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

Abstract

The Mona Complex in Anglesey-Lleyn, Wales formed by Avalonian subduction and accretion from the latest Neoproterozoic to early Paleozoic. It comprises an ophiolite, high-pressure metamorphic rocks, volcaniclastic sediments and mélanges. However, understanding of the tectonic evolution has been held back by the paucity of age constraints, thus only a major geochronological study will be sufficient to decipher the subduction, accretion and exhumation history of this significant Avalonian orogen. We conducted U-Pb dating of detrital zircons in the Monian Supergroup in order to constrain the maximum depositional ages, and undertook K-Ar dating of phengites and U-Pb dating of detrital zircons in the Blueschist unit, the Central Shear Zone (CSZ), and the New Harbour Group to estimate the timing and duration of the metamorphic events, and to constrain the minimum depositional ages of the Gwna Group.Our geochronological data give minimum (K-Ar) depositional ages of 578–530Ma on phengites and maximum (U-Pb zircon) depositional ages of 878–550Ma, 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 548–515Ma, this study) and earlier than in the lowermost South Stack Group (maximum depositional ages of 569–522Ma, this study). Phengite-rich schists in the Blueschist unit and the CSZ show indistinct K-Ar ages ranging from ca. 578 to 530Ma. However, the K-Ar age of the New Harbour Group is ca. 474Ma, 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 474Ma.