Late Mesozoic magmatism of the Jiurui mineralization district in the Middle-Lower Yangtze River Metallogenic Belt, Eastern China: Precise U-Pb ages and geodynamic implications

摘要

The Middle-Lower Yangtze River metallogenic belt (MLYRMB), extending from Daye in Hubei Province in the west to Zhenjiang in Jiangsu Province in the east, hosts a number of large polymetallic (Cu-Au-Mo, Fe, Zn, Pb, and Ag) deposits and constitutes one of the most important metallogenic belts in China. The Cu-Au-Mo deposits in the Jiurui district are an important component of the MLYRMB. In this study we carried out precise and detailed zircon U-Pb dating for all types of magmatic rocks from the Wushan ore deposit in the Jiurui district. Three samples of Cu-Au-Mo-related porphyries from different ore belts at Wushan were analyzed and yielded zircon U-Pb ages of 148.0 ± 1.0. Ma, 145.4 ± 0.9. Ma and 147.3 ± 0.9. Ma, respectively. A series of dykes were emplaced immediately following the Cu-Au-Mo-related porphyries at Wushan. A dark-colored basic dyke which intruded into the granodiorite porphyry at Wujia gold deposit near Wushan was dated at 144.5 ± 1.2. Ma. Two lamprophyre dykes taken from the north ore belt at Wushan underground mining stops were dated at 143.6 ± 0.9. Ma and 144.3. ± 0.9. Ma, respectively. A late-stage dyke which was also taken from the Wushan north ore belt yielded an age of 142.6 ± 1.0. Ma and might represent the end of magmatism in the Wushan ore deposit. These new geochronological data demonstrate that the time range of magmatism in the Wushan ore deposit is approximately between 148. Ma and 143. Ma, showing that the magmatic activity at Wushan was rapid and intensive. The ages of Cu-Au-Mo-related porphyries from other areas in the Jiurui district, such as the Dongleiwan, Yangjishan and Chengmenba ore deposits, were also measured and yielded zircon U-Pb ages of 141.5 ± 1.7. Ma, 143.4 ± 1.4. Ma and 146.6 ± 1.0. Ma, respectively. Combined with those previously reported zircon U-Pb age results from the Jiurui district, the present age data set demonstrates that extensive magmatism in the Jiurui district was coeval and intensive, marked by a magmatic activity in the age range of 148 to 138. Ma and peaked between 148. Ma and 142. Ma. According to the statistics of all those precise zircon U-Pb ages, the Cu-Au-Mo-related porphyries in the Edong and Tongling districts in the MLYRMB show similar ages, and they have a slightly younger peak age and a longer duration than that of the Jiurui district. The geographic shape of the MLYRMB in the Cretaceous shows an arcuate structure, the Jiurui district is located at the transitional point of the arcuate structure and the Edong and Tongling districts are situated on both sides of the arcuate structure. Considering that the Jiurui district has a slightly older peak age and a shorter duration of magmatic activity than that in the Edong and Tongling districts, it seems that the arcuate structure of the MLYRMB played an important role in the formation of these Cu-Au-Mo-related porphyries. Consequently, we suggest that the genesis of the Late Mesozoic magmatic rocks along the MLYRMB may have been due to a tectonic activity developed from southeast to northwest, which probably has a close relationship with the subduction of the paleo-Pacific plate beneath the Eurasian plate in Mesozoic times.