The newly discovered Xiweizigou mafic—ultramafic rocks are cropped out in Xinyang town of Tianshui area at the conjunction of the Qinling—Qilian orogenic belts．The ultramafic rocks are composed of serpentinization olivine—pyroxenite and diopsidite，and characterized by low SiO2 and high MgO contents，and similar to that of primitive mantle in rare earth element and trace element．The mafic rocks are composed of meta gabbro and meta gabbrodiorite and have the characteristics of cumulates，and are characterized by low SiO2 and TiO2 contents，high MgO content and Mg#，low Ti/V ratio（20±），high abundances of rare earth elements and relatively flat in HREE．The mafic rocks belong to the tholeiitic series according to their geochemical characteristics．The abundance and variation of high field strength elements and ratios of associated elements suggest that the mafic rocks show characteristics of E type MORB，and are derived from enriched lithosphere mantle which had been hybridized by the crust．Taking the tectonic setting and regional tectonic background into account，the geochemistry characteristics indicate that the Xiweizigou mafic—ultramafic rocks are relatively typical SSZ type ophiolite related to arc—island setting．LA ICP MS zircon U Pb dating yields a 206Pb/238U weighted average age of 443.6±1.8 Ma，indicating that the SSZ type ophiolite should be formed in the Late Ordovician．The formation of the SSZ type ophiolite is closely related to the northward subduction of Tianshui—Wushan Ocean，and suggest a tectonic framework with an integrated ocean—arc—basin system in the Early Paleozoic at the conjunction of the Qinling—Qilian orogenic belts．