The age of blueschist metamorphism in Anglesey, North Wales: evidence from 40 Ar/ 39 Ar mineral dates of the Penmynydd schists

Abstract

The 'Penmynydd' schists of SE Anglesey include several high P /low T lithologies including: (1) metasedimentary quartz–phengite schist; and (2) mafic blueschist with barroisite/crossite assemblages that appear to have developed from an earlier actinolitic greenschist facies protolith. One phengite and three amphibole concentrates have been analysed by incremental-release 40 Ar/ 39 Ar dating methods and display discordant age spectra with anomalously young apparent ages recorded in low T increments. Ages increase systematically throughout intermediate T portions of the analyses to define high T plateaux of c . 550–560 Ma (barroisite/crossite-rich concentrates) and c. 580–590 Ma (actinolite-rich concentrate). The phengite concentrate also displays an internally discordant spectrum with intermediate T increments defining ages similar to those recorded by barroisite/crossite and high T increments giving ages similar to those of actinolite. The amphibole and phengite ages are interpreted as dating post-metamorphic cooling following a regional low- P Ml greenschist metamorphism ( c . 580–590 Ma) and a high- P M2 metamorphism ( c . 550–560 Ma). 40 Ar/ 39 Ar ages suggest multiple metamorphism of oceanic crust before the arrival of the blueschist terrane and its accretion to the late Precambrian basement of southern Britain immediately prior to development of the Welsh Basin. An age of c. 550–560 Ma for blueschist metamorphism on Anglesey provides a likely maximum age for ignimbrites within the pre-late Lower Cambrian Arfon Group exposed in NW Wales.