Mesoproterozoic olivine gabbronorites of the Bashkirian anticlinorium,the South Urals: Parental melts and specifics of magma evolution

This paper is devoted to detailed study of picritic rocks (olivine melanogabbronorites) and comagmatic gabbrodolerites from sills and dikes in the central part of the Bashkirian meganticlinorium. These rocks are ascribed to the Kama-Belsk magmatic province (KBP) that was formed in the eastern East European Platform (EEP) in the Mesoproterozoic time. The study of minerals (EMPA, SIMS), rocks, and their oxygen isotope compositions showed the contribution of crustal contamination, fractional crystallization and cumulus processes in their formation. The geochemical indicators of crustal contamination (Nb/Nb*, (Nb/La) _n , δ¹⁸O, and others) show strong variations, which indicates uneven crustal contribution in the parental melts during rock formation (10–25%). The study of weakly contaminated (δ¹⁸O = 5.3‰) olivine melanogabbronorites (MgO = 22.55 wt %) from the small Ishlya-1 subvolcanic body, which contain subordinate amount of cumulus (24%), high-magnesian olivine (Fo91.3), and high-Cr spinel (cr# 0.67), as well as HREE depleted clinopyroxenes, allowed us to retrieve the composition of parental melt. The latter contained about 20 wt % MgO and was formed by 19–26% melting of mantle source (potential mantle temperature T _m of 1530–1545°C). Geochemical characteristics of KBP reflect the formation of primary melts by melting of mantle column at different depths, mixing of the melts, and significant contamination by crustal material. The dominant role in the formation of the rocks of the Ishlya area and Mashak Complex was played by derivatives of spinel peridotites, while the rocks of the Bakal-Satka area were derived from garnet peridotites.