Ar-40/Ar-39 age constraints on tectonothermal events in the Shaw area of the eastern Pilbara granite-greenstone terrain (W Australia): 700 Ma of Archean tectonic evolution

The Shaw area in the eastern Pilbara Craton, consists of granitoids with in trusion ages ranging from 3470 Ma to 2850 Ma, and supracrustal rocks with a ges ranging from 3515 Ma to 3100 Ma as determined by U-Pb and Pb-Pb dating methods. In this terrain several major deformation events have recently bee n recognized, including late strike-slip deformation, compression and early extension. In order to provide time constraints for these events, twenty-f our samples from the Shaw area have been used for Ar-40/Ar-39 step heating experiments (twenty-two hornblendes, one muscovite, one biotite), resulting in spectra defining ages between 3520 and 2800 Ma. The late strike-slip ev ent occurred prior to 2944 +/- 9 Ma. The argon cooling ages provide time co nstraints for the compressional event between 3325 Ma and 3200 Ma. Cooling ages between 3300 and 3200 Ma cannot be related to granitoid intrusion in t his area and are interpreted as cooling after regional metamorphism related to the compressional event. The best time constraint from Ar-40/Ar-39 ages for the Split Rock Shear Zone, a major ductile detachment in the Shaw bath olith associated with early extension, is that it was last active prior to 3222 +/- 13 Ma. A 3341 +/- 13 Ma plateau age of a metabasalt sampled in con tact with,granites of the northwestern Corunna Downs Batholith provides a m inimum age for granitoid intrusion and a minimum age for the Warrery Shear Zone. Hornblendes from the Black Range Dolerite Dyke gave a plateau age of 2796 +/- 9 Ma, providing a constraint on the time of cratonization of the P ilbara granitoid-greenstone terrain. Most of the cooling ages, including a 3466 +/- 13 Ma age for the contact zone of the North Shaw Granodiorite, are related to intrusion of granitoids. In the Shaw Batholith, a large number, but significantly, not all of the sample population has been reset during the intrusion of late to post-tectonic adamellites between 3050 Ma and 2850 Ma. Preservation of such a large range (700 Ma) in argon isotopic ages in a relatively small area is unusual. The resetting of argon ages in the gran itoid domain is best explained by a combination of preferential intrusion o f younger granites and the difference in heat conductivity of granite versu s basalt. In the greenstone domain the resetting effect is more localized i nto a network of fluid flow channels, mostly situated in shear zones. (C) 1 999 Elsevier Science B.V. All rights reserved.