated site of eruption of the ARCs of the SIP in an intra-continental rift at a place where an ocean was soon to open. We suggest that the SIP ARCs formed in this intracontinental rift by decompression melting involving DARCs that had lain in the underlying lithospheric mantle within the along-strike extension of the LKSZ for more than one billion years. Thrusting and metamorphism during the Scandian collision between Laurentia and Baltica transformed the SIP ARCs into DARCs that occupy the immediate footwall to the suture zone between Laurentia and Baltica. At the collision, the SIP rocks were carried south-southeastwards from their then location on the outermost continental margin of Baltica to their current location in the highest level of a sandstone-dominated nappe complex. The ARCs and DARCs of Kola and Finnmark provide independent evidence of the operation of two complete Wilson cycles of the opening and the closing of oceans in the region. doi:10.1029/2006TC002052.

Many tuffs contain crystal, glass, and rock fragments that are supported by calcite cement, and some of these have recently been classified as carbonatites, i.e., igneous rocks dominated by magmatic carbonate minerals. In samples studied here, the cementing calcite has lower Sr and higher Mg concentrations, and higher [sup.18]O/[sup.16]O but lower [sup.13]C/[sup.12]C, than magmatic calcite. These chemical data, together with textural evidence, indicate that the cementing calcite in these tuffs was precipitated as coatings on the pyroclastic fragments by meteoric water. A single thin section of a calcite-cemented tuff (whether it holds carbonatite or silicate lapilli) can contain as many as five texturally and chemically distinct kinds of carbonate, precipitated sequentially, and never in equilibrium with each other. The secondary calcite in some occurrences may have formed by reaction of lime calcined by hot silicate magma from limestone wall rocks. Pyroclastic deposits that contain calcite only in pore-filling cement or replacing silicates should not be called carbonatites. Keywords: calcite cement, lapilli tuff, carbonatite.

Magmatic fluorapatites of five African carbonatite complexes were analyzed for rare-earth (REE) and trace elements by electron microprobe and high-resolution synchrotron micro-XRF to explore the fluorapatite composition during different stages of carbonatite magma evolution. Early crystallized fluorapatites have La concentrations mostly below 1,500 ppm and low [SIGMA]REE. They display convex-upward shaped REE patterns with [(La/Nd).sub.cn] [less than or equal to] 1 and low [(La/Yb).sub.cn] ratios < 100. In contrast, fluorapatites from fractionated carbonatites have straight REE patterns with [(La/Nd).sub.cn] > 1 and [(La/Nd).sub.cn] generally above 100, and have La up to 1 wt% at a high [SIGMA]REE. Model calculations with the fractionating mineral assemblage fluorapatite + calcite [+ or -] clinopyroxqne suggest REE distribution coefficients for fluorapatite/carbonatite melt with a positive slope throughout from La to Lu, in order to meet the relationships observed in the natural fluorapatites. The calculations oppose closed system conditions of magma fractionation along the liquid lines of descent, but suggest periods of instantaneous fluorapatite crystallization. Fluorapatite trace element characteristics are therefore thought to be indicative for carbonatite evolution, and can reflect the relative degree of magma fractionation. We suggest that the [(Eu/Eu*).sub.cn] and Y evolution in the fluorapatites is a manifestation of an aqueous fluid immiscibly coexisting with the carbonatite magma from early evolution on, which is able to continuously extract divalent Eu and Y from the carbonatite magma.

The only significant silicate intrusive rock type in the Dicker Willem carbonatite complex is trachyte, forming, in places, an anastomosing array of minor intrusions cutting basement gneiss close to the carbonatite contact. Bodies are predominantly composite breccias, composed of trachyte clasts, commonly in the form of ellipsoidal pellets, enclosed within, and sharply delineated from, a matrix of carbonatite. Despite close temporal and spatial relationships to carbonatite magmatism, the ultrapotassic, quartz-normative composition and isotope systematics of the trachytes preclude any genetic derivation from the carbonatitic and ijolitic rocks of the central complex. Sr, Nd and Pb isotope ratios of trachytes strongly resemble those of the highest grade, potassic fenites, whose metasomatic trend converges from the unaltered basement gneiss towards the homogeneous signature of the nepheline sovite-sovite-ijolite suite. Trachytes are interpreted as forming by melting of a cupola of high-grade fenite in response to the advective heat flux from rising carbonatite magma or fluid. Mixed carbonatite and trachyte were emplaced in a fluidised system as contemporaneous, but genetically unrelated, immiscible magmas.

New experimental data in CaO-MgO-Si[O.sub.2]-C[O.sub.2] at 1 GPa define the vapor-saturated silicate-carbonate liquidus field boundary involving primary minerals calcite, forsterite and diopside. The eutectic reaction for melting of model calcite (1% MC)-wehrlite at 1 GPa is at 1100 [degrees] C, with liquid composition (by weight) 72% CaC[O.sub.3] (CC), 9% MgC[O.sub.3] (MC), and 18% CaMg[Si.sub.2][O.sub.6] (Di). These data combined with previous results permit construction of the isotherm-contoured vapor-saturated liquidus surface for the calcite/dolomite field, and part of the adjacent forsterite and diopside fields. Nearly pure calcite crystals in mantle xenoliths cannot represent equilibrium liquids. We recently determined the complete vapor-saturated liquidus surface between carbonates and model peridotites at 2.7 GPa; the peritectic reaction for dolomite (25% MC)-wehrlite at 2.7 GPa occurs at 1300 [degrees] C, with liquid composition 60% CC, 29% MC, and 11% Di. The liquidus field boundaries on these two surfaces provide the road-map for interpretation of magmatic processes in various peridotite-C[O.sub.2] systems at depths between the Moho and about 100 km. Relationships among kimberlites, melilitites, carbonatites and the liquidus phase boundaries are discussed. Experimental data for carbonatite liquid protected by metasomatic wehrlite have been reported. The liquid trends directly from dolomitic towards CaC[O.sub.3] with decreasing pressure. The 1.5 GPa liquid contains 87% CC and 4% Di, much lower in silicate components than our phase boundary. However, the liquids contain approximately the same CaC[O.sub.3] (90 [+ or -] 1 wt%) in terms of only carbonate components. For C[O.sub.2]-bearing mantle, all magmas at depth must pass through initial dolomitic compositions. Rising dolomitic carbonatite melt will vesiculate and may erupt as primary magmas through cracks from about ~70 km. If it percolates through metasomatic wehrlite from 70 km toward the Moho at 35-40 km, primary calcic siliceous carbonatite magma can be generated with silicate content at least 11-18% (70-40 km) on the silicate-carbonate boundary.