Rheological behaviour of mafic dykes deformed in a granite host, Wanna, Eyre Peninsula, South Australia

The Tournefort Metadolerite dyke swarm, emplaced at ~1810 Ma into granites now exposed on the coast of Eyre Peninsula, South Australia, were deformed under granulite metamorphic conditions during the Kimban Orogeny (~1730–1690 Ma). Products of strain localization within the mafic dykes and flattening of dyke margins have allowed us to develop a structural chronology. During the initial deformation (KD1), in a dextral shear regime, the mafic dykes were boudinaged and became elongate rafts in a variably deformed granite. The mafic dykes were initially weaker than the host-rock granites, with grain size reduction during early shearing on dyke margins (shear strain, γ ≥ 20), which was accompanied by leucosome development. The surrounding granite is undeformed as a strain shadow, with shear strains increasing away from the dyke contact into a strongly deformed megacrystic felsic gneiss. Deformation progressively changed to a flattening strain (KD2), responsible for pinch-and-swell structures, further boudinage and development of late shear bands. Structural complexities associated with the mafic dykes were produced by the amplification and refraction of a compressive stress across rheological interfaces. These are most complex adjacent to boudin necks and where greater rheological differences existed between the dyke and the granite host-rock. With progressive deformation there was a relative change in the rheology of the mafic dykes vs. granite that was a function not only of mineralogy, but of the metamorphic and deformation histories in the two lithologies. •Mafic dykes and granites are deformed under granulite facies conditions.•Exceptional exposures show dyke-parallel shear followed by flattening strain.•Dyke margins are sheared and ‘weaker’ than granite host and dyke-centre.•Competency contrasts control a chronology of structural development.•A model is proposed to explain boudinage and leucosomes in mafic dyke necks.