The dacite cropping out at Jilusu on the northern margin of Qaidam basin has lots of adakitic features: SiO263.7%~64.3%, Al2O315.07%~15.27 %, MgO 3.47%~3.72 %, K2O/Na2O ratio is about 0.14~0.25, Y and Yb are both low: Y is 17.82×10-6~18.10×10-6, 17.96×10-6on average, and Yb is 1.47×10-6~1.55×10-6, Sr is high (527×10-6~537×10-6), HREE is depleted and Eu and Sr negative anomalies are relatively weak, but the content of MgO is higher than that of the typical adakite in the world. Generally,the presence of adakite signifies the beginning of slab subduction. The age of adakitic dacite is about 514.2±8.5 Ma, and the age of eclogite from the northern margin of Qaidam basin is 494.6±6.5 Ma. Therefore, it is considered that the slab started its subduction in Late Cambrian, that the adakitic magma was formed at the subduction depth of about 75~85 km, and that the partial melting residues with garnet facies continued subducting and finally formed eclogite in Early Ordovician. The discovery of adakitic dacite confirms that ocean-continent subduction occurred in Early Palaeozoic on the northern margin of Qaidam basin.