The western part of the West Siberian petroleum megabasin; geologic history and structure of the basement

On the basis of complex research into the western part of the West Siberian Plate, a new scheme of the structure-formation zones of its basement has been developed and a new geological map of its pre-Jurassic basement has been compiled. Ophiolites and other mafic-ultramafic complexes (fragments of the oceanic Earth's crust) were studied. The most complete Late Ordovician ophiolite complex comprising a melange of serpentinites, gabbroids, plagiogranites, and basalt with jasper interbeds lies in the Shaim area. Weakly altered spinel lherzolites were also examined there. The research has revealed not only Ordovician but also Devonian basalts as well as gabbroid complexes (392 Ma, Sm-Nd dating). The Paleozoic geodynamic history of the region was terminated with collision, which was accompanied by tectonic piling and the intrusion of granite plutons, metamorphism, and formation of new continental crust. These crucial events, which consolidated Paleozoic complexes throughout the vast area of the future West Siberian megabasin, date from the Early Permian. The composition, age, and nature of granitoid plutons in western West Siberia were studied, and isochrons and isotope characteristics were obtained, which give an insight into both the genesis and age of granite plutons and their subsequent transformations.