Potassium–argon ages of slates and phyllites from south-west England

Abstract

Potassium-argon dates from slates and phyllites will give the date of dynamic metamorphism provided that all radiogenic argon was expelled from the rocks at the time of metamorphism and none has been lost since. Where whole-rock slate ages can be compared with ages on separated mica the agreement is normally quite good. Some instances of incomplete argon loss are known, but these are mostly associated with other evidence of incomplete recrystallization. Significant losses due to continuous diffusion in a stable geological environment have not been observed, but post-metamorphic losses may occur in a tectonic environment. In general K–Ar dates from slates are thought to give useful younger limits to the age of metamorphism, provided that certain statistical and petrological criteria can be satisfied. Sixty slates from south-west England have been dated by the K–Ar method. Material from the Torquay area and north Devon gave scattered results, but the remainder fall into four quite well defined regional groups. (1) A 365–345 m.y. group from the Dodman phyllites and Gramscatho Beds in south Cornwall, associated with sw – ne structural trends, probably corresponds to a ‘Bretonic’ phase of folding in the Upper Devonian or Lower Carboniferous. (2) A group in the range 340–320 m.y. from the e–w Devonian slate belt on the southern limb of the synclinorium, considered in relation to stratigraphical evidence, suggests a Namurian (‘Sudetic’) phase of tectonism. (3) The Start schists and associated slates give 290-310 m.y., but the character of the event which these figures represent is not known. (4) Dates in the range 270–310 m.y. from Port Isaac, the Boscastle–Tintagel area and the southern boundary of Culm, represent minimum ages for the known late Carboniferous folding, and may in part date uplift and cooling at the end of the orogeny.