Genesis and Evolution of Mantle Melts of the Devonian Mafic-Ultramafic Rocks from the Eastern Azov Region (Dnieper-Donets Rift, Ukraine): Evidence from Clinopyroxene Geochemistry

The paper is dedicated to the study of the Devonian magmatic association of the Eastern Azov region, which is a part of the Pripyat-Dnieper-Donets rift zone. The association includes gabbros, peridotites, pyroxenites, and lamprophyre dikes of the Pokrovo-Kireevsky massif (PKM) and picrites, picrobasalts, and basalts of the Anton-Taram Formation (ATF). Analysis of clinopyroxenes of different generations from the PKM micaceous gabbro and the ATF alkaline picrite provided insight into the mantle source of these rocks and the evolution of melts, which determined the close spatiotemporal association of kimberlites, basites, ultramafic rocks, including alkaline varieties. Clinopyroxenes from the micaceous gabbro are composed of Cpx1 (Mg # = 0.87–0.88) or Сpx2 (Mg # = 0.80–0.81) cores and Cpx3 external zones (Mg # = 0.70–0.76). Clinopyroxenes from alkaline picrite are composed of Сpx2 cores (Mg # = 0.80–0.84) and external Cpx3 zones (Mg # = 0.71–0.78). Clinopyroxenes in general have an upward concave multielement pattern, with enrichment in LREE, depletion in Ba, Nb, and HREE, Zr–Hf negative anomaly, and, additionally, negative Sr-anomaly in Cpx2 and Cpx3. The calculated equilibrium melts for Сpx2 from the micaceous gabbro are very close in composition to this gabbro, and those for Cpx2 from the alkaline picrite coincide in composition with this picrite, and in general are close to ATF picritic lavas. The high Mg# and Cr content in cores Cpx1 indicate that this mineral was derived from the earliest weakly differentiated magma close to the primary melt. The presence of a negative Zr–Hf anomaly in Cpx1 geochemical patterns at Zr_PM РМ suggests that their equilibrium melts were derived through melting of metasomatized, possibly carbonated garnet-bearing peridotites. Cores Cpx1 are likely relics of phenocrysts that crystallized from the earliest melt during the formation of the PKM and accompanying ATF volcanic rocks. An important feature of the Eastern Azov rocks is the very high Ti content (up to 7.3 wt.% TiO₂) in the highest Mg (Mg# 0.48–0.65) and deepest seated (CaO/Al₂O₃> 0.8) melts, which generated picrobasalts and lamprophyres. The geochemical similarity of the early Cpx1 cores with clinopyroxenes from ilmenite-bearing mantle metasomatites is consistent with the assumption that ultrahigh-Ti primary melts of the Eastern Azov lamprophyres were derived from carbonated ilmenite-bearing, and likely, phlogopite-bearing (PIC) peridotites.