New constraints on the timing of tectonic activity in the Archaean Central Pilbara Craton, Western Australia

K.A. Beintema, P.R.D. Mason, D.R. Nelson, S.H. White, and J.R. Wijbrans

The Archaean Pilbara Craton in Western Australia has a domainal architecture which has been interpreted to reflect a history of accretion. The Tabba Tabba Shear Zone is the major division between the East and West Pilbara blocks: this interpretation is based on significant differences in the tectono-thermal histories of the bordering terranes. New laser ablation ICP-MS and SHRIMP U-Pb zircon geochronological data, coupled with trace element data for the same core parts of the sampled mineral grains, indicate a range of magmatic crystallization ages for representative igneous rocks emplaced before, during or after shearing. Results from both dating techniques agree for two separate homogeneous samples to within analytical error (2s). Our data indicate that a granodioritic suite intruded the area at about 3250 Ma, followed by gabbroic suite at 3235 Ma. The area was subsequently affected by an early dextral compressive event during which the Tabba Tabba Shear Zone was formed, and the granodiorites and gabbros were incorporated into the Tabba Tabba Shear Zone. A granitoid suite intruded the shear zone at 2940 Ma, with xenocrystic populations of 3115 Ma and 3015 Ma, a possibly West Pilbara association. The East and West Pilbara terranes may thus have been relatively close to each other between 3250 and 3115 Ma. The Tabba Tabba Shear Zone currently forms the eastern bounding fault of the Mallina Basin. The last major activity in the structure occurred during a major phase of oblique movement, corresponding to closure of the Mallina Basin. Ages of late syn-kinematic granitic intrusions indicate that this occurred at about 2940 Ma.