The northern zone of West Sayan, made up of Vendian, Early Cambrian, and Middle Cambrian volcanosedimentary rocks, is a fragment of a large island-arc system on the southwestern periphery of the Siberian craton. The specifics of this zone is a wide occurrence of plagiogranitoids of the Maina tonalite-plagiogranite complex, which are spatially associated only with the deposits of the Early Cambrian Nizhny (Lower) Monok Formation. By now, the nature and age of the Maina plagiogranites have been debatable because of the absence of reliable isotope-geochronological data and drastic diversity of opinions regarding the nature of pebble of plagiogranitoids from conglomerates of the Lower and Middle Cambrian volcanosedimentary deposits.
Plagiogranitoids of the Maina complex were compared with plagiogranitoid pebble from the lower beds of the Verkhny (Upper) Monok Formation, which contain paleontologic remains of Early Cambrian age. It has been established that some pebble varieties do not differ in petrographic, petrochemical, and trace-element compositions from plagiogranites of the Yenisei and Tabat plutons. Results of U-Pb and Ar-Ar isotope dating of the plagiogranitoids point to their Early Cambrian age (523.8±2.1 Ma). Isotope-geochemical modeling of the composition of the source of plagiogranites showed that their primary melts were produced from metabasic substrates corresponding in trace-element composition to island-arc tholeiitic basalts at low (<10 kbar) pressures.