A THERMAL-GRADIENT AT CONSTANT-PRESSURE - IMPLICATIONS FOR LOW-PRESSURE TO MEDIUM-PRESSURE METAMORPHISM IN A COMPRESSIONAL TECTONIC SETTING, FLIN-FLON AND KISSEYNEW DOMAINS, TRANS-HUDSON OROGEN, CENTRAL CANADA

The Kisseynew Domain, a major tectonostratigraphic domain in the inter nal Trans-Hudson Orogen of central Canada, originated as a sedimentary basin filled with turbidite of the Burntwood Suite at 1.86-1.84 Ga. R ocks of the Kisseynew Domain were thrust southwestward over, and struc turally interleaved with, the 1.9-Ga Snow Lake island are and ocean-fl oor assemblage during convergence of the Trans-Hudson Orogen and the A rchean Superior craton at similar to 1.84-1.81 Ga. At Snow Lake, the z one of tectonic interleaving records polyphase deformation (F-1-F-4) a nd is characterized by a northerly increase in peak temperatures of me tamorphism at 4-6 kbar pressure. The central Kisseynew Domain was meta morphosed at uniform high grade of 750 +/- 50 degrees C and 5-6 kbar. Pressure and temperature calculations on 13 representative samples fro m the Snow Lake area in a critical temperature window of 500-700 degre es C (staurolite and sillimanite zones) yield evidence for two success ive thermal regimes that varied in time and intensity from south to no rth. F-1 (1.842-1.835 Ga) was the major burial event, and may have bee n followed by thermal relaxation in a 15-35 Ma time interval between F -1 and F-2. Garnet commenced growing during early F-2 (1.82-1.805 Ga) at similar to 500 degrees C and similar to 4 kbar throughout the study area. During F-2, temperature and pressure in the staurolite zone inc reased by similar to 50 degrees C and 1-1.5 kbar to peak conditions. I n the staurolite zone, cooling had commenced by the time of F-3, as in dicated by chlorite-grade F-3 assemblages. In the sillimanite zone, ho wever, metamorphism was related to a thermal event in the Kisseynew Do main. Here, peak conditions continued until after F-3 (duration of mor e than 10 Ma), on the basis of isograds and isotherms cross-cutting la rge F-3 structures. We suggest that low-to medium-pressure, high-tempe rature metamorphism in the Kisseynew Domain resulted from heat advecte d by sheets of similar to 1.815 Ga peraluminous granitic rocks. The so lidi of the granitic rocks buffered the peak conditions of metamorphis m. A possible cause for the mobilization of granitic magma in the lowe r crust of the Kisseynew Domain was high basal heat-flow resulting fro m convective thinning of the lithosphere during thickening of the crus t.