Evolution of melt during the crystallization of the Bele pipe basanites (North Minusa depression)

The paper is devoted to the mineralogy and thermobarogeochemistry of the Bele pipe basanites. We report the chemical composition of phenocrysts (olivine, clinopyroxene, and plagioclase) and minerals of basanite groundmass (olivine, clinopyroxene, magnetite, plagioclase, biotite, apatite, amphibole, etc.). The minimum temperatures of mineral formation were established by the homogenization temperatures of primary melt inclusions. The groundmass minerals crystallized in the sequence: olivine (>1200-1230°C) → clinopyroxene (1100-1160°C) → plagioclase (990-1140°C), magnetite → apatite (1040-1100°C) → biotite, ilmenite, amphibole, (nepheline), K-feldspar. The presence of rhönite as a daughter phase of olivine-hosted melt inclusions is ample evidence of subsurface crystallization of the groundmass minerals (P≤0.6 kbar). Electron microprobe analysis of the homogenized and residual inclusion glasses in the minerals has shown that the evolution of basanitic melt during crystallization had a miaskitic character and was accompanied by its enrichment with SiO₂, Al₂O₃, and alkalies and depletion with mafic components. Oxygen fugacity during the basanite crystallization was higher than the QFM buffer values. Studies of the chemical composition of the largest olivine and clinopyroxene phenocrysts have shown that their cores are possibly relics of disintegrated mantle xenoliths. Zoned phenocrysts of clinopyroxene might have reacted with basanite melt at T≥1210°C, which resulted in "pseudoprimary" inclusions in their cores.