According to geological, petrological, geochemical, and geochronological studies, the Haraa Gol terrane in the western Hentiyn Mts. is dominated by two rock assemblages of different ages, associated with the initiation and development of the island arcs and marginal spreading seas of the Mongol–Okhotsk Ocean. The Late Cambrian, Early Ordovician, and Middle Ordovician were marked by the effusion of basalt and basaltic andesite and the formation of gabbro and gabbro-dolerite in back-arc spreading basins. In the Late Silurian–Devonian, after a short pause, tectonomagmatic processes were activated, with the formation of differentiated island-arc volcanics, gabbro, and granitoids. Their absolute 40Ar–39Ar age is given in the paper. The model age of the TNd(DM) protolith of the Haraa Gol igneous rocks corresponds to the composition of the Mesoproterozoic juvenile crust.