Flow Differentiation in the Xenolithic Ultrabasic Dykes of the Cuillins and the Strathaird Peninsula, Isle of Skye, Scotland

The xenolithic Tertiary ultrabasic dykes of south-west Skyeare composed principally of forsteritic olivine, plagioclase,and clinopyroxene with accessory chrome spinel and magnetite.The plagioclase: pyroxene ratios are relatively constant withineach dyke but the absolute amounts of these minerals are dependenton the olivine content which varies, often considerably, acrossthe dykes. Five main types of olivine distribution are recordedfrom the dykes and in all five there is a concentration of olivinecrystals towards the centre of the dyke. The average size ofthe olivine crystals also varies across the dykes with increasesin size generally corresponding to increases in the olivinecontent. The petrogenetic hypothesis of composite intrusion previouslyproposed for these dykes by Bowen (1928) is re-examined andfound to be inadequate. It is suggested that the dykes, whichwere intruded as magmatic suspensions of olivine crystals androck fragments, are unlikely to have been differentiated intheir present positions and the possibility that the differentiationoccurred during their emplacement is examined. It is shown thatthe mineral distributions and crystal size variations occurringin the dykes are, on the whole, analogous to those expectedto arise during laminar flow in vertical conduits, and it isconcluded that flow differentiation occurred during the intrusionof the ultrabasic dykes of south-west Skye. ¹Present address:Department of Geology,University of Toronto,Toronto5,Ontario,Canada.