40AR 39AR TRAVERSE - GRENVILLE FRONT TECTONIC ZONE TO BRITT DOMAIN, GRENVILLE PROVINCE, ONTARIO, CANADA/

Ar-40/Ar-39 data (on hornblende, muscovite and K-feldspar) are present ed for samples from the western Grenville Province taken along a 140-k m traverse from the Grenville Front into the Britt domain. Our interpr etation is based on 28 new analyses, synthesized with 20 previously re ported from the traverse area. In regions where comparisons are possib le, muscovite and (large domain) K-feldspar apparent ages appear simil ar (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 degrees C temperature range, c. 2 degrees C Myr(-1), is appropriate for exhumation controlled by post-orogenic erosional unro ofing. At the Grenville Front Tectonic Zone (GFTZ)- Britt domain bound ary there is a c. 25-Myr offset in both hornblende and muscovite/K-fel dspar ages. We interpret the lower ages in the Britt domain to reflect variations in crustal thickness and geothermal gradient between the f lank and interior of a thick orogen. The argon data from the GFTZ are interpreted in the context of an asymmetric crustal-scale antiformal s tructure developed during a late episode of convergence. Hornblende fr om rocks on either side of the core of the antiform has an apparent ag e 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 asso ciated with the development of the crustal-scale antiform previously p ostulated. In the east, the ages reflect the cooling of material broug ht toward the surface in the flank of the antiform. Hornblendes from t he 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.