SHRIMP U-Pb ZIRCON DATING OF THE JIUYISHAN COMPOSITE GRANITE IN HUNAN AND ITS GEOLOGICAL SIGNIFICANCE

The SHRIMP (sensitive high resolution ion microprobe) zircon U-Pb method was used to determine the ages of main rock masses in the Jiuyishan composite granite, Hunan Province. The Xuehuading granite was dated at 432±21 Ma, the Jingjiling granite, 156±2 Ma, the Shaziling granite, 157±1 Ma, and the Xishan volcanic-intrusive complex, 156±2 Ma. The results show that the Xuehuading granite and the Jingjiling granite belong to Caledonian epoch and early Yanshanian epoch, respectively, and the Shaziling granite and the Xishan volcanic-intrusive complex, which were originally considered to be of Indosinian epoch and late Yanshanian epoch, respectively, actually belong to the early Yanshanian epoch, too. The inherited zircons of 1579 Ma, 2108 Ma and 2669Ma suggests that there were derived from the Paleo-Mesoproterozoic or Archean basement crust. The formation event of lots of early Neoproterozoic (912 Ma) zircons is an indirect evidence that the collisional belt between the Yangtze block and the Cathaysia block passed through the studied area.