Hf isotopic composition and age constraints on the evolution of the Archean central Uchi Subprovince, Ontario, Canada

The Archean Uchi Subprovince, located in the northwestern Superior Province, comprises greenstone belts and granitoid complexes that formed in the course of a multistage history between about 3000 and 2690 Ma. The earliest volcanic-plutonic complexes were assembled into a microcontinent, which then progressively grew by the addition of magmatic area and tectonic slivers. A major compressional orogeny concluded this evolution at around 2700 Ma. Lu-Hf isotopic compositions for well dated zircon populations from volcanic and plutonic rocks of the central part of the subprovince have epsilon(Hf) values that range from +6.1 to +1.4. These results show that the bulk of the province was derived predominantly from depleted mantle sources but there is a superimposed trend showing involvement of the oldest greenstone and crustal sequences in the generation of the youngest magmatic complexes. The temporal and isotopic relationships of the Uchi Subprovince contrast with the situation observed in the southern Abitibi greenstone belt, which developed within a time span of less than 80 m.y. in an intraoceanic setting, and exhibits quite uniform, depleted-mantle-like isotopic values revealing only very subordinate degrees of recycling of older crust.