This work presents the first results of petrographic, petrogeochemical, and geochronological studies of boulder-pebble tuffaceous conglomerates of the Otukdash Formation of the Ordovician Sistigkhem Trough (Tuva segment of the Central Asian Orogenic Belt). The boulders contain fragments of intrusive rocks of the sub-alkaline series: quartz monzonites, monzodiorites, montzoleucogranites and alaskites. U‒Th‒Pb dating of the monzodiorite and monzoleucogranite fragments revealed two stages of different-age magmatism: Late Riphean (~690 Ma) and Early Ordovician (~480 Ma). Ancient fragments of leucoplagiogranites dating back to ~630 Ma were previously identified in the tuffaceous conglomerates of the Cambrian Bayan-Kol Formation in the Sistigkhem Trough. Currently, no magmatic rocks of similar age have been identified in the Tuva segment. The Early Ordovician montzoleucogranites geochemically and geochronologically correlate with the rocks of the Bashkymugur Complex of the Western Sangilen, which allows us to conclude that a single tectonic-magmatic event occurred during their formation. Thus, we obtained direct evidence of the existence of sub-alkaline magmatic complexes in the Late Precambrian and Ordovician in the the Sistigkhem Trough, which served as a source of material for the studied coarse-grained formations.
Bulgakova et al. (Fri,) studied this question.