LA-ICPMS Zircon U-Pb Dating for Baimashan and Jintan Indosinian Granitic Plutons and Its Petrogenetic Implications

The Indosinian tectonic setting of the South China Block is long-debated due to poor understanding of precise geochronology of the associated magmatism. Our studies of highly precise LA-ICPMS zircon U-Pb geochronology on Indosinian granitic plutons in the Baimashan (Hunan) and Jintan areas(Jiangxi) show that the biotite monzonitic granite from the Jintan area yielded a weighted mean 206Pb/238U age of (222.1±1.6)Ma,and biotite monzonitic granite and biotite granodiorite from the Baimashan area yielded the concordant ages of (223.3±1.4)Ma and (204.5±2.2)Ma,respectively. These ages represent the formation age of the granitic plutons. In conjunction with other geochronological data,it is inferred that there may be three-episode magma activities for Hunan Baimashan intrusion during the Indosinian involving the first intrusion at about 240Ma,and subsequent intrusion activities of 220Ma and 204Ma,respectively. It is additionally showed an extensive tectono-magmatic activity at about 220Ma for the hinterland of the South China Block.