Magmatic versus metasomatic origin of HFSE- and REE-enriched peridotite-melilitolite-foidolite-carbonatite complexes – a melt inclusion study from Gardiner (Greenland) and Kovdor (Russia)

In some composite alkaline complexes, large amounts of ultramafic rocks occur together with melilitolites, carbonatites and other alkaline silicate rocks. These rock associations are of significant economic interest due to their common enrichment in critical and strategic elements like Ti, P, HFSE, or REEs. Key examples for such systems are the Gardiner (Greenland) and Kovdor (Russia) complexes. The timing of enrichment of HFSE during the evolution of these two complexes is still under debate. Carbonatites at Kovdor (Russia) are HFSE-mineralized, with only minor HFSE-phases in the associated silicate rocks. In contrast, melilitolites from Gardiner show ore-grade enrichment of perovskite, while the associated carbonatites are barren.

Previous work assumed similar parental melts due to similar homogenization temperatures and daughter mineral assemblages in olivine-hosted melt inclusions from ultramafic rocks. However, it was recently shown that dunites, peridotites, pyroxenites, and melilitolites from Gardiner represent orthomagmatic cumulate rocks, whereas some of the dunites and melilitolites at Kovdor are rather of metasomatic origin.

To reconcile these different origins, compositions and homogenization temperatures of crystallized melt inclusions from dunites, pyroxenites, and melilitolites are revisited in detail. Subtle differences in temperature and the abundance of carbonates and HFSE-phases in olivine-hosted melt inclusions from peridotites may display the metasomatic evolution. The comparison between the Gardiner and Kovdor complex will shed light on the nature of these rocks at variable evolutionary stages and the subsequent ore forming processes.