U-Pb and 40Ar/39Ar geochronology of the Saint-Urbain and Lac Allard (Havre-Saint-Pierre) anorthosites and their associated Fe-Ti oxide ores, Québec: Evidence for emplacement and slow cooling during the collisional Ottawan orogeny in the Grenville Province

Intrusive suites containing anorthosite-mangerite-charnockite-granite (AMCG) are a characteristic feature of Mesoproterozoic magmatic activity in the Grenville Province of eastern North America. In this study, the U-Pb and 40Ar/39Ar geochronology of two AMCG suites (Saint-Urbain and Havre-Saint-Pierre), and associated rutile-bearing hemo-ilmenite deposits, within the allochthonous polycyclic belt of the Grenville Province in Québec are used to constrain their crystallization ages and cooling histories, and to assess their tectonic setting. Both AMCG suites, separated by 700 km along the St. Lawrence River, have similar crystallization ages (ca. 1050-1060 Ma) and durations of magmatism (10-15 million years). The 450 km2 Saint-Urbain anorthosite is a composite intrusion with a distinctly younger central leuconorite (1046.2 ± 3.1 Ma) compared with anorthosite to the north and south (1055.0 ± 2.4 Ma and 1053.6 ± 2.6 Ma, respectively). The age of a deformed oxide-apatite gabbronorite (1057.4 ± 1.5 Ma) along the northwest margin of the intrusion is within analytical uncertainty of the anorthosites, whereas the oldest intrusion in the Saint-Urbain area is the regionally extensive Saint-Anne du Nord orthopyroxene granodiorite (1060.8 ± 2.8 Ma). The large 11,000 km2 Havre-Saint-Pierre anorthosite suite comprises a number of intrusive phases, including the 2700 km2 Lac Allard anorthosite, which contains the giant Lac Tio ilmenite deposit and Big Island ilmenitite dyke. U-Pb zircon ages of three anorthosites from the Lac Allard intrusion, including the host to the Lac Tio deposit, overlap within error (1061.6 ± 3.0 Ma, 1060.5 ± 1.9 Ma, 1057.4 ± 8.4 Ma) and the crosscutting Big Island dyke yields an age of 1052.9 ± 6.5 Ma. These results indicate that the Havre-Saint-Pierre anorthosite should be divided into an older geon 11 part (ca. 1129 Ma) and a younger geon 10 part (ca. 1060 Ma). Average cooling rates of 3-4 °C/million years were estimated for the two AMCG suites by combining crystallization ages (U-Pb zircon) and cooling ages (U-Pb rutile, 40Ar/39Ar biotite/plagioclase) and reflect emplacement of the intrusions into a hot, long-duration orogen. Emplacement of these two AMCG suites at ca. 1050-1060 Ma is related to regional extensional tectonics near the end of peak metamorphism associated with the collisional Ottawan orogenic phase of the Grenville orogeny. The results of this study provide strong support for proposals that link AMCG magmatism associated with the Grenville orogeny to convective thinning or delamination of the lithosphere and subsequent melting of upwelling asthenospheric mantle.