Incremental pluton emplacement during inclined transpression

Abstract

Multidisciplinary investigations suggest that many igneous plutons were assembled incrementally, through emplacement of discrete magma batches. Internal pluton contacts between different units can be remarkably cryptic and/or complex in nature. This presents a major challenge in understanding both how repeated intrusions were emplaced relative to each other, and in their relationships with regional deformation.Here we present the results of a multidisciplinary study from a Neoarchean pluton exposed in the Yilgarn Craton of Western Australia. Our new structural, geophysical, geochemical and geochronology datasets indicate that the pluton was emplaced incrementally along an active, inclined transpressional shear zone. Structural relationships indicate that magmatic to high-temperature solid-state fabrics locally postdate a mid- to upper-greenschist tectonic fabric in an adjacent older unit. We consider these relationships as diagnostic of incremental emplacement, indicating that the shearzone recorded several cycles of strain softening and hardening, in relation to the syntectonic emplacement of discrete magma pulses, followed by syndeformational cooling down to upper-greenschist facies conditions. Our data also suggest that pluton growth occurred through over-accretion of progressively more evolved magma pulses.