40 Ar– 39 Ar step-heating ages of the Tertiary igneous rocks of Mull, Scotland

Abstract

Twenty-one samples from throughout the Mull Tertiary igneous succession were dated. Of these, eight gave a satisfactory age plateau and the dates show a systematic decrease throughout the succession. Extrusion of the lavas began c . 60 Ma ago, contemporaneous with the bulk of activity in the British Tertiary igneous province. Igneous activity continued until c . 57 Ma. Probably the lavas were extruded during the reversely magnetized chron 26r; development of the central igneous complex continued into the normal chron 26, while the late dykes were intruded during 25r. If this is so then, according to the polarity timescale of Harland et al. 1982, the duration of igneous activity lies between 0.7 and 4.7 Ma, consistent with the dates. The usefulness of the age plateau selection criteria of Lanphere & Dalrymple (1978) is supported by the results.