Cumulate relations in the ‘Younger Basic’ masses of the Huntly–Portsoy area, Grampian Region

Abstract

Synopsis The ‘Younger Basic’ masses of the Huntly–Portsoy area have been found to contain a number of separate cumulate bodies ranging up to 3–4 km in dimensions. Olivine, plagioclase and augite are the dominant cumulus phases throughout, and although the composition of these phases varies from one body to another, consistencies in the compositional variation suggest that the individual cumulate units are probably fragments of a large stratiform intrusion that has been subjected to post-magmatic disruption. Although some disturbance may have occurred when a later pulse of magma was emplaced to form the granular noritic and gabbroic rocks associated with the cumulates, it is concluded that the fragmentation of the latter was mainly associated with a regional episode of post-‘Younger Basic’ shearing and mylonitization.