Ondor Sum Group is situated in the southern area of Suzuoqi,Inner Mongola, which is divided into Baoyintu Group in Early Proterozoic ear(1 910Ma), Ondor Sum Group in Middle Late Proterozoic ear(1 511Ma—825Ma), and ophiolitic melange in Early Paleozoic era(409Ma).This region is a magmatic passive continent margin in Middle-Late Proterozoic ear. An ocean basin between continents is formed in Early Paleozoic era. The ophiolitic melange occurs in Wuletu-Wulanhuduge-Chaganwula area. The authors consider that the ophiolitic melange is a dismembered and broken ophiolitic suite. The matrix consists of sericite-and serpentine-schist, including many block fragments such as dolomite, silicolite, ultramafic rock, basic volcanic rock and limestone. The MgO/FeO* and MgO/(MgO+FeO*)ratios of the ultramafic rocks are close to those of most metaperidotites around the world. The basic volcanic rocks are similar to the oceanic tholeiite in composition. Geochemistry of major and rare-earth elements of the basic volcanic rocks indicates the features of an ocean basin between continents. The Sm-Nd whole-rock isochron for the ophiolitic melange yields an age of 409Ma. This is consistent with the fact that the ophiolitic melange is unconformably covered by the Late Devonian Seribayanaobao Formation. It is evident that the ophiolitic melange was formed in Middle-Late Silurian and tectonically emplaced between Early and Middle Devonian.