LAST GASP OF THE GRENVILLE OROGENY - THERMOCHRONOLOGY OF THE GRENVILLE FRONT TECTONIC ZONE NEAR KILLARNEY, ONTARIO

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 Grenvi lle Front Tectonic Zone (GFTZ) near Killarney, Ontario. High-grade met amorphic assemblages (approximately 1450 Ma) in this part of the GFTZ pre-date the Grenvillian orogeny and were primarily exhumed, with litt le or no metamorphic overprinting, by Grenvillian deformation. The tit anite 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 ar e interpreted in terms of partial lead loss during a short-lived therm al event that increased in intensity from west to east across the tran sect. Ar-40/Ar-39 data from hornblende indicate cooling through approx imately 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-degree s-C at approximately 930 Ma. Biotite data are not easily interpreted o wing to the effects of partial resetting and/or excess Ar-40. The ther mochronological data suggest that a thermal event with peak temperatur es of 500-600-degrees-C affected the GFTZ at approximately 980 Ma, fol lowed by very rapid cooling to approximately 350-degrees-C. We interpr et the data in terms of a tectonic model involving rapid exhumation of GFTZ rocks (in response to erosion) in the hangingwall of a crustal-s cale shear zone developed during a approximately 980 Ma episode of con vergence.