Mesoproterozoic within-plate igneous province of the western urals: Main petrogenetic rock types and their origin

The Mesoproterozoic (1.38–1.30 Ga) Kama-Belsk igneous province (KBP) was formed at the eastern margin of the East European Platform (EEP), in the Volga-Ural area and Bashkirian anticlinorium. It is made up of plutonic, volcanic, and subvolcanic (numerous dike and sill swarms) rocks of bimodal composition. KBP, as most of large igneous provinces, contains two geochemical types of basites: high-titanium (HTi) rocks with TiO2 > 1.5 wt % and low-titanium (LTi) rocks with TiO2 < 1.5–2.0 wt %. They demonstrate zoned distribution, were derived from different mantle sources at different regimes of their partial melting. The high and low-titanium basites significantly differ in geochemical and isotopic (Sr, Nd, O) parameters. The HTi rocks are characterized by Ti/Y > 400, (Gd/Yb) n = 1.62–4.08, (Dy/Yb) n = 1.31–2.43; Nb/Nb* from 0.5 to 1.3, while the LTi rocks have Ti/Y < 400, (Gd/Yb) n = 1.23–1.51, (Dy/Yb)n = 1.01–1.26, and Nb/Nb* from 0.3 to 0.9. The HTi rocks have ɛNd(T) from + 1.3 to −2.4, while the LTi rocks are characterized by ɛNd(T) from + 0.5 to −6.1. The oxygen isotopic composition δ18O is 5.0–5.9‰ in the LTi rocks and 7.0‰ in the HTi picrobasalts. According to obtained estimates, the parental melts for the LTi type (Mg# = 0.76) are comparable with high-Mg primary melts inferred for within-plate picrites. The parental melts for the HTi type (Mg# = 0.69) had higher Fe contents, which in combination with lowered Al2O3 and elevated TiO2, Na2O, and P2O5 make these rocks similar to ferropicrites. The HTi melts were presumably derived by partial melting of a pyroxenite in equilibrium with garnet-bearing residue, whereas the LTi melts were generated from peridotite protolith and left spinel-bearing residuum. Both the varieties of the basites were contaminated mainly by Paleoproterozoic crustal material.