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In many alkaline complexes, large amounts of ultramafic rocks occur together with carbonatites, melilitolites and other alkaline silicate rocks. There is an ongoing debate if and how these contrasting lithologies were formed by differentiation of a common, mantle-derived silicate magma or rather by metasomatic processes between carbonatite and country rocks. In order to find petrological evidence for one or the other, two key examples, the Gardiner (E Greenland) and Kovdor (Russia) complexes are compared in this study. Despite their similar tectonic setting and succession of rock types, they show significant differences in the texture and mineral composition of ultramafic rocks. Ultramafic rocks from Kovdor include calcite- and biotite-rich dunites and pyroxenites without typical cumulate textures. They consist of Ni-poor forsterite, Cr-poor diopside and Ni-Cr-poor spinel and are possibly metasomatic reaction products between mantle-derived carbonatite melts and silicic host rocks. Similar ultramafic rocks are associated with carbonatites e. g. at Palabora (South Africa), Afrikanda (Russia), and Salitre (Brazil). In contrast, the ultramafic rocks from Gardiner show well-preserved cumulate textures and consist of Ni-rich forsterite, Cr-rich diopside as well as Cr-Ni-Ti-rich spinel and also contain F-Cl-rich apatite. They record an increase in aSiO2 from dunite to pyroxenite at similar f O2 (ΔFMQ ~ +1.2, with FMQ = fayalite-magnetite-quartz buffer), indicating that these rocks represent cumulates of an evolving, moderately oxidized mafic melt derived from a Ti-rich mantle source, similar to other rocks of the North Atlantic igneous province. In contrast to systems like Kovdor where carbonatite metasomatism is likely dominant, Ti-rich parental silicate magmas can abundantly crystallize Ti phases, as recorded by massive perovskite cumulates in Gardiner melilitolites. This can effectively scavenge HFSE from the magmatic system early in its evolution and likely explains HFSE-barren carbonatites at Gardiner, while those from Kovdor are highly HFSE-enriched. In summary, the results of our study provide strong textural and mineral chemical evidence that ultramafic rocks in alkaline complexes can be of both cumulate and metasomatic origin; the specific type has an important bearing on their HFSE enrichment and on the types of ores present in such complexes. [ABSTRACT FROM AUTHOR]

Carbonatites exist as intrusive and extrusive rocks, with the former dominating the rock record. The geochemical link between intrusive and extrusive equivalents and the processes during eruption of carbonatite melt are essentially unknown. This contribution aims at providing new insights into the transition from intrusive to extrusive carbonatites with special emphasis on the trace element budget. The Gross Brukkaros natural laboratory in central Namibia was chosen to study the interface between fine-grained dolomite‑carbonatite dykes and associated diatremes. Whole rock geochemistry combined with micro-scale petrography via BSE imaging and microXRF mapping provide evidence that the carbonatite dykes contain significant amounts of silica and aluminium, which is inherited from crustal xenoliths by resorption and leaching. At the transition from carbonatite dyke to diatreme, the CO 2 component (likely together with most of the other volatiles e.g. Cl, H 2 O, F, S) of the carbonatitic brine-melt vaporised and was released into the atmosphere. During this process the Si, Ca, Mg and Fe transported together with a high amount of trace elements became precipitated from a resulting primarily magmatic fluid as a mixture of cryptocrystalline quartz (quartz I) and aegirine-augite (plus minor magnetite) with frequently occurring microcrystalline quartz I grains. This mineral assemblage forms the matrix in the diatreme breccia and precipitated by rapid temperature drop in the course of decompression from the hydrothermal fluid. In the post-eruption stage the influx of meteoric waters (into the system), which mixed with the remaining magmatic fluid, caused precipitation of a second quartz generation (quartz II; euhedral grains), which is depleted in trace elements. All measured trace elements show significantly higher contents in quartz I compared to quartz II (with exception of Li). The application of the TitaniQ geothermometer indicates that Ti incorporation in quartz is kinetically controlled for quartz I. During quartz II formation the system thermodynamically equilibrates and ends up with realistic precipitation temperatures of 290–350 °C. Therefore, this study shed light on the trace element behaviour at the interface between feeder dykes and diatreme roots of a strongly contaminated carbonatite melt. [ABSTRACT FROM AUTHOR]