Yanjiadian dioritic pluton, located in the east of the Qilian orogenic belt, is the largest Early Paleozoic composite dioritic pluton in the conjunction area of the Qinling and Qilian orogenic belts, and consists mainly of quart diorites and diorites. Previous studies suggest that the samples from northern Yanjiadian diorite has a formation age of ~440Ma by UPb zircon dating, produced by the northward subduction of the interarc rift(or initial backarc basin) represented by Hongtubao basic volcanic rock during the MiddleLate Ordovician period. Regional geology survey shows that s paralleloriented structure is well developed from north to south within the Yanjiadian dioritie with mylonitic texture and gneissic structure. LAICPMS UPb zircon dating of the Yanjiadian dioritie in the south indicates that the diorite formed at 418±32Ma. Geochemical analysis of major and trace elements display high SiO2, Al2O3, Na2O and Sr contents, low Yb and Y contents, high ratios of Sr/Y and (La/Yb)N, and no or weakly Eu anomalies, similar to the geochemical characteristics of the adakites. Low ratios of Na/K, Nb/U and Ce/Pb, and nagtive εHf(t) values suggest that these adakitic rocks resulted from the partial melting of the continental crust. High ratios of Y/Yb, Ho/Y, and Nb/Ta imply that the rocks in the source have been metamorphosed into the eclogite facies. Highdensity, eclogitefacies mafic lower crust may be detached from crust and sink into the mantle due to gravitational instability, and subsequent reaction with mantle peridotites results in the high MgO and Mg# of the samples from the southern of the Yanjiadian diorite. In conclusion, the crust thickness in the conjunction area of QinlingQilian progressively increased during the period of ~440Ma to ~418Ma, and ~418 Ma diorite at Yanjiadian formed in the postsubuduction and collision orogenic stage. Both events occurred at the same geodynamic setting and time framework as the North Qilian tectonic belt did.