Timing of the formation of the Tianhuashan Basin in northern Wuyi as constrained by geochronology of volcanic and plutonic rocks

Abstract

The Tianhuashan Basin is one of the most important volcanic basins in the northern Wuyi, southeastern China, comprising two successive volcanic units, the Daguding Formation and the overlying Ehuling Formation, along with several small associated igneous intrusions. The Lengshuikeng super-large-scale Ag-Pb-Zn deposit, which is closely related to these volcanic-intrusive rocks, is located in the northwestern part of the basin. In order to understand the basin evolution and magmatism, we determined LA-ICP-MS U-Pb zircon ages for the volcanic successions and associated intrusive rocks. U-Pb zircon dating of volcanic units yielded precise ages of 144±1 Ma for crystal tuff in the lower member of the Daguding Formation, 142±1 Ma for andesite within the upper member of the Daguding Formation, 140±1 Ma for tuffite of the first (i.e., lowermost) member of the Ehuling Formation, and 137±1 Ma for rhyolitic ignimbrite within the third volcano-stratigraphic member of the Ehuling Formation. Three types of intrusive igneous rocks (quartz syenite porphyry, K-feldspar granite porphyry, and rhyolite porphyry) yielded precise weighted mean 206Pb/238U ages of 144±1, 140±1, and 140±1 Ma, respectively, suggesting that these intrusions along with the aforementioned volcanics were all emplaced during the Early Cretaceous. In addition, the weighted mean 206Pb/238U ages determined on zircon from two samples of a granite porphyry intrusion, which hosts ore mineralization of the Lengshuikeng Ag-Pb-Zn deposit, are 158±1 and 157±1 Ma, indicating emplacement in the Late Jurassic. These new geochronological results for igneous rocks of the Tianhuashan Basin constrain the timing of volcanic and plutonic activity in the basin, and have important implications for our understanding the tectonic history of the region, and for identifying metallogenic types and the timing of ore deposition of the Lengshuikeng deposit.