40 Ar/ 39 Ar traverse — Grenville Front Tectonic Zone to Britt Domain, Grenville Province, Ontario, Canada *

40 Ar/ 39 Ar data (on hornblende, muscovite and K‐feldspar) are presented for samples from the western Grenville Province taken along a 140‐km traverse from the Grenville Front into the Britt domain. Our interpretation is based on 28 new analyses, synthesized with 20 previously reported from the traverse area. In regions where comparisons are possible, muscovite and (large domain) K‐feldspar apparent ages appear similar (at c. 920–930 Ma), but throughout the traverse, these are c. 60–70 Myr younger than the hornblende ages. The inferred cooling rate over the c. 350–500°C temperature range, c. 2°C Myr ‐1 , is appropriate for exhumation controlled by post‐orogenic erosional unroofing. At the Grenville Front Tectonic Zone (GFTZ) — Britt domain boundary there is a c. 25‐Myr offset in both hornblende and muscovite/K‐feldspar ages. We interpret the lower ages in the Britt domain to reflect variations in crustal thickness and geothermal gradient between the flank and interior of a thick orogen. The argon data from the GFTZ are interpreted in the context of an asymmetric crustal‐scale antiformal structure developed during a late episode of convergence. Hornblende from rocks on either side of the core of the antiform has an apparent age of c. 990 Ma, our estimate of the age of the compressional event. In the west, we infer that these date the short‐lived thermal event associated with the development of the crustal‐scale antiform previously postulated. In the east, the ages reflect the cooling of material brought toward the surface in the flank of the antiform. Hornblendes from the antiform core appear to contain excess radiogenic argon. We suggest that this was the ambient argon in rocks transported from depth that was subsequently trapped when the rocks cooled rapidly.