The 40 Ar‐ 39 Ar laser analysis of K‐feldspar: Constraints on the uplift history of the Grenville Province in Ontario and New York

A comprehensive geochronologic database describes a period of late extension across the Metasedimentary Belt (MB) of the Grenville Province in northeastern North America. Because extension continued through much of the young unroofing history of the region, these data do not bracket the timing of final extension across all shear zones and do not constrain the timing of final juxtaposition of terranes. Analysis of K‐feldspar in the MB by the 40 Ar‐ 39 Ar method can give information on the very latest perturbations in the area since K‐feldspars have multiple, low closure temperatures, ranging from about 150° to 300°C. These multiple diffusion domains require temperature‐time modeling to fully interpret argon release spectra. This modeling requires precise knowledge of the temperature of degassing of each step, which usually requires the use of a resistance furnace for sample analysis. We establish a first‐order relationship between laser power and temperature, which can be used for multidiffusion domain modeling of K‐feldspars. Comparison of samples analyzed by both methods reveals virtually no difference between the systems, supporting the validity of K‐feldspar analyses by laser step heating. Our data suggest that using the laser for K‐feldspars can give results that are geologically reasonable, precise, and easier to collect. In this case, these data are then applied to the cooling history of the North American Grenville Province. Our K‐feldspar analyses show that the latest extensional motion along shear zones in the MB is after 900 Ma, and the region is uplifting as a uniform block by 780 Ma.