Abstract
The paper presents the results of geochemical and isotopic-geochemical (Sm-Nd) studies of the Upper Riphean (?) (in the Stratigraphic Scale of Russia) sedimentary rocks of the Urulungui and Dyrbylkey formations of the Daur Series in the Argun continental Massif, as well as U-Th-Pb and Lu–Hf isotopic studies of detrital zircons from these formations. Rocks of the Urulungui and Dyrbylkey formations differ significantly in mineral composition and content of the main elements, which is probably due to the degree of chemical weathering of the source rocks. Analysis of concentrations of trace elements in sedimentary rocks of the Urulungui and Dyrbylkey formations enabled to establish that they were formed in a subduction-related setting. The presence of conglomerate-breccias, conglomerates, gravelstones, fragments of igneous and volcanic rocks, a weak degree of roundness of clastic material, as well as abundant Neoproterozoic detrital zircons in the formations is evidence of tectonic and igneous activities in the region during sedimentation. Sedimentary rocks of the Urulungui Formation are characterized by two-stage model ages TNd(DM2) = 2.0–2.1 Ga, and the Dyrbylkei Formation – TNd(DM2) = 1.6–1.8 Ga according to the results of Sm-Nd isotopic-geochemical studies. The abundant Neoproterozoic zircons in sandstones of the Urulungui and Dyrbylkey formations suggest that Neoproterozoic igneous rocks widely developed within the western Argun continental Massif were the main source of clastic materials. At the same time, the presence of Early Precambrian detrital zircons is evidence that Early Precambrian rocks were also present in the source area. However, Early Precambrian igneous and metamorphic associations whose age would be reliably determined from geochronological data have not yet been found so far within the Argun continental Massif, and thus it is not out of the question that rocks of the Daur Series form a «detached block» intruded in the Argun Massif.
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