Abstract

Integrated structural and petrological data are used to trace the evolution of the Northern Arran Coarse Granite during the latter stages of its ascent and its emplacement. Structural analysis of the aureole of the granite suggests that it rose as a single body of magma, which is confirmed by petrological data. During its ascent, the pluton intersected the downward-dipping limb of an existing synform, refolding and displacing it upward to form a concentric or rim synform. This direct evidence of vertical displacement of the country rocks confirms that the pluton is a diapir. Diapiric ascent was permitted by a reduction in wall-rock viscosity and hence viscous drag forces on the pluton, aided by heat loss due to crystallization during ascent. Most of the deformation within the aureole can be attributed to ascent of the pluton. The early rim synform is overprinted by flattening strains and faulting caused by in situ radial expansion, which represents the ultimate evolution of a diapiric body. The final shape of the pluton is determined by inhomogeneous deformation of the aureole, which is partly controlled by pre-existing faults.

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