Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Four A-type granitoid plutons from the Saguenay-Lac-Saint-Jean region of the Grenville Province, lacking regionally imposed solid-state deformation, give UPb zircon igneous crystallisation ages of 1146 ± 3 Ma, 1082 ± 3 Ma, 1067 ± 3 Ma, and 1020+4−3 Ma. These ages indicate that the region has not been affected by the Ottawan period of the Grenville orogeny. A compilation of these and existing UPb age data reveals three periods of Mesoproterozoic magmatism in this region: 1160-1140 Ma, 1082-1050 Ma and 1020-1010 Ma. Each of these periods appears to have had some or all of the components of the well-known anorthosite-mangerite-charnockite-granite suite, and the first two periods can be correlated with magmatism elsewhere in the Grenville Province. Sparse UPb metamorphic ages also fall within these periods and are interpreted to reflect regional-scale contact metamorphism produced by the large plutons. In addition, the few ages of strike-slip movements on major shear zones also fall within these time periods. It is difficult to interpret these data as reflecting separate orogenic events, as there is little definitive evidence of calc-alkaline magmatism, thrusting or regional metamorphism in this region. Perhaps during the interval 1160 to 1010 Ma magmatism was driven by repeated upwelling of hot mantle, either in a plume or above a sinking lithospheric slab, but access to the mid-levels of the crust currently exposed was controlled by strike-slip movements on major vertical shear zones that resulted from plate-tectonic activity.