Namuaiv Lamprophyre Pipe in the Khibina Massif: Mechanism of Formation and Implications for the Nature of the Mantle Source of Late-Stage Magmatism in the Kola Alkaline Province

This study presents petrographic, major and trace-element, and Sr-Nd isotopic data for rocks from the Namuaiv explosion pipe, which intrudes the Khibina massif in the Kola Alkaline Province (KAP). These rocks record the late stage magmatic event in the KAP’s evolution. The results provide insights into the formation mechanisms of alkaline-ultramafic explosion pipes and constrain the nature of the mantle source during the province’s late magmatic stages. The pipe’s formation involved two distinct lamprophyric magmas—aillikite and monchiquite—as well as associated hydrothermal processes. The initial aillikite magma pulse underwent fluid fragmentation, whereas the subsequent monchiquite magma produced a hybrid rock— monchiquite breccia with aillikite magmaclasts. The fluid phases produced during explosive emplacement of aillikite formed a breccia with a natrolite-rich matrix. Some magmaclasts that were not incorporated into the monchiquite matrix were instead cemented into hydrothermal natrolite breccias. Geochemical and isotopic contrasts between early pre-Khibina lamprophyre dikes (Terskiy Coast)—coeval with alkaline-ultramafic carbonatite massifs—and later dikes and pipes (Khibina massif) suggest a shift in the composition of carbonate-bearing metasomatic assemblages in the mantle source. Early melts involved K-Na amphibole, but this metasomatic phase was exhausted during large-scale melting, leading to source depletion. Late-stage melts were instead derived from a phlogopite-bearing source, formed by metasomatic overprinting of potassium-rich melts generated by incongruent amphibole melting. K-Na amphibole was involved in the generation of the early melts, but this metasomatic phase was exhausted during large-scale melting, leading to source depletion. Instead, late stage melts were derived from a phlogopite-bearing source formed by metasomatic overprinting of the early depleted source. The metasomatic agent was potassium-rich melts derived from incongruent melting of K-Na amphibole.