Dating the onset of volcanism at the Rum Igneous Centre, NW Scotland

Major volcanic activity on the Isle of Rum commenced with the eruption of thick (>100 m) intracaldera rhyodacite ash-flow sheets fed from steep-sided feeder conduits in the proximity of the Main Ring Fault. Twenty plagioclase phenocrysts of the rhyodacite were analysed using single crystal Ar-40/Ar-39 laser dating, yielding a mean apparent age of 60.83 +/- 0.27 Ma (MSWD = 3.65). On an age v probability plot the feldspars do not, however, show a simple Gaussian distribution, but a major peak at 60.33 +/- 0.21 Ma and two smaller shoulders at c. 61.4 Ma and 63 Ma. These older ages are interpreted to represent recycled and largely re-equilibrated feldspars. The age peak at 60.33 +/- 0.21 Ma is interpreted to represent the intrusion and eruption age of the rhyodacites. This new age constraint overlaps with that for the ultrabasic intrusion, implying that the latter was already forming at depth and supplying necessary heat during the early felsic activity phase, and quickly thereafter migrated upwards to shallow structural levels and intruded the volcano's earlier deposits. Combined with previously published ages, these new age data highlight an extremely rapid succession of events at the Rum centre, the whole sequence occurring in potentially <500 ka.