Abstract：This paper report the LA-ICP-MS zircon U-Pb age data of the alkaline rocks and lamprophyres in the Gejiu tin district, Yunnan Province. Combined with the character of local geology and geophysical of this area, authors try to discuss the geodynamic setting of western Cathaysia block during Late Yanshan epoch. The dating data shows the alkaline rocks formed at 79.9 Ma and the age of lamprophyre is 82.0 Ma, both of which emplaced during late Yanshanian epoch. Based on the previous geophysical profile, tectonic evolution, and the data achieved this time as well as the rock formation we propose that the Gejiu giant tin deposit formed in an extensional setting in Late Yanshanian epoch.