Three generations of AMCG magmatism, contact metamorphism and tectonism in the Saguenay-Lac-St-Jean region, Grenville Province, Canada.

Four A-type granitoid plutons from the Saguenay-Lac-St-Jean region of the Grenville Province, lacking regionally imposed solid-state deformation, give U-Pb zircon igneous crystallisation ages of 1146±3, 1082±4, 1068±4, and 1020±4 Ma. 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 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, such as 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, in turn reflecting periods of enhanced tectonic activity.