FORMATION OF THE ABITIBI GREENSTONE-BELT BY ARC-TRENCH MIGRATION

Volcanic and plutonic rocks of Archean greenstone belts in the Abitibi subprovince become younger southward; plutonic rocks are similar to 1 0-30 m.y. younger than volcanic rocks. The Abitibi greenstone belt has an upper crust of similar to 2.75-2.70 Ga oceanic supracrustal assemb lages and a middle crust of similar to 2.70-2.68 Ga granitic and gneis sic rocks. The greenstone belt upper crust is greenschist facies with upright structures and low reflectivity, and the middle crust is amphi bolite facies with moderate to shallowly dipping structures and strong reflectivity. We propose that the age progression and crustal layerin g resulted from accretion of oceanic assemblages followed by extensive mid-crustal magmatism linked to southward migration of an arc-trench system. Thermal softening of the middle crust during magmatism caused ductile flow now represented by strong, mid-crustal reflectivity. Upwa rd heat flow following magmatism caused late-tectonic (2.67-2.65 Ga) l ow-pressure metamorphism observed in the upper crust.