Detailed field, petrological and geochemical studies of the Jiugequan ophiolite and its overlying rock suites, North Qilian, are reported in this paper. The volcan ic rocks are made up of cycles of pyroclastic rocks, basalts and tuffs. The ophi ol ite is covered conformably with a suite of volcanicsedimentary rocks, namely t h e ophiolite's overlying rock suites (OORS). The basalts from the ophiolite are t ypical Ntype MORBs, whereas those from the OORS are Etype MORBs. The highly/ mo derately incompatible trace elements are more and more enriched up wards within the OORS. Geochemisty of the basalts and petrography of the sediments b oth record the geodynamic processes during the oceanic crust's moving from the s preading axis to the continental margin. It is inferred that the time interval w as relatively short between the birth of the Jiugequan ophiolite and its obducti on.