Two cleaved, epizone-grade Palaeozoic mudrock suites from Snowdonia yield a mean Rb-Sr whole-rock age of 409 ± 11 Ma. This is consistent with resetting of the Rb-Sr whole-rock isotope systems of the mudrocks during cleavage development in end-Silurian/Early Devonian (Acadian) deformation. In contrast, cleaved Palaeozoic anchizone mudrocks from north and central Wales give a Silurian mean age of 430 ± 9 Ma. This dates either a pre-tectonic isotopic event preserved through Acadian anchizone metamorphism, or mid-Silurian deformation and metamorphism at anchizone grade unaffected by the Acadian event.
The precise timing of the main Caledonian deformation in Wales has never been established firmly. Powell & Phillips (1985) suggest that it occurred in north and south Wales in latest Silurian time, whereas Ashgill and end-Llandovery tectonic events were dominant in central Wales (see also George 1963). However, Woodcock (1984,, 1987) and Soper et al. 1987 have argued in favour of a single tectonic event of early to middle Devonian age. This chronological study was undertaken to address this controversy.
The only previous attempt to define precisely the timing of cleavage development in Wales was the K-Ar dating of cleavage-parallel chlorite from a volcanic greywacke from north Wales (Fitch et al. 1969). When recalculated using modern decay constants, these data give an age of 399 ± 10 Ma. Bath (1974) demonstrated the feasibility of obtaining Rb-Sr ages from cleaved mudrocks of the Welsh Caledonides, and this work produced ‘Caledonian’ ages of 409 ± 20Ma and 393 ± 22Ma from north Wales and ‘late