Objectives: On the basis of 1∶50000 regional geological survey, it is discovered that pyroxene and gabbro are widely exposed in the shizishan copper orefield, Yimen County, Yunnan Province. The studies on basic magmatic rocks in the area are relatively less and the diabase formation time data are absent. In this paper, we carried out zircon laser denudation (LA- ICP- MS) dating and geochemical analysis on the diabase in Shizishan Copper orefield, Yimen County. Results and Conclusions: The weighted average age of zircon U- Pb is 1858±18 Ma (MSWD=0.90,n=13). It represents the formation and emplacement of the diabase. The whole rock geochemical characteristics of the diabase are similar to those of the OIB, and show that the diabase formed in a extensional tectonic environment. So, the Yangtze plate was in the post- collisional intraplate extensional tectonic environment at 1858 Ma ago, which should be related to the break- up of Columbia supercontinents. The sedimentary time of the “Dongchuan Group” intruded by the diabase in the Shizishan orefield must be earlier than that of the diabase emplacement. Combined with the lithologic assemblage and regional data, the “Dongchuan Group” should be divided into the Mesoarchaean Yuanjiang Group.