A review of mesoscopic magmatic structures and their potential for evaluating the hypersolidus evolution of intrusive complexes

Abstract

The perception that plutonic bodies are structurally simple is disappearing with the recognition of an array of magmatic structures useful for constraining hypersolidus temporal histories, evolving rheologies, strain fields, flow directions, growth and cooling patterns, tilting and syn-emplacement tectonism. These histories provide a powerful means of testing an array of growth, emplacement, chamber evolution and tectonic models. Important points include that: (1) many structures form in “hydrogranular” or congested magma slurries during magma mush avalanching, local convection and late hypersolidus strain challenging the notion that magmas must have ≤55% crystals to convect/fractionate and form compositional diversity in upper crustal magma chambers; (2) magmatic fabrics reflect transient strain in these slurries rather than flow directly: the latter must be inferred by linking geometries with temporal and kinematic information; (3) caution is needed when using anisotropy of magnetic susceptibility and similar quantitative tools that characterize, through a single ellipsoid, preferred orientations of mineral grains given the increasing recognition of multiple fabrics in plutons; and (4) that future studies are particularly needed in plutons focusing on the distribution and styles of compositionally defined structures, magmatic folding, shear zones and faults, multiple fabrics and of the physical/chemical behaviors in hydrogranular slurries.