Extensional structures during deposition of the 3460 Ma Warrawoona Group in the eastern Pilbara Craton, Western Australia

Abstract

Structures in the granite-greenstone terrain of the eastern Pilbara were mapped in both the Coongan Belt greenstones and Shaw Batholith granitoids. Important structures that are coeval with an early magmatic event, in both the granitoids and the greenstones, were identified. In the Coongan greenstones a brittle extensional fault array is syn-kinematic with felsic volcanics of the Duffer Formation, dated at 3452 ± 16 Ma (Pidgeon, 1978a). In the underlying Shaw Batholith a major ductile shear zone was mapped at the contact with the Coongan Belt. Although the foliation in the shear zone forms part of a domal structure, the stretching lineations constantly plunge to the ENE. The sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the EW-trending part and dextral movement on the NS-trending part. A wedge-shaped granitoid intruded into the shear zone during deformation. This granitoid is part of the North Shaw Suite which has been dated at 3467 ± 6 Ma (McNaughton et al., 1988). The structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels. The structural/kinematic, magmatic, metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry. In turn, this implies that the domal geometry of the Shaw Batholith was initiated, at mid crustal levels, at a very early stage (namely during deposition of the Warrawoona Group) in the evolution of the Pilbara craton. As extension occurred during intrusion and extrusion of large volumes of magma, the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry. The initial domal shape of the Shaw Batholith has been enhanced during later deformation phases, which caused steepening of NS-trending structures.