The Ardara pluton, Ireland: deflating an expanded intrusion

Abstract

The Ardara pluton, north-western Ireland, is a circular monzodiorite-granodiorite intrusion with a “tail.” It has been regarded as a typical example of a pluton emplaced entirely by expansion (“balloning”) after initial intrusion. Most of the foliation in the pluton is magmatic, with weak superimposed solid-state features, apart from a thin rind of strong solid-state deformation. The solid-state deformation in the pluton appears to be largely, if not entirely, of post-intrusive, tectonic origin. Flattened microgranitoid enclaves, a concentric magmatic flow foliation and compositional zoning in the pluton are consistent with some expansion of magma prior to crystallization. However, strains in the wall rocks indicate that only a small amount (25%–38%) of the volume of the pluton can be ascribed to ballooning. This relatively small amount of expansion could account for (a) observed structural concordance and strong syn-metamorphic deformation in the aureole, (b) flattening of microgranitoid enclaves (magma globules) in the pluton and possibly (c) a small amount of solid-state deformation in the pluton during “lock-up” of magmatic flow. Evidence of solid-state deformation superimposed on a strong magmatic flow foliation in the tail of the pluton implies that a fault was active during emplacement and may have assisted the intrusion. Further space for the pluton may have been made by a complex interaction of processes, many of which cannot be constrained from a two-dimensional cross-section through the area. These possibilities highlight the limitations of attempting to constrain emplacement mechanisms on the sole basis of structures adjacent to plutons.