Incremental dilation of magma filled fractures: evidence from dykes on the Isle of Skye, Scotland

Localised swarms of narrow (10–120 mm) basaltic dykes occur at offsets and terminations of some larger basaltic dykes in Skye, Scotland. These narrow dykes cut and chill against each other and the larger dyke and its country rock. The narrow dykes can only be traced a short distance into the larger dyke before terminating. Some of these terminations show the central parts of both dykes merging as the chilled margins vanish, at other terminations the later dyke outcrop narrows progressively to a point. These all show that after initial dilation and some crystallisation of material in the dyke fissure there were additional (up to 11) increments of dilation. Increments were sufficiently spaced in time for preceding injections to have become solid. Time spans separating initial and later dilation may be sufficient for local crystallisation of dykes up to 3.2 m thick. Away from these terminations the dyke fissure remained an open void occupied by flowing magma during the successive increments of dilation. This paper demonstrates that fluid filled dyke fissures can vary in width with time, an observation consistent with the modern theoretical models of dyke emplacement that relate dyke width to magma input pressures and other magmatic parameters. Dyke offsets and terminations with late dilational increments are shown to provide a variety of other information on flow, crystallisation and sealing of dyke fissures.