Abstract: In order to deepen the understanding about the tectonic evolution of Dunhuang land mass, and to understand the tectonic deformation history of the alteration rock type gold depositin Xiaoxigong shear zone before the mineralization, the mylonitized quartz diorite body from the south side of Xiaoxigong gold deposit have been selected to carry out the LA-ICP-MS zircon U-Pb chronology and geochemical studes. In this paper, 24 zircons have been analyzed totally, their Th and U contents range from 46.5×10^-6 to 266.4×10^-6 and from 196.3×10^-6 to 496.3×10^-6, respectively, their Th/U ratios vary from 0.24 to 0.53, and their LA-ICP-MS U-Pb ages are (411.3±5.8)Ma (MSWD=2.7), with the formation age of early Devonian.The SiO₂ and TiO₂ contents of these mylonitized quartz diorites vary from 57.88% to 58.26% and from 0.81% to 0.86% respectively, their σ values range from 1.38 to 1.56, their alkali indexes(NK/A) change from 0.4 to 0.41, showing the features of calc-alkaline rocks.Their aluminum saturations(A/CNK) vary from 0.95 to 0.98, belonging to the subaluminous rock series, with I-type granite characteristics.These mylonitized quartz diorites have weak Eu anomalies, their ΣREE range from 73.85×10^-6 to 160.37×10^-6, with moderate low ΣREE content. Their ΣLREE/ΣHREE ratios change from 8.3 to 11.25, with the relative loss of HREE. The contents of Ba, Sr and Zr are higher, while the contents of Rb, Th and U are lower. The tectonic setting for these mylonitized quartz diorites is the volcanic arc environment, which is considered as the response of the Dunhuang block the subducted and collised with the Beishan orogenic belt during the Early Paleozoic to Early Late Mesozoic. The obtained dating data suggests that the subduction collision was lasted to the Early Devonian in this area.