Structural evolution of the Warrawoona Greenstone Belt and adjoining granitoid complexes, Pilbara Craton, Australia: implications for Archaean tectonic processes

The Pilbara granite-greenstone terrain (3.6-2.8 Ga), Australia, is quoted a s the key area formed by passive, gravity-driven solid-state diapirism. But , contrasting views have been reported recently, which led us to re-examine the key area: the Warrawoona Greenstone Belt and the adjoining Mt Edgar an d Corunna Downs Granitoid Complexes. We found: (i) an asymmetric, non-syncl inal distribution of metamorphic grade and deformation intensity across the Warrawoona Greenstone Belt; (ii) a major shear zone on the margin of the M t Edgar granitoid complex with a unidirectional stretching lineation patter n; (iii) a NE-trending transcurrent shear zone within the central gneiss co mplex of the Mt Edgar Granitoid Complex; (iv) coeval magmatic activity and doming of the granitoid complexes; and (v) the loss of about 9 km of strati graphic units in the Warrawoona Greenstone Belt. The area has had a prolong ed history with at least four deformation phases between ca. 3.47 and 2.8 G a. The main domal geometry of the Mt Edgar granitoid complex was obtained d uring active NE-SW extension at 3.31 Ga (Dx+2), which appears to been in re sponse to E-W compression. Possibly, the E-W compression resulted in crusta l thickening and partial melting, probably of a 3.46-3.43 Ga granitic basem ent, causing crustal weakening, collapse and extrusion of the 3.31 Ga grani toids. Extension at 3.31 Ga was accommodated by the Mt Edgar Shear Zone, a mid-crustal detachment, and by the associated Beaton Well Zone, which acted as a transfer zone within the gneissic basement. Doming of the Mt Edgar Gr anitoid Complex is interpreted to have occurred in an active, extensional c ore complex-type mode. An indication for deformation prior to 3.32 Ga comes from syntectonically intruded gabbro/diorite and dolerite sills within the Mt Edgar Shear Zone, which intruded possibly as early as 3.46-3.45 Ga (Dx1). Ultramafic schistose xenoliths within the gabbro/diorite intrusions are the oldest observed deformed rocks, formed during D,. Post-doming deformat ion involved NE-SW compression, D-x+3,D- followed by dextral transcurrent d eformation, Dx+4, during NW-SE compression. The latter may be related to th e regional phase of deformation at 2.93 Ga, during which the eastern and we stern Pilbara were amalgamated. Comparison of this deformation sequence to others within the Marble Bar Domain suggests a common tectonic history from 3.46 Ga onwards. Our structural and kinematic data, especially the 3.31 Ga unidirectional stretching lineation pattern, lead us to reject solid-state diapirism due to convectional overturn as the main cause for doming of the Mt Edgar Granitoid Complex. We suggest that the ca-use lies in a plate tec tonic process. (C) 2001 Elsevier Science B.V. All rights reserved.