The development of contrasting structures during the cooling and crystallisation of a syn-kinematic pluton

Abstract

Structural elements of a syn-kinematic pluton in the Pilbara Craton of Western Australia reflect evolving rheology during progressive crystallisation, representing a transition from magmatic, through high-temperature sub-solidus to low-temperature deformation. Magmatic elements include: (1) NNE-trending, sub-vertical, kilometre-scale folds (outlined by xenolith trains) that have a magmatic axial planar foliation defined by aligned K-feldspar, (2) cross-cutting, ENE-trending, sub-vertical shear zones intruded by numerous ≤metre-scale granite sheets, and (3) a narrow, concordant magmatic K-feldspar foliation in the granite host-rock adjacent to the shear zones. Microstructures, such as fractured feldspars healed by quartz that is continuous with matrix grains, further suggest that deformation occurred while the pluton was magmatic. High-temperature structural elements include ‘checkerboard’ sub-grain boundaries in quartz, whereas deformation at lower temperatures is indicated by kinked feldspar, recrystallised microcline and undulose quartz. A NNE-trending, sub-vertical sub-solidus foliation, defined by elongate quartz aggregates, overprints all the above mentioned features. Generation of these sequential structural elements correspond to the transition from Newtonian-fluid, through Bingham-type, to Newtonian-solid behaviour as the crystal fraction of the felsic magma increased. The orientation of the structures is consistent with a regional ESE–WNW shortening direction throughout magma crystallisation. This study indicates how evolving magma rheology and regional strain regimes control the micro- to macro-structural elements of a syn-tectonic pluton.