SHRIMP U-Pb ages of zircons from the Lengshuikeng ore field, North Wuyi Mountains, South China: Implications for dating volcaniclastic rocks

The Lengshuikeng Ag-Pb-Zn ore field is located in the North Wuyi Mesozoic volcanic belt south of the Qinzhou-Hangzhou suture zone between the Yangtze and Cathaysia paleo-plates. Previous zircon U-Pb geochronological studies on ignimbrites and tuffs from this area have yielded conflicting ages of 157-161 Ma (Early Upper Jurassic) and 137-144 Ma (Early Lower Cretaceous). Volcanic rocks in the ore field have even been proposed to include both ages. Our SHRIMP zircon U-Pb dating of the ignimbrite and tuff samples from the ore field, along with field observations and results from geochronological work on other volcanic and sub-volcanic rocks in the region, shows that two populations of magmatic zircons, one autocrystic and the other xenocrystic, are present in the pyroclastic rocks. The autocrystic zircons have ages suggesting formation/eruption at approximately 140 Ma, whereas the xenocrystic zircons give ages of 155-159 Ma, indicating intrusion of granitic porphyries in the Early Upper Jurassic. Therefore, the pyroclastic rocks in the Lengshuikeng Ag-Pb-Zn ore field formed in the Early Lower Cretaceous. The youngest zircon U-Pb ages from pyroclastic rocks may not represent the formation/eruption ages of the host rock, depending most likely on the existence and/or abundance of juvenile or vitric pyroclasts in the rocks.