Volume 33, Nº 4 (2025)

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 generationof the early melts, but this metasomatic phase was exhausted during large-scale melting, leading to sourcedepletion. Instead, late stage melts were derived from a phlogopite-bearing source formed by metasomaticoverprinting of the early depleted source. The metasomatic agent was potassium-rich melts derived fromincongruent melting of K-Na amphibole.

In the southeastern fragment of the Raahe-Ladoga suture zone in Russia, within the Meyeri tectonic zone, increased pressures (“overpressure”) were revealed, caused by structural-metamorphic transformations of rocks during collisional interaction of allochthonous and autochthonous blocks. It is assumed that tectonic interaction of the rigid crustal block of the Archean basement of the Karelian craton (autochthon) and the Proterozoic granulite block of the Svecofennian belt (allochthon) controls the conditions for the formation of superlithostatic pressure anomalies. Methods of mineral geobarometry and numerical thermomechanical modeling in the rocks of the thrust zone recorded pressures up to 9–11 kbar, while lithostatic pressure not exceeding 4–6 kbar. The obtained results allow us to consider that the nature of the local superlithostatic pressure up to 7–9 kbar, established by mineral geobarometers and numerical thermomechanical modeling, can be explained by the tectonic interaction of blocks with heterogeneous physical and mechanical properties, and not reflect the error of the applied mineral geobarometry instruments.

Metabasite melting is a large-scale geological process that contributes to the formation of felsic volcanics and, to a greater extent, tonalite-trondhjemite-granodiorite (TTG) complexes, which make up a significant part of ancient continental crust. Based on the results of phase equilibrium modelling using the Perple_X software package, melting parameterisation was performed for three compositions: anhydrous mid-ocean ridge basalt (MORB), MORB-H₂O (2.78 wt % H₂O) and hydrated basalt (AOC, altered oceanic crust, 2.78 wt % H₂O) for temperatures of 500–1600°С and pressures of 0.0001–3 GPa. The obtained expressions are in good agreement with the few experimental data and show that for hydrous compositions (MORB-H₂O and AOC) there is a sharp increase in melt volume (up to 20 vol %) in the first 20–30°C after passing the water solidus temperature, the subsequent increase in temperature leads to a more restrained increase in the degree of melting. Modelling has shown that near-solidus melts in hydrous systems have rhyolitic and trachydacite compositions. A further increase in the degree of melting leads to a decrease in SiO₂ and alkaline elements and an increase in CaO, MgO and FeO. The change in volume and composition of the melt is considered in the context of peritectic reactions, as well as changes in H₂O content. Application of the melting parameterisation to metabasalts from subducting slabs in the Cascadia and Central Aleutian subduction hot zones has revealed different degrees of melting of these rocks along the corresponding geotherms; the products of such melting are adakite magmas. The proposed parameterisation of rock melting can be used to analyse the mechanisms of felsic rock formation in different geodynamic settings and can be integrated into existing petrological and petrological-thermomechanical models.