K–Ar results from Western Ireland and their bearing on the timing and siting of Thulean magmatism

Synopsis Potassium-argon ages measured for several small igneous bodies which outcrop in western Ireland range from 55 m.y. to 61 m.y. and confirm a Tertiary age for these intrusions. These results together with geochronological information available for localities elsewhere in the British Tertiary Province are critically assessed. It is apparent that major igneous activity was confined to the interval 60 m.y.–50 m.y. The distribution in time probably comprises a major phase of extrusive and plutonic activity with dyke injection c. 59 m.y., with a later phase, predominantly of dyke intrusion, c. 52 m.y. The distribution in space appears to have been controlled by a series of NW.–SE. and N.–S. trending lineaments. The former may result from plate separation but both trends and the line of parting may have been determined by pre-existing lines of crustal weakness. These were preferentially exploited by bursts of igneous activity c. ? 80 m.y., 59 m.y. and 52 m.y. which correlate with the incidence and modification of sea-floor spreading during the early evolution of the North Atlantic. There is no evidence for systematic magmatic migration with time, thus denying an origin for the Province by drift in the vicinity of a localised mantle “plume” or “hot-spot”.