Oriented Crystal Growth in some Tertiary Dykes

Abstract

DURING investigations of Tertiary dykes of the Skye swarm in the Strathaird and Sleat peninsulas, layers of oriented crystals were found in the feldsparphyric dolerite dykes¹. The dykes contain abundant plagioclase phenocrysts (< 2 cm), less abundant olivine phenocrysts (< 5 mm) and rare pyroxene phenocrysts. Layers have been found in seven dykes (width 1.5–10 m), each of which has two well defined layers parallel to its length and roughly symmetrical about its centre plane. These layers, which persist throughout the exposed length of each dyke (up to 50 m), are 0.2–4 cm wide and contain relatively large elongate crystals (up to 2 cm) oriented at high angles to the plane of the layer. The elongate phases are clinopyroxene, plagioclase and olivine which, when all three are present, succeed each other inwards with some overlap. In some dykes, the olivine and plagioclase layers are absent. Elongate phases comprise up to 70 per cent of a layer, the groundmass of which is similar to and continuous with the fine grained aphyric sub-ophitic dolerite bounding its outer margin. Elongate pyroxene and plagioclase crystals widen towards the dyke centre, whereas the olivines have an intensely dendritic habit (Fig. 1) with dendrites branching towards the dyke centre. These features, together with the sharply defined and planar outer margins of the layers, indicate inward growth of the elongate crystals from a stable substrate.