The Palaeogene Bracken Bay-Straiton dyke: composition and controls on intrusion

The Bracken Bay–Straiton dyke, a member of the Palaeogene Mull dyke swarm, is one of the Solitary dykes of southern Scotland and northern England recognized by Geikie (1897). It shows a remarkable (unique?) change of strike direction, deviating from the regional NW–SE trend to propagate along the Southern Upland Fault for 16 km before apparently resuming its southeasterly course in the Southern Uplands. The dyke is a clinopyroxene–plagioclase–phyric quartz tholeiite, showing little petrographic variation along strike. However, geochemical data indicate that the dyke segment south of the Southern Upland Fault is not comagmatic with the main dyke; it is here distinguished as the Kello Water dyke. The reason for the dyke’s deviation at the fault is uncertain but may be related to a significant role for crustal heterogeneities, particularly Caledonian fractures which had earlier been utilized by the Late Carboniferous quartz dolerite dyke swarm.