New isotopic age data constrain the depositional age and accretionary history of the Neoproterozoic-Ordovician Mona Complex (Anglesey-Lleyn, Wales)

Our geochronological data give minimum (K-Ar) depositional ages of 578-530 Ma on phengites and maximum (U-Pb zircon) depositional ages of 878-550 Ma, which indicates that the sediments in the structurally uppermost Gwna Group were deposited earlier than those in the middle New Harbour Group (maximum depositional ages of 548-515 Ma, this study) and earlier than in the lowermost South Stack Group (maximum depositional ages of 569-522 Ma, this study). Phengite-rich schists in the Blueschist unit and the CSZ show indistinct K-Ar ages ranging from ca. 578 to 530 Ma. However, the K-Ar age of the New Harbour Group is ca. 474 Ma, which we interpret to reflect a later metamorphic event. In a larger perspective, our new ages are broadly contemporaneous with the calc-alkaline continental arc magmatism in NW Wales and Central England that formed by successive eastward subduction from ca.711 to 474 Ma.