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SHRIMP U-Pb dating of zircons from volcanic-intrusive complexes in the Xiangshan uranium orefield, Jiangxi Province, and its geological implications

CHEN Zheng-le 1 , WANG Yong 1 , ZHOU Yong-gui 1 , HAN Feng-bin 1 , WANG Ping-an 1 , GONG Hong-liang 1 , SHAO Fei 2 , TANG Xiang-sheng 2 , XU Jin-shan 2 (1. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China; 2. Research Institute No.270, CNNC, Nanchang 330200, Jiangxi, China)  
The volcanic-intrusive complexes were dated by the SHRIMP U-Pb dating method of zircons from the Xiangshan uranium orefield, Jiangxi Province. SHRIMP U -Pb dating of zircons from 7 volcanic rocks yielded 206Pb/238U concordant ages from 133Ma to 142Ma. The weighted mean ages of two porphyroclastic lava samples are 134.6±1.0Ma (MSWD=1.5) and 134.1±1.0Ma (MSWD=0.65), respectively, which constrain the latest volcanic eruption in Xiangshan area at 134Ma. Two sub-porphyritic granites from Baquan in northern and Wuzhang in southern Xiangshan uranium orefield have ages of 133.3±0.8Ma (MSWD=0.82) and 134.7±0.9Ma (MSWD=1.08). The consistent ages of granite porphyry with porphyroclastic lava within the error range indicate that these two types of rocks were approximately simultaneously erupted or/and then intruded, probably with similar origins, as also supported by field evidence and previous geochemical analyses. A rhyodacite sample obtained from drill hole No.ZK111A -1 in the upper part of the Daguding Formation at Shutang in western Xiangshan has age of 141.6±1.7Ma (MSWD=0.9), suggesting that the first-cycle of the volcanic eruption should have ended at about 142Ma, most probably during the Late Jurassic. The intrusive age of a quartz -monzonite granite-porphyry sample from drill hole No. ZK19-2 in Gangshangying area is 136.4±1.0Ma (MSWD=1.7), and the age of rhyodacite-porphyry from the Ruyiting section is 137.4±1.7Ma (MSWD=1.11), indicating an interval age of 136-137Ma between the first and the second cycle of volcanic eruption, and both the granite-porphyry and the rhyodacite -porphyry intruded during the interval time of two volcanic movement cycles. The dating results obtained by the authors and the previous researchers show that the second-cycle of the volcanic-intrusive movement in Xiangshan area ranged from 136Ma to 133Ma in time, belonging to early Cretaceous volcanic event. It is also proposed that the Ehuling Formation in Xiangshan area should be Early Cretaceous in age.
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