Novel age constraints on offshore islets around Taiwan with implications for the northern Taiwan volcanic zone

Determining the exact age of volcanic activity in the Northern Taiwan Volcanic Zone (NTVZ) is essential for comprehending Taiwan's tectonic evolution. The timing and duration of the volcanic processes that formed the NTVZ remain uncertain despite the availability of geochronological, geochemical, and geophysical data. While extensive efforts have been directed towards investigating the age of the Tatun Volcanic Group, the primary feature of the NTVZ, many other aspects require further examination. This study concentrates on two offshore islets in the northern region of the NTVZ. We present new ⁴⁰Ar/³⁹Ar ages of 0.252 ± 0.018 Ma for a lava flow, representing an early stage of volcanic activity, and 0.118 ± 0.012 Ma for a dike, representing the latest stage of volcanic activity around Pengjia Islet. A sample collected from Mianhua Islet did not yield a reasonable age due to high uncertainty, likely attributable to young and low radiogenic argon. Field observations support the interpretation that volcanic activity on both islets is considerably younger than previously estimated. This study corroborates the hypothesis from geophysical studies that submarine NTVZ features to record a potentially active magmatic reservoir offshore of northern Taiwan. By refining our understanding of spatio-temporal patterns in volcanic activity, this study contributes to volcanic hazard assessment in Taiwan and surrounding areas.