Mineralogical, geochemical, and isotopic data of a new special agpaitic dyke, enriched in high field strength elements (Eastern Part of Baltic Shield, Russia)

We present the petrography, bulk major and trace elements, and isotopic compositions for the recently identified Mohnatye Roga agpaitic syenite dyke, located 30 km from the Kovdor carbonatite complex, Kola Peninsula, Russia. The dyke was identified below Quaternary deposits as the result of a gravimetric survey and subsequent drilling. A K–Ar age on lamprophyllite of 368 ± 9 Ma, coincides with the age of the Devonian Kola Alkaline Carbonatite Province (KACP). In the total alkalis vs. silica diagram, the Mohnatye Roga dyke exhibits a compositional trend from lamprophyre to peralkaline phonolite as well as mineralogical and geochemical affinities with the nearby Niva intrusion. The dyke is composed of orthoclase, Ti-rich aegirine-augite, aenigmatite, lamprophyllite-group minerals, K-amphibole, and astrophyllite with accessory noonkanbahite, calcite, ilmenite, rutile, barite, apatite, and sulfides. Late stage minerals include natrolite and ferripyrophyllite. A notable feature of this dуke is its extremely high agpaicity and enrichment in HFSEs (high-field strength elements). The concentrations of a number of HFSEs are higher in this dyke than in the Lovozero rare metal deposit. Mohnatye Roga dyke and Niva intrusion are examples of a gradual transition from melt to hydrothermal fluid. Isotopic data indicate a mantle source for the dyke, and seemingly an absence of contamination by continental crust during its formation. The geochemical and isotopic data suggest that the source of the KACP rocks, including the Mohnatye Roga dyke, is FOZO mantle which is believed to be related to plume magmatism. • Origin and age of the recently identified lamprophyllite-bearing agpaitic syenite Mohnatye Roga dyke (Kola peninsula) • Great similarity with the Niva intrusion and the link to Devonian Kola Alkaline Carbonatite province • Agpaitic SiO 2 -undersatured melts preserving the depleted FOZO plume signature • Enrichment in HFSEs of Mohnatye Roga dyke and comparison with gigant Lovozero and Khibiny deposits.