Mineral chemistry and thermobarometry of the pre-rift Upper Cretaceous to Paleocene melilite-bearing dykes from the northern part of the Bohemian Massif (Ploučnice River region): Implications for compositional variations of spinels from ultracalcic melts

The Ploučnice River region (polzenite group) is uniquely characterized by its melilite-bearing subvolcanic rocks located in the northern section of the Bohemian Massif. They are the crystallization products of ultracalcic melts during the pre-rift evolution of the Ohře/Eger Rift, which is the easternmost part of the European Cenozoic Rift System. The melt was produced by low-degree partial melting of carbonate-bearing garnet peridotite and pyroxenite at a depth of approximately 100 km (P ~ 3.0 Gpa). The rapid ascent of the ultracalcic melts through the lithosphere was accompanied by fractional crystallization of olivine + spinel ± clinopyroxene mainly within the upper to middle crustal storage zone at depths between 12 and 24 km (0.3–0.6 Gpa). Notably, olivine crystallized generally at higher temperatures (1257–1356 °C) compared to clinopyroxene (1156–1203 °C) and plagioclase (1099–1112 °C). The calculated oxygen fugacity during fractional crystallization (perovskite, −4.8 to +3.9 ΔNNO) decreases at the late-stage of crystallization due to residual magma exsolving oxidizing fluids and decreased fO₂ (oxygen fugacity) of the magmas from which monticellite was crystallized (ΔNNO −6.0 to −3.9). The rounded shapes and chemical composition (Cr/(Cr + Al) 0.52–0.82) of partially resorbed chromite xenocrystic cores in subhedral to euhedral spinel grains indicate that they originated in the mantle. The first stage of magmatic evolution for the studied rocks is related to the Cr-spinel (Cr/(Cr + Al) 0.35–0.50) crystallization, which successively changed to a high-alumina composition (Cr/(Cr + Al) 0.25–0.30). Magnetite (magnetite–ulvöspinel solid solution) forms an atoll texture or small euhedral crystals in the groundmass. Both textural types of magnetite crystallized during the late-stage magmatic evolution of the ultracalcic melt. Carbonate or quartz-rich xenoliths were incorporated during magma emplacement under the upper crust. Sr-Nd isotopic data, mineral composition, and whole-rock chemical composition all verified that the assimilation of the xenoliths only affected the chemical composition of the host magma in the immediate neighborhood of the contract (up to a few millimeters around the xenolith).