Abstract

Dr M. D. Max & Dr J. C. Roddick write: Dallmeyer and Gibbons (1987) have provided evidence on the timing of blueschist metamorphism in eastern Anglesey. We present a further 40Ar/39Ar amphibole mineral analysis that supports this timing and examine the wider implications of the data.

Figure D1 is an age spectrum for amphibole from a massive crossite-epidote rock (Geological Survey of Ireland specimen 77–1745) taken from a quarry along the northern side of the old Bangor-Holyhead road about 250m to the ESE of the Marquess of Anglesey’s Monument (537 715 on 1 : 50 000 sheet 114, Anglesey). This is about 1 km from the localities sampled by Dallmeyer and Gibbons. The rock was composed of about 70% felted, schistose, light blue to light violet crossite, up to (rarely) 1 mm in length, with about 22% epidote group minerals and about 8% chlorite. There was neither apparent retrogression nor intergrowth with other amphibole species. The analysed separate consisted of 74 mg of 100-250 pm grains of essentially pure crossite.

The age spectrum is broadly similar to that of sample 1 of Dallmeyer and Gibbons, giving a maximum age of 550 Ma. For the final 20% gas the ages decrease to 517 Ma, a phenomenon seen in other amphibole analyses (Harrison & McDougall 1981) and possibly related to the degassing of a high Ca region within the crossite, as reflected in this case by the low K/Ca ratio (Fig. Dl). The peak age of 550 Ma is considered to provide