Interplay between deformation and magmatism during doming of the Archaean Shaw granitoid complex, Pilbara craton, western Australia

The Shaw Granitoid Complex, one of the classic granitoid domes of the Archaean Pilbara Craton, Western Australia, evolved from a plutonic complex into a ∼60 km diameter steep-sided dome during migmatisation of the deep crust. Consistent stratigraphic younging of greenstones away from the dome and underlying outward-dipping domical foliations indicate that the present exposure is a crustal section. The section passes downward through progressively older greenstones, into a near-concordant, high-level, sheeted granite complex, the ca. 3470 Ma North Shaw Suite (NSS), to a younger sill-dyke complex of leucogranite, the Pilga Leucogranite. Farther below, the leucogranite grades via diatexite and a network of leucogranitic dykes into the lowermost unit of migmatitic orthogneisses, which have protolith ages that overlap with the NSS. A semi-continuous, solid-state domal foliation extends downward from the greenstones, through the NSS into the migmatitic orthogneiss, but is less evident in the intervening Pilga Leucogranite. A set of NNW-trending upright folds with axial planar leucogranite dykes exist in both the Pilga Leucogranite and underlying orthogneisses, but not in the NSS or greenstones. Field relations indicate that much of the solid-state, dome-concordant foliation developed in a sub-horizontal attitude before the leucogranite sheets were emplaced above the migmatitic orthogneisses. Emplacement occurred along a sub-horizontal high-strain zone, probably at a rheological boundary between the relatively rigid NSS and softer underlying migmatitic orthogneisses. The sheeted leucogranite complex and underlying orthogneisses were subsequently folded with later leucogranite magmas injected along the axial surfaces of the folds. These leucogranite magmas locally extended into the overlying deformed NSS, where they also acquired a weak domical fabric, suggesting syn-doming emplacement. Although presently estimated to be 3445–3410 Ma old, stratigraphic constraints suggest that the leucogranite and the dome could be as young as 3300 Ma. The contrasting strain patterns above and below the Pilga Leucogranite are not expected for horizontal tectonic models invoking thrusting, core-complex formation or cross-folding, but are similar to one observed for gravity-driven, vertical tectonic models. Thermal softening initiated deformation, and then deformation localised melts at a specific rheological interface. Once established, this interface controlled the locus of Pilga Leucogranite intrusion during dome evolution, demonstrating the interplay between deformation and magmatism.