A 40AR 39AR STUDY OF POSTTECTONIC COOLING IN THE BRITT DOMAIN OF THE GRENVILLE PROVINCE, ONTARIO

We report the results of a 40Ar/39Ar study of rocks from the Britt domain, northwestern Grenville Province. Most of the age spectra obtained for micas and amphiboles have well defined age plateaus. Typically, K-feldspars yielded spectra with age gradients and Arrhenius plots with kinks or offsets. Hence, some of the data were interpreted in terms of a multi-diffusion domain-size model. This 40Ar/39Ar data base (5 amphiboles, 9 micas, 4 K-feldspar), along with some recently acquired U-Pb data, provide reasonable constraints on the post-peak metamorphic cooling history which began at depth in the upper amphibolite facies. Comparison of the data with exhumation paths generated by thermal models suggest that cooling, taking place over approximately 400 Ma, was for the most part due to erosionally controlled exhumation. Although the decompression may have initiated with a short period of rapid (extensional?) uplift, the entire cooling history may have been erosionally controlled. There is an apparent difference in cooling ages between north and south in the 40Ar/39Ar mica and K-feldspar data for the Britt domain, which may be due to somewhat higher exhumation rates in the north and/or an elevated geotherm in the south.