LATE-STAGE PROPAGATION OF THE GRENVILLE OROGEN: THERMOBAROMETRY AND TIMES OF PEAK METAMORPHISM FROM SUDBURY METADIABASE, GEORGIAN BAY, ONTARIO

Abstract

Did the final (Rigolet) phase of the Grenvillian Orogeny result from renewed convergence or post-convergent ductile flow? This question remains unresolved in part because of limited data on the timing of late Grenvillian tectonism in the parautochthonous Britt Domain (BD) in central Ontario, which occupies the critical region between the Grenville Front and the orogenic core. This thesis presents P-T-t data from Sudbury metadiabase (SmD), part of a ca. 1235 Ma dyke swarm that intruded Laurentian crust after pre-Grenvillian tectonism, and records exclusively Grenvillian (< 1100 Ma) metamorphism. Samples were collected along a 40 km transect representing the southern, central, and northern BD along Georgian Bay. All samples preserve relict igneous textures overprinted by granulite-facies corona assemblages including garnet + clinopyroxene ± orthopyroxene ± pargasite. Thermobarometry on equilibrated subdomains yielded T-P estimates of 785 °C and 15 kbar, 763 °C and 13.1 kbar, and 718 °C and 13.4 kbar for rocks from HA Gray Island in the southern BD, Byng Inlet in the central BD and Key Harbour in the northern BD, respectively. U-Pb geochronology of metamorphic zircon yielded weighted mean 206Pb/238U ages of 1033 ± 12 Ma, 1020 ± 8.7 Ma and 1011 ± 11 Ma for the southern, central, and northern BD samples, respectively. Metamorphic ages from the southern and central BD (ca. 1033 and 1020 Ma) overlap with previously reported ages of syn-kinematic pegmatite dykes in the Shawanaga Shear Zone (ca. 1042-1019 Ma), which record normal-sense ductile reactivation of the Allochthon Boundary Thrust. The northern BD age (ca. 1011 Ma) is similar to other metamorphic ages from the Grenville Front Tectonic Zone (ca. 1010-980 Ma). These new ages are interpreted to reflect ca. 2 mm/y propagation of Grenvillian metamorphism and deformation from SE to NW across the parautochthon, compatible with model predictions for post-convergent ductile flow.