New zircon ages, together with a revision of the lithostratigraphy of "suspect" terranes in SW Mongolia, suggest that the collage of continental and oceanic units in this region resulted from recurrent magmatic reworking and deformation of Silurian–early Devonian proximal and distal passive margin sequences of the Paleo-Asian Ocean. Tectonic evolution of studied area during Palaeozioc and Mesozoic is discussed in detail.