Alkaline magmatism of the Vitim province, West Transbaikalia, Russia: Age, mineralogical, geochemical and isotope (О, C, D, Sr and Nd) data

In this paper, we study the geochronology, mineral chemistry, and whole-rock elemental, stable (O, C, D) and Sr-Nd isotopic data for alkaline ultrabasic–basic massifs of the Vitim alkaline province (Sayzhenski complex) in the Central Asian Orogenic Belt, near the boundary with the Siberian craton, to evaluate their petrogenesis and geodynamic significance. U-Pb zircon dating results in Early Paleozoic (520–486 Ma) and Late Paleozoic (306–294 Ma) stages of alkaline rock formation. The mineralogy and geochemistry exhibit a wide range of SiO2 (38–73 wt.%), enrichment in Sr, Ba, LREE and Ta and, most significantly, in Na and Al. The rocks crystallized from a parental CO2- and H2O-rich silica-undersaturated melt. Isotopically, the rocks are highly variable, with (87Sr/86Sr)i – 0.705595–0.707729 and (143Nd/144Nd)i – 0.512237–0.512643. The geochemical and isotope data suggest that the rocks were derived from a source composed of three distinct components: PREMA, EM II and marine carbonate. Additionally, stable (O, C, D) isotope data display the shifting influence of assimilated organic sediments in the source of melts and a partial secondary isotope exchange between the late-magmatic fluids and minerals.