Last Gasp of the Grenville Orogeny - Thermochronology of the Grenville Front Tectonic Zone Near Killarney, Ontario

Abstract

We present U-Pb (titanite, zircon) and Ar-40/Ar-39 (hornblende, mica, K-feldspar) data from a transect across the western part of the Grenville Front Tectonic Zone (GFTZ) near Killarney, Ontario. High-grade metamorphic assemblages (approximately 1450 Ma) in this part of the GFTZ pre-date the Grenvillian orogeny and were primarily exhumed, with little or no metamorphic overprinting, by Grenvillian deformation. The titanite and zircon data form a discordant array with an upper intercept of 1454 +/- 8 Ma and a lower intercept of 978 +/- 13 Ma. These data are interpreted in terms of partial lead loss during a short-lived thermal event that increased in intensity from west to east across the transect. Ar-40/Ar-39 data from hornblende indicate cooling through approximately 450-degrees-C at approximately 993-979 Ma, multiple diffusion domain models for the interpretation of discordant K-feldspar spectra indicate cooling through approximately 365-340-degrees-C at 990-960 Ma, and muscovite data indicate cooling through approximately 320-degrees-C at approximately 930 Ma. Biotite data are not easily interpreted owing to the effects of partial resetting and/or excess Ar-40. The thermochronological data suggest that a thermal event with peak temperatures of 500-600-degrees-C affected the GFTZ at approximately 980 Ma, followed by very rapid cooling to approximately 350-degrees-C. We interpret the data in terms of a tectonic model involving rapid exhumation of GFTZ rocks (in response to erosion) in the hangingwall of a crustal-scale shear zone developed during a approximately 980 Ma episode of convergence.