A petrological investigation of the xenolithic ultrabasic dykes of the Cuillins and the Strathaird Peninsula, Isle of Skye

Abstract

The Tertiary ultrabasic dykes of the Cuillins and the Strathaird peninsula are studied and classified on the bases of their content of cognate xenoliths and textures. Two principal types are, recognised, the Coire Lagan type (similar to those studied by Drever and Johnston, 1958) and the xenolithic Ben Cleat type. The dykes of the Ben Cleat type are investigated in detail. The Ben Cleat dykes are composed principally of olivine (Fa11), plagioclase (Ang4 with normally zoned margins) and clinopyroxene (Ca43 Mg46 Fe11) with accessory chrome spinel: the compositions of these minerals are constant throughout the dykes. The transverse variations in modal amount and crystal size of the three principal minerals within selected representative dykes have been determined and it is established beyond doubt that the dykes are differentiated. The petrogenetic hypothesis of composite intrusion previously proposed for these dykes by Bowen (l928) is examined in the light of this evidence and found to be inadequate. It is suggested that the dykes were intruded as suspensions of olivine crystals and rock fragments in an ultrabasic liquid from which plagioclase, pyroxene and a small amount of olivine subsequently crystallised. It is demonstrated that the dykes are unlikely to have been differentiated in their present positions and the possibility that the differentiation occurred during their emplacement is examined. The processes by which, crystals might migrate in flowing magma are considered, and it is shown that the mineral distributions and crystal size variations occurring in the dykes are analogous to those expected, from the results of theoretical and experimental investigations, to arise during laminar flow of suspensions of solid particles in a viscous fluid in vertical conduits. Several of the apparently anomalous differentiation phenomena are also interpreted in the light of theoretical fluid mechanics as the results of flow in near-vertical fissures. It is concluded that (i) the dykes were each intruded as a single pulse of ultrabasic liquid containing large amounts of olivine phenocsrysts,?(ii) the distinctive type of differentiation which is characteristic of these dykes occurred during their intrusion and (iii) the mechanism involved was flowage differentiation. The petrographiea of the cognate xenoliths are studied and their distribution and orientation explained on the basis of laminar flow of the dyke magma. The age of the xenolithic ultrabasic dykes in relation to the other intrusions of the Cuillin Tertiary igneous complex is reconsidered and it is suggested that the dykes were emplaced contemporaneously with the Sgurr Dubh ultrabasic intrusion. Petrogenetic hypotheses are advanced for both the dykes; and the cognate xenoliths and it is tentatively proposed that both were derived by partial fusion of a deep-seated peridotitic rook.