The origin of the 1.73-1.70 Ga anorogenic Ulkan volcano-plutonic complex, Siberian platform, Russia: inferences from geochronological, geochemical and Nd-Sr-Pb isotopic data

The Ulkan volcano-plutonic complex, a part of a 750 km Bilyakchian-Ulkan anorogenic belt, is located in the eastern part of the Archean-Paleoproterozoic Aldan shield. The tectonic position and geochemistry indicate that the Ulkan Complex is a typical A-type or intraplate magmatic association. The felsic volcanics of the Uian Group and granitoids of the North Uchur Massif, the major igneous components of the Ulkan Complex, have U-Pb zircon and monazite ages between 1721±1 Ma and 1703±18 Ma. Together with the spatially associated 1736±6 Ma Dzhugdzhur anorthosite massif, the Ulkan Complex forms a typical Proterozoic anorthosite-granite-volcanic association with the minimum duration of formation of 12 m.y. Initial ε_Nd values between 0 and 1.1, similar for the Uian felsic volcanics, early granitoid phases of the North Uchur Massif and high-grade metamorphic basement rocks, indicate, along with geochemical data, that the crustal source of the Ulkan parental magmas may be similar to the basement rocks. The higher ε_Nd(T) values of -0.3 to +1.9 in the later North Uchur granitoids and associated ore-bearing metasomatites, and relatively low time-integrated Rb/Sr, U/Pb, and Th/U estimated for their sources, may demonstrate involvement of variable amounts of a depleted mantle-derived component in the generation of later phases of the North Uchur Massif. The preferred model of formation of magmas parental to the Ulkan Complex involves thermal interaction of an uprising mantle diapir with Paleoproterozoic lower crust, which was accompanied by chemical interaction between a fluid derived from the diapir, with the lower crustal rocks.